LT3505 1.2A, Step-Down Switching Regulator in 3mm × 3mm DFN FEATURES DESCRIPTION ! Wide Input Range: 3.6V to 36V Operating, The LT®3505 is a current mode PWM step-down DC/DC 40V Maximum converter with an internal 1.4A power switch. The wide ! Up to 1.2A Output Current operating input range of 3.6V to 36V (40V maximum) ! Resistor-Programmable Fixed-Frequency Operation makes the LT3505 ideal for regulating power from a wide from 200kHz to 3MHz variety of sources, including unregulated wall transform- ! Output Adjustable Down to 780mV ers, 24V industrial supplies and automotive batteries. The ! Short-Circuit Robust oscillator can be programmed for high frequency operation ! Uses Tiny Capacitors and Inductors allowing the use of tiny, low cost external components or ! Soft-Start it can be programmed for lower frequency operation to ! Low Shutdown Current: <2µA maximize effi ciency. ! Low V Switch: 350mV at 1A CESAT Cycle-by-cycle current limit provides protection against ! Thermally Enhanced, Low Profi le 3mm x 3mm shorted outputs and soft-start eliminates input current DFN-8 and MSOP-8 Packages surge during start-up. The low current (<2µA) shutdown mode provides output disconnect, enabling easy power APPLICATIONS management in battery-powered systems. ! Automotive Battery Regulation , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. ! Industrial Control Supplies ! Wall Transformer Regulation ! Distributed Supply Regulation ! Battery-Powered Equipment TYPICAL APPLICATION 750kHz, 3.3V Step-Down Converter Effi ciency 4.2V TO 3V6IVN VIN BOOST V3.O3UVT 90 0.1µF 10µH 11..12AA,, VVIINN >> 58VV 85 ON OFF SHDN SW 80 LT3505 36.5k 22pF %) 75 FB Y ( C RT GND VC 11.3k 10µF FFICIEN 7605 E 75.0k 69.8k 60 VIN = 12V VOUT = 3.3V 1µF 68pF 55 fSW = 750kHz L = 10 H 50 0 0.2 0.4 0.6 0.8 1.0 1.2 3505 TA01 LOAD CURRENT (A) 3505fc 1

LT3505 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Voltage (V ) ....................................................40V Operating Temperature Range (Note 2) IN BOOST Pin Voltage ..................................................50V LT3505E ..............................................–40°C to 85°C BOOST Pin Above SW Pin .........................................25V LT3505I .............................................–40°C to 125°C SHDN Pin ..................................................................40V Maximum Junction Temperature ..........................125°C FB Pin .........................................................................6V Storage Temperature Range ...................–65°C to 150°C V Pin .........................................................................3V C R Pin .........................................................................3V T PIN CONFIGURATION TOP VIEW BOOST 1 8 VC TOP VIEW SW 2 7 FB BOOST 1 8VC 9 SW 2 7FB VIN 3 6 RT VIN 3 9 6RT SHDN 4 5 GND SHDN 4 5GND MS8E PACKAGE 8-LEAD PLASTIC MSOP DD PACKAGE 8-LEAD (3mm × 3mm) PLASTIC DFN TJMAX = 125°C, θJA = 40°C/W, θJC = 5°C/W EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB TJMAX = 125°C, θJA = 43°C/W, θJC = 5°C/W EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LT3505EDD#PBF LT3505EDD#TRPBF LCHB 8-Lead (3mm x 3mm) Plastic DFN –40°C to 85°C LT3505IDD#PBF LT3505IDD#TRPBF LCHC 8-Lead (3mm x 3mm) Plastic DFN –40°C to 125°C LT3505EMS8E#PBF LT3505EMS8E#TRPBF LTCNX 8-Lead Plastic MSOP –40°C to 85°C LT3505IMS8E#PBF LT3505IMS8E#TRPBF LTCNY 8-Lead Plastic MSOP –40°C to 125°C LEAD BASED FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LT3505EDD LT3505EDD#TR LCHB 8-Lead (3mm x 3mm) Plastic DFN –40°C to 85°C LT3505IDD LT3505IDD#TR LCHC 8-Lead (3mm x 3mm) Plastic DFN –40°C to 125°C LT3505EMS8E LT3505EMS8E#TR LTCNX 8-Lead Plastic MSOP –40°C to 85°C LT3505IMS8E LT3505IMS8E#TR LTCNY 8-Lead Plastic MSOP –40°C to 125°C Consult LTC Marketing for parts specifi ed with wider operating temperature ranges. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifi cations, go to: http://www.linear.com/tapeandreel/ 3505fc 2

LT3505 ELECTRICAL CHARACTERISTICS The " denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at T = 25°C, V = 12V, V = 17V, unless otherwise noted. (Note 2). A IN BOOST PARAMETER CONDITIONS MIN TYP MAX UNITS V Operating Range 3.6 36 V IN Undervoltage Lockout 3.1 3.35 3.6 V Feedback Voltage " 765 780 795 mV FB Pin Bias Current V = Measured V (Note 4) " 55 150 nA FB REF Quiescent Current Not Switching, R = 75.0k 2.0 2.7 mA T Quiescent Current in Shutdown V = 0V 0.01 2 µA SHDN Reference Line Regulation V = 5V to 36V 0.007 %/V IN Switching Frequency V = 0.7V, R = 13.7k 2.70 3.01 3.30 MHz FB T V = 0.7V, R = 75.0k 675 750 825 kHz FB T V = 0.7V, R = 357k 180 200 220 kHz FB T Maximum Duty Cycle R = 75.0k " 90 94 % T Error Amp Transconductance V = 0.78V 200 µA/V FB Error Amp Voltage Gain V = 0.78V 400 V/V FB V Source Current V = 0V, V = 1.5V 10 µA C FB C V Sink Current V = 1V, V = 1.5V 14 µA C FB C V Switching Threshold Voltage I = 0mA 0.9 V C OUT V Clamp Voltage V = 0V 1.7 V C FB R Bias Voltage V = 0.6V 0.5 V T FB V = 0V, R = 75.0k 50 mV FB T Switch Current Limit (Note 3) 1.4 1.75 2.2 A Switch V I = 1A 350 mV CESAT SW Switch Leakage Current 0.1 2 µA Minimum Boost Voltage Above Switch I = 1A 1.6 2.2 V SW BOOST Pin Current I = 1A 24 50 mA SW SHDN Input Voltage High 2.3 V SHDN Input Voltage Low 0.3 V SHDN Bias Current V = 2.3V (Note 5) 6 20 µA SHDN V = 0V 0.01 0.1 µA SHDN Note 1: Stresses beyond those listed under Absolute Maximum Ratings Note 3: Current limit guaranteed by design and/or correlation to static test. may cause permanent damage to the device. Exposure to any Absolute Slope compensation reduces current limit at higher duty cycle. Maximum Rating condition for extended periods may affect device Note 4: Current fl ows out of pin. reliability and lifetime. Note 5: Current fl ows into pin. Note 2: The LT3505E is guaranteed to meet performance specifi cations from 0°C to 85°C. Specifi cations over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LT3505I specifi cations are guaranteed over the –40°C to 125°C temperature range. 3505fc 3

LT3505 TYPICAL PERFORMANCE CHARACTERISTICS Effi ciency (V = 5V, L = 10µH, Effi ciency (V = 3.3V, L = 10µH, Effi ciency (V = 3.3V, OUT OUT OUT f = 750kHz) f = 750kHz) L = 4.7µH, f = 2.2MHz) SW SW SW 95 95 95 TA = 25°C TA = 25°C TA = 25°C 90 90 90 85 85 85 %) 80 %) 80 %) 80 Y ( 75 Y ( 75 Y ( 75 C C C N N N CIE 70 CIE 70 CIE 70 EFFI 65 EFFI 65 EFFI 65 60 60 60 VIN = 8V VIN = 8V 55 VIN = 12V 55 VIN = 12V 55 VIN = 8V VIN = 24V VIN = 24V VIN = 12V 50 50 50 0 0.2 0.4 0.6 0.8 1.0 1.2 0 0.2 0.4 0.6 0.8 1.0 1.2 0 0.2 0.4 0.6 0.8 1.0 1.2 LOAD CURRENT (A) LOAD CURRENT (A) LOAD CURRENT (A) Effi ciency (V = 5V, L = 4.7µH, Max Load Current (V = 3.3V, Max Load Current (V = 5V, OUT OUT OUT f = 2.2MHz) L = 6.8µH, f = 750kHz) f = 750kHz) SW SW SW 95 1.7 1.7 90 TA = 25°C 1.6 TA = 25°C 1.6 TA = 25°C TYPICAL, L = 22µH TYPICAL 85 1.5 1.5 EFFICIENCY (%) 67785050 OUTPUT CURRENT (A) 1111....2341 *10% MDRINOIPMOUUMT OUTPUT CURRENT (A) 1111....2341 *10% DRMOTYPINPOIIUMCTAULM, ,L L = = 1 100µµHH 60 1.0 1.0 55 VIN = 8V 0.9 0.9 VIN = 12V 50 0.8 0.8 0 0.2 0.4 0.6 0.8 1.0 1.2 5 7 9 11 13 15 17 19 6 8 10 12 14 16 18 20 22 24 26 28 30 LOAD CURRENT (A) INPUT VOLTAGE (V) INPUT VOLTAGE (V) 3505 G05 3505 G06 Max Load Current (V = 3.3V, Max Load Current (V = 5V, OUT OUT L = 2.2µH, f = 2.2MHz) L = 3.3µH, f = 2.2MHz) Switch Voltage Drop SW SW 1.8 1.80 500 TA = 25°C TA = 25°C 1.7 1.70 450 1.6 1.60 400 RRENT (A) 11..54 TYPICAL RRENT (A) 11..4500 TYPICAL (mV)H)330500 TA = 25°TCA = 85°C OUTPUT CU 111...123 MINIMUM OUTPUT CU 111...123000 MINIMUM VCE(SWITC221055000 TA = –45°C 1.0 1.00 100 *10% DROPOUT *10% DROPOUT 0.9 0.90 50 0.8 0.80 0 5 6 7 8 9 10 11 12 7 8 9 10 11 12 14 16 18 0 300 600 900 1200 1500 INPUT VOLTAGE (V) INPUT VOLTAGE (V) SWITCH CURRENT (mA) 3505 G07 3505 G08 3505 G09 3505fc 4

LT3505 TYPICAL PERFORMANCE CHARACTERISTICS Undervoltage Lockout Switching Frequency Frequency Foldback, R = 75.0k T 4.00 2.40 0.6 TA = 25°C 3.90 2.20 3.80 MHz)2.00 RT = 21k V) 0.5 UVLO (V)3333....64570000 G FREQUENCY (111...468000 RT = 30.1k BIAS VOLTAGE ( 00..34 33..2300 WITCHIN11..0200 R PIN T 0.2 S 0.1 3.10 0.80 RT = 75.0k 3.00 0.60 0 –50 –25 0 25 50 75 100 125 –50 –25 0 25 50 75 100 125 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 TEMPERATURE (°C) TEMPERATURE (°C) FB VOLTAGE (V) 3505 G10 3505 G12 Typical Minimum Input Voltage, Soft-Start SHDN Pin Current (V = 5V, f = 750kHz) OUT SW 2.0 50 7.2 TA = 25°C TA = 25°C TA = 25°C 1.8 45 7.0 TO START A) 1.6 40 6.8 WITCH CURRENT LIMIT ( 00111.....68042 I (A)µSHDN 3223100555 INPUT VOLTAGE (V) 66656.....04682 TO RUN S 0.4 10 5.6 0.2 5 5.4 0 0 5.2 0 0.25 0.50 0.75 1 1.25 1.50 1.75 2 0 2 4 6 8 10 12 14 16 18 20 1 10 100 1000 SHDN PIN VOLTAGE (V) VSHDN (V) LOAD CURRENT (mA) 3505 G14 3505 G15 Typical Minimum Input Voltage, Typical Minimum Input Voltage, Typical Minimum Input Voltage, (V = 3.3V, f = 750kHz) (V = 3.3V, f = 2.2MHz) (V = 5V, f = 2.2MHz) OUT SW OUT SW OUT SW 5.5 7.0 TA = 25°C TA = 25°C 5.3 5.5 6.8 TO START TO START TO START 5.1 6.6 V) 4.9 V) 5.0 V) 6.4 TO RUN GE ( 4.7 GE ( GE ( 6.2 A A A VOLT 4.5 VOLT 4.5 VOLT 6.0 UT 4.3 UT TO RUN UT 5.8 P P P IN 4.1 TO RUN IN IN 5.6 4.0 3.9 5.4 3.7 5.2 TA = 25°C 3.5 3.5 5.0 1 10 100 1000 1 10 100 1000 1 10 100 1000 LOAD CURRENT (mA) LOAD CURRENT (mA) LOAD CURRENT (mA) 3505fc 5

LT3505 TYPICAL PERFORMANCE CHARACTERISTICS Switch Current Limit Switch Current Limit, R = 75.0k Typical Minimum On Time T 2.0 1.8 160 TA = 25°C 1.9 140 1.7 MIT (A) 11..78 MIT (A) 1.6 E (ns) 120 RRENT LI 11..56 RRENT LI 1.5 M ON TIM 18000 WITCH CU 11..34 WITCH CU 1.4 MINIMU 4600 S 1.2 S 1.3 20 1.1 1.0 1.2 0 –50 –25 0 25 50 75 100 125 0 10 20 30 40 50 60 70 80 90 100 –50 –25 0 25 50 75 100 125 TEMPERATURE (°C) DUTY CYCLE (%) TEMPERATURE (°C) 3505 G20 R Pin Bias Voltage Switching Frequency Switching Frequency T 505 3.0 VFB = 0.78V TA = 25°C TA = 25°C 500 Hz) 2.5 Hz) M M mV) CY ( 2.0 CY ( 1 GE ( 495 UEN UEN A Q Q BIAS VOLT 490 CHING FRE 11..05 CHING FRE T T 485 WI WI S 0.5 S 480 0 0.1 –50 –25 0 25 50 75 100 125 0 5 10 15 20 25 30 35 10 100 TEMPERATURE(°C) RT PIN BIAS CURRENT (µA) RT PIN RESISTANCE (kΩ) 3505 G24 Operating Waveforms, Operating Waveforms Discontinuous Mode VSW VSW 5V/DIV 5V/DIV 0.5A/DIIVL IL 0.5A/DIV 0 0 VOUT VOUT 20mV/DIV 20mV/DIV VIN = 12V 1µs/DIV 3505 G18 VIN = 12V 1µs/DIV 3505 F26 VOUT = 3.3V VOUT = 3.3V IOUT = 0.5A IOUT = 50mA L = 10µH L = 10µH COUT = 10µF COUT = 10µF RT = 75.0k RT = 75.0k 3505fc 6

LT3505 PIN FUNCTIONS BOOST (Pin 1): The BOOST pin is used to provide a drive R (Pin 6): The R pin is used to program the switching T T voltage, higher than the input voltage, to the internal bipolar frequency of the LT3505 by connecting a resistor from NPN power switch. this pin to ground. The Applications Information section of the data sheet includes a table to determine the resistance SW (Pin 2): The SW pin is the output of the internal power value based on the desired switching frequency. Minimize switch. Connect this pin to the inductor, catch diode and capacitance at this pin. boost capacitor. FB (Pin 7): The LT3505 regulates its feedback pin to 780mV. V (Pin 3): The V pin supplies current to the LT3505’s IN IN Connect the feedback resistor divider tap to this pin. Set internal regulator and to the internal power switch. This the output voltage by selecting R1 according to: pin must be locally bypassed. SHDN (Pin 4): The SHDN pin is used to put the LT3505 in R1=R2 VOUT –1   shutdown mode. Tie to ground to shut down the LT3505. 0.78V  Tie to 2.3V or more for normal operation. If the shutdown feature is not used, tie this pin to the V pin. SHDN also A good value for R2 is 10.0k. IN provides a soft-start function; see the Applications Infor- V (Pin 8): The V pin is used to compensate the LT3505 C C mation section. control loop by tying an external RC network from this GND (Pin 5): Tie the GND pin to a local ground plane pin to ground. below the LT3505 and the circuit components. Return the Exposed Pad (Pin 9): The Exposed Pad must be soldered feedback divider to this pin. to the PCB and electrically connected to ground. Use a large ground plane and thermal vias to optimize thermal performance. 3505fc 7

LT3505 BLOCK DIAGRAM VIN VIN 3 C2 INT REG AND UVLO D2 Σ BOOST ON OFF 1 SLOPE R3 COMP R Q SHDN C3 4 S Q DRIVER Q1 C4 L1 OSC SW 2 VOUT D1 C1 FREQUENCY FOLDBACK VC gm 780mV GND VC FB RT 5 8 7 6 R1 3505 BD R2 OPERATION (Refer to Block Diagram) The LT3505 is a constant frequency, current mode step- An internal regulator provides power to the control circuitry. down regulator. A resistor-programmed oscillator enables This regulator includes an undervoltage lockout to prevent an RS fl ip-fl op, turning on the internal 1.4A power switch switching when V is less than ~3.4V. The SHDN pin is IN Q1. An amplifi er and comparator monitor the current used to place the LT3505 in shutdown, disconnecting the fl owing between the V and SW pins, turning the switch output and reducing the input current to less than 2µA. IN off when this current reaches a level determined by the The switch driver operates from either the input or from voltage at the V pin. An error amplifi er measures the C the BOOST pin. An external capacitor and diode are used output voltage through an external resistor divider tied to to generate a voltage at the BOOST pin that is higher than the FB pin and servos the V node. If the error amplifi er’s C the input supply. This allows the driver to fully saturate output increases, more current is delivered to the output; the internal bipolar NPN power switch for effi cient opera- if it decreases, less current is delivered. An active clamp tion. (not shown) on the V node provides current limit. The C VC node is also clamped to the voltage on the SHDN pin; When the FB pin is low, the voltage at the RT pin decreases soft-start is implemented by generating a voltage ramp at to reduce the oscillator frequency. This frequency foldback the SHDN pin using an external resistor and capacitor. helps to control the output current during start-up and overload. 3505fc 8

LT3505 APPLICATIONS INFORMATION FB Resistor Network where f is the switching frequency in hertz and t is SW ON(MIN) the worst-case minimum on-time in seconds. The minimum The output voltage is programmed with a resistor divider on-time of the LT3505 is a strong function of temperature. between the output and the FB pin. Choose the 1% resis- The typical performance characteristics section of the tors according to: datasheet contains a graph of minimum on-time versus  VOUT  temperature to help determine the worst-case minimum R1=R2 –1 on-time for the intended application. 0.78V  If the input voltage is high enough that the duty-cycle R2 should be 20k or less to avoid bias current errors. requirement is lower than DC , the part enters pulse- MIN Reference designators refer to the Block Diagram. skipping mode. Specifi cally, the onset of pulse-skipping occurs at: Input Voltage Range V = (V + V ) / DC – V + V The input voltage range for LT3505 applications depends IN(PS) OUT D MIN D SW on the output voltage, on the absolute maximum ratings Above V the part turns on for brief periods of time IN(PS) of the V and BOOST pins, and on the programmed to control the inductor current and regulate the output IN switching frequency. voltage, possibly producing a spectrum of frequencies below the programmed switching frequency. To remain The minimum input voltage is determined by either the in constant-frequency operation the input voltage should LT3505’s minimum operating voltage of 3.6V, or by its remain below V . See the “Minimum On Time” sec- maximum duty cycle. The duty cycle is the fraction of IN(PS) tion of the data sheet for more information on operating time that the internal switch is on and is determined by above V . the input and output voltages: IN(PS) Note that this is a restriction on the operating input voltage V +V DC= OUT D to remain in constant-frequency operation; the circuit will VIN–VSW +VD tolerate brief transient inputs up to the absolute maximum ratings of the V and BOOST pins when the output is in IN where V is the forward voltage drop of the catch diode D regulation. The input voltage should be limited to V IN(PS) (~0.4V) and V is the voltage drop of the internal switch SW during overload conditions (short-circuit or start-up). (~0.4V at maximum load). This leads to a minimum input voltage of: Minimum On Time V +V OUT D For switching frequencies less than 750kHz, the part V = –V +V IN(MIN) D SW DC will still regulate the output at input voltages that exceed MAX V (up to 40V), however, the output voltage ripple IN(PS) with DC = 1 – f /8.33, where f is in MHz. MAX SW SW increases as the input voltage is increased. Figure 1 il- The maximum input voltage is determined by the abso- lustrates switching waveforms in continuous mode for a lute maximum ratings of the VIN and BOOST pins. For 3V output application near VIN(PS) = 33V. constant-frequency operation, the maximum input voltage As the input voltage is increased, the part is required to is determined by the minimum duty cycle requirement. switch for shorter periods of time. Delays associated with As the input voltage increases, the required duty cycle turning off the power switch determine the minimum on to regulate the output voltage decreases. The minimum time of the part. The worst-case typical minimum on-time duty-cycle is: is 130ns. Figure 2 illustrates the switching waveforms DCMIN = fSW • tON(MIN) when the input voltage is increased to VIN = 35V. 3505fc 9

LT3505 APPLICATIONS INFORMATION Now the required on time has decreased below the minimum on time of 130ns. Instead of the switch pulse VSW width becoming narrower to accommodate the lower duty 20V/DIV cycle requirement, the switch pulse width remains fi xed at 130ns. In Figure 2 the inductor current ramps up to a IL value exceeding the load current and the output ripple 0.5A/DIV increases to ~200mV. The part then remains off until the output voltage dips below 100% of the programmed value VOUT 200mV/DIV before it begins switching again. AC COUPLED 2 s/DIV 3505 F01 For switching frequencies above 750kHz, the input voltage COUT = 10 F ILOAD = 0.75A must not exceed V . See the “Input Voltage Frequency VOUT = 3V L = 10 H IN(PS) VIN = 30V RT = 75.0k Foldback” section of the datasheet for a circuit solution that provides safe operation above V at switching Figure 1 IN(PS) frequencies exceeding 750kHz. For switching frequencies below 750kHz, operation above V is safe and will IN(PS) not damage the part as long as the output voltage stays in regulation and the inductor does not saturate. Figure VSW 20V/DIV 3 shows the switching waveforms of a 750kHz applica- tion when the input voltage is increased to its absolute IL maximum rating of 40V. 0.5A/DIV As the input voltage increases, the inductor current ramp rate increases, the number of skipped pulses increases VOUT 200mV/DIV and the output voltage ripple increases. The part is robust AC COUPLED enough to survive prolonged operation under these condi- 2 s/DIV 3505 F02 tions as long as the programmed switching frequency is COUT = 10 F ILOAD = 0.75A VOUT = 3V L = 10 H less than 750kHz and the peak inductor current does not VIN = 35V RT = 75.0k exceed 2.2A. Inductor current saturation may further limit Figure 2 performance in this operating regime. Frequency Selection The maximum frequency that the LT3505 can be pro- grammed to is 3MHz. The minimum frequency that the VSW LT3505 can be programmed to is 200kHz. The switching 20V/DIV frequency is programmed by tying a 1% resistor from the R T pin to ground. Table 1 can be used to select the value of R . T IL Minimum on-time and edge loss must be taken into consid- 0.5A/DIV eration when selecting the intended frequency of operation. Higher switching frequency increases power dissipation VOUT 200mV/DIV and lowers effi ciency. AC COUPLED 2µs/DIV 3505 F03 COUT = 10µF ILOAD = 0.75A VOUT = 3V L = 10µH VIN = 40V RT = 75.0k Figure 3 3505fc 10

LT3505 APPLICATIONS INFORMATION Finite transistor bandwidth limits the speed at which the When the input voltage is below 16V, the zener diode power switch can be turned on and off, effectively setting path conducts no current and the current fl owing out the minimum on-time of the LT3505. For a given output volt- of the R pin (and through R4) is nominally 0.5V/20k = T age, the minimum on-time determines the maximum input 25µA, which programs a 2.2MHz switching frequency. voltage to remain in continuous mode operation, V . As the input voltage is increased above 16V, the zener IN(PS) See the “Input Voltage Range” section of the datasheet for diode begins to conduct and gradually reduces the cur- more information on determining V . For switching rent fl owing out of the R pin. This mechanism reduces IN(PS) T frequencies below 750kHz, operation above V (up the switching frequency as the input voltage is increased IN(PS) to 40V) is safe provided that the system will tolerate the above 16V (up to 36V) to ensure that the part constantly pulse-skipping behavior outlined in the “Minimum On operates in continuous mode without skipping pulses, Time” section of the datasheet. At switching frequencies thereby preventing the excessive die temperature rise exceeding 750kHz, edge loss limits operation to input encountered in pulse-skipping mode. voltages below V . IN(PS) Although the circuit can be operated indefi nitely above Finite transition time results in a small amount of power V , this frequency foldback method is intended to ZENER dissipation each time the power switch turns on and off protect circuits during temporary periods of high input (edge loss). Edge loss increases with frequency, switch voltage. For example, in many automotive systems, the current, and input voltage. normal operating input range might be 9V to 16V, and the LT3505 can be programmed to operate above the Input Voltage Frequency Foldback AM band (>1.8MHz). At the same time, the circuit must be able to withstand higher input voltages due to load In constant frequency operation (below V ) edge IN(PS) dump or double-battery jump starts. During these brief loss only reduces the application effi ciency. However, at periods, it is usually acceptable to switch at a frequency high switching frequencies exceeding 750kHz and input within the AM band. voltages exceeding V , the part operates in pulse-skip- IN(PS) ping mode and the switch current can increase above the If the output is shorted while the input voltage is greater than current limit of the part, 1.75A. This further increases the V , the switching frequency will be reduced to 30kHz ZENER power dissipated during switch transitions and increases and the part will not be able to recover from the short until die temperature. To remedy the situation a single resis- the input voltage is reduced below V (see the following ZENER tor (R5) and a zener diode (D3) can be added to a typical discussion). LT3505 circuit as shown in Figure 4. D2 2.50 1N4148 6.7V TO 3V6IVN VIN BOOST C0.31µF V5VOUT ent [A] 22..2050 ON ODF3F SHDN LT3505 SW 6.L81µH R611.9k C225pF Load Curr 11..5705 BZT52C16T 81R0665Vk RT GND VFBC DM1BRM140 R112.5k C101µF ncy [MHz] / 101...270550 Switching que 0.50 Frequency R4 R3 Fre Maximum 20.0k 100k 0.25 Load Current C2 C4 0 1µF 22pF 0 5 10 15 20 25 30 35 40 Input Voltage [V] LTC3505 • F04b 3505 F04 Figure 4. 2.2MHz, 5V Application with Input Voltage Frequency Foldback Circuit 3505fc 11

LT3505 APPLICATIONS INFORMATION Component Selection for Input Voltage Frequency and V is the voltage drop of the internal power switch SW Foldback Circuit (~0.4V at maximum load), V is the maximum input IN(MAX) voltage for the application (must be less than 36V), and To determine the values of R4, R5, and D3 for a specifi c t is the worst-case minimum on-time for the in- application follow the procedure outlined in this section. ON(MIN) tended application. The worst-case minimum on-time can First select the value of R4 from Table 1. be determined from the graphs in the “Typical Performance Table 1. R Pin Resistance Characteristics” section of the datasheet. Next look up the T RT PIN RESISTANCE (kΩ) SWITCHING FREQUENCY (MHz) resistance that corresponds to fSW(MIN) in Table 1. This 357 0.20 resistance is R , the effective resistance from the R T(MAX) T 237 0.30 pin to ground at V that programs the oscillator to IN(MAX) 165 0.40 a switching frequency equal to f . SW(MIN) 124 0.50 100 0.60 Finally determine R5 from the following equation: 84.5 0.69 R5 = 2 • (V – V )/(1/R4 – 1/R ) 71.5 0.80 IN(MAX) ZENER T(MAX) 61.9 0.91 where V is the zener diode breakdown voltage, ZENER 54.9 1.00 and V is the maximum input voltage that will be IN(MAX) 48.7 1.11 applied to the V pin. V must not exceed 36V, the IN IN(MAX) 44.2 1.21 maximum operating input voltage of the LT3505. The 40.2 1.31 equation to determine R5 assumes that R5 will com- 37.4 1.39 pensate a percentage of the current fl owing through R4 34.0 1.50 31.6 1.60 equal to R4/RT(MAX). Be careful not to select a value of 29.4 1.70 R5 much less than that determined by the equation above 27.4 1.80 because it may become possible for R5 to compensate 25.5 1.90 100% of the current fl owing through R4 reducing the 23.7 2.02 frequency to 30kHz. In this state the part is not able to 22.6 2.10 start into large output current loads. 21.0 2.22 20.0 2.31 Whenever the voltage at the FB pin is below 600mV, the 19.1 2.39 LT3505 folds back the switching frequency by reducing 18.2 2.48 the bias voltage at the R pin. If the input voltage is higher T 16.9 2.62 than the zener voltage, the reduced voltage at the R pin 16.2 2.71 T results in a larger voltage drop across R5, and a reduced 15.4 2.81 voltage drop across R4. The current carried by R5 may 14.7 2.90 13.7 3.01 be large enough to completely compensate the current fl owing through R4, reducing the frequency to 30kHz. In Second, determine the value of V from the equation IN(PS) this situation the input voltage will have to be reduced until in the “Input Voltage Range” section of the data sheet. the input voltage is less than the zener voltage. Select the zener diode, D3, to have a breakdown voltage (V ) below V . Next determine the desired foldback Note that when V is above V and the frequency is ZENER IN(PS) IN ZENER frequency from the following equation: reduced, the inductor ripple current will be higher and the maximum load that the LT3505 can regulate will be lower. f = (V + V )/[t • (V + V – V )] SW(MIN) OUT D ON(MIN) IN(MAX) D SW See the Inductor Selection and Maximum Output Current where VD is the forward drop of the catch diode (~0.4V), section of this data sheet for more information. 3505fc 12

LT3505 APPLICATIONS INFORMATION Inductor Selection and Maximum Output Current Catch Diode A good fi rst choice for the inductor value is: Depending on load current, a 1A to 2A Schottky diode is recommended for the catch diode, D1. The diode must L = 1.2 (V + V )/f OUT D SW have a reverse voltage rating equal to or greater than the where VD is the voltage drop of the catch diode (~0.4V), maximum input voltage. The ON Semiconductor MBRM140 L is in µH and fSW is in MHz. With this value there will is a good choice; it is rated for 1A continuous forward be no subharmonic oscillation for applications with 50% current and a maximum reverse voltage of 40V. or greater duty cycle. The inductor’s RMS current rating must be greater than your maximum load current and Input Capacitor its saturation current should be about 30% higher. For The input of the LT3505 circuit must be bypassed with a robust operation in fault conditions, the saturation current X7R or X5R type ceramic capacitor. Y5V types have poor should be above 2.2A. To keep effi ciency high, the series performance over temperature and applied voltage and resistance (DCR) should be less than 0.1 . Table 2 lists should not be used. For switching frequencies higher than several vendors and types that are suitable. 750kHz, bypass the input with a 1µF or higher value ceramic Of course, such a simple design guide will not always capacitor. For switching frequencies below 750kHz, bypass result in the optimum inductor for your application. A the input with a 2.2µF or higher value ceramic capacitor. larger value provides a higher maximum load current and If the input power source has high impedance, or there is reduces output voltage ripple at the expense of slower signifi cant inductance due to long wires or cables, additional transient response. If your load is lower than 1.2A, then bulk capacitance may be necessary. This can be provided you can decrease the value of the inductor and operate with a low performance electrolytic capacitor. with higher ripple current. This allows you to use a physi- Step-down regulators draw current from the input sup- cally smaller inductor, or one with a lower DCR resulting ply in pulses with very fast rise and fall times. The input in higher effi ciency. There are several graphs in the Typical capacitor is required to reduce the resulting voltage Performance Characteristics section of this data sheet that ripple at the LT3505 and to force this very high frequency show the maximum load current as a function of input switching current into a tight local loop, minimizing EMI. voltage and inductor value for several popular output volt- To accomplish this task, the input bypass capacitor must ages. Low inductance may result in discontinuous mode be placed close to the LT3505 and the catch diode; see operation, which is okay, but further reduces maximum the PCB Layout section. A second precaution regarding load current. For details on maximum output current and the ceramic input capacitor concerns the maximum input discontinuous mode operation, see Linear Technology voltage rating of the LT3505. A ceramic input capacitor Application Note 44. combined with trace or cable inductance forms a high quality (underdamped) tank circuit. If the LT3505 circuit is plugged into a live supply, the input voltage can ring to Table 2. Inductor Vendors VENDOR URL PART SERIES INDUCTANCE RANGE (µH) Size (mm) Sumida www.sumida.com CDRH4D28 1.2 to 4.7 4.5 × 4.5 CDRH5D28 2.5 to 10 5.5 × 5.5 CDRH5D28 2.5 to 33 8.3 × 8.3 Toko www.toko.com A916CY 2 to 12 6.3 × 6.2 D585LC 1.1 to 39 8.1 × 8.0 Würth Elektronik www.we-online.com WE-TPC(M) 1 to 10 4.8 × 4.8 WE-PD2(M) 2.2 to 22 5.2 × 5.8 WE-PD(S) 1 to 27 7.3 × 7.3 3505fc 13

LT3505 APPLICATIONS INFORMATION twice its nominal value, possibly exceeding the LT3505’s High performance electrolytic capacitors can be used for voltage rating. This situation can be easily avoided; see the output capacitor. Low ESR is important, so choose one the Hot Plugging Safely section. that is intended for use in switching regulators. The ESR should be specifi ed by the supplier and should be 0.1Ω Output Capacitor or less. Such a capacitor will be larger than a ceramic capacitor and will have a larger capacitance, because the The output capacitor has two essential functions. Along capacitor must be large to achieve low ESR. Table 3 lists with the inductor, it fi lters the square wave generated by several capacitor vendors. the LT3505 to produce the DC output. In this role it deter- mines the output ripple so low impedance at the switching Figure 5 shows the transient response of the LT3505 with frequency is important. The second function is to store several output capacitor choices. The output is 3.3V. The energy in order to satisfy transient loads and stabilize the load current is stepped from 500mA to 1.2A and back LT3505’s control loop. to 500mA and the oscilloscope traces show the output voltage. The upper photo shows the recommended value. Ceramic capacitors have very low equivalent series re- The second photo shows the improved response (less sistance (ESR) and provide the best ripple performance. voltage drop) resulting from a larger output capacitor A good value is: and a larger phase lead capacitor. The last photo shows C = 49/(V • f ) OUT OUT SW the response to a high performance electrolytic capaci- where C is in µF and f is in MHz. Use X5R or X7R tor. Transient performance is improved due to the large OUT SW types and keep in mind that a ceramic capacitor biased output capacitance. with V will have less than its nominal capacitance. This OUT BOOST Pin Considerations choice will provide low output ripple and good transient response. Transient performance can be improved with a Capacitor C3 and diode D2 are used to generate a boost high value capacitor, if the compensation network is also voltage that is higher than the input voltage. In most cases adjusted to maintain the loop bandwidth. a 0.1µF capacitor and fast switching diode (such as the 1N4148 or 1N914) will work well. Figure 6 shows two A lower value of output capacitor can be used, but tran- ways to arrange the boost circuit. The BOOST pin must sient performance will suffer unless the compensation be at least 2.3V above the SW pin for best effi ciency. For network is adjusted to reduce the loop gain. Also, a lower outputs of 3.3V and above, the standard circuit (Figure 6a) value output capacitor may result in increased sensitivity is best. For outputs between 3V and 3.3V, use a 0.22µF to noise which can be alleviated by adding a 22pF phase capacitor. For outputs between 2.5V and 3V, use a 0.47µF lead capacitor from FB to V . OUT Table 3. Capacitor Vendors VENDOR PHONE URL PART SERIES COMMENTS Panasonic (714) 373-7366 www.panasonic.com Ceramic, EEF Series Polymer, Tantalum Kemet (864) 963-6300 www.kemet.com Ceramic, T494, T495 Tantalum Sanyo (408) 749-9714 www.sanyovideo.com Ceramic, POSCAP Polymer, Tantalum Murata (404) 436-1300 www.murata.com Ceramic AVX www.avxcorp.com Ceramic, TPS Series Tantalum Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic 3505fc 14

LT3505 APPLICATIONS INFORMATION ILOAD 1A/DIV VOUT 32.4k 22pF FB 10µF VC VOUT 10.0k 20mV/DIV 100k AC COUPLED 22pF 10µs/DIV 3505 F05a ILOAD 1A/DIV VOUT 32.4k 44pF 10µF FB ×2 VC 10.0k 20mVV/ODUIVT 100k AC COUPLED 22pF 10µs/DIV 3505 F05b ILOAD VOUT 1A/DIV 32.4k 66pF + FB 120µF VC 301k10.0k KEMET 20mVV/ODUIVT A700D127M006ATE015 AC COUPLED 22pF 10µs/DIV 3505 F05c Figure 5. Transient Load Response of the LT3505 with Different Output Capacitors as the Load Current is Stepped from 500mA to 1.2A. V = 12V, V = 3.3V, L = 2µH, R = 20.0k IN OUT T D2 D2 BOOST C3 BOOST C3 LT3505 LT3505 VIN VIN SW VOUT VIN VIN SW VOUT GND GND VBOOST – VSW ≅ VOUT 3505 F06a VBOOST – VSW ≅ VIN 3505 F06b MAX VBOOST ≅ VIN + VOUT MAX VBOOST ≅ 2VIN (6a) (6b) Figure 6. Two Circuits for Generating the Boost Voltage 3505fc 15

LT3505 APPLICATIONS INFORMATION capacitor and a small Schottky diode (such as the BAT-54). of the BOOST pin. For lower output voltages tie a Schottky diode to the input At light loads, the inductor current becomes discontinu- (Figure 6b). The circuit in Figure 6a is more effi cient because ous and the effective duty cycle can be very high. This the BOOST pin current comes from a lower voltage source. reduces the minimum input voltage to approximately You must also be sure that the maximum voltage rating 400mV above V . At higher load currents, the inductor OUT of the BOOST pin is not exceeded. current is continuous and the duty cycle is limited by the The minimum operating voltage of an LT3505 applica- maximum duty cycle of the LT3505, requiring a higher tion is limited by the undervoltage lockout (3.6V) and by input voltage to maintain regulation. the maximum duty cycle as outlined above. For proper start-up, the minimum input voltage is also limited by Soft-Start the boost circuit. If the input voltage is ramped slowly, The SHDN pin can be used to soft-start the LT3505, reducing or the LT3505 is turned on with its SHDN pin when the the maximum input current during start-up. The SHDN pin output is already in regulation, then the boost capacitor is driven through an external RC fi lter to create a voltage may not be fully charged. Because the boost capacitor is ramp at this pin. Figure 8 shows the start-up waveforms charged with the energy stored in the inductor, the circuit with and without the soft-start circuit. By choosing a large will rely on some minimum load current to get the boost RC time constant, the peak start up current can be reduced circuit running properly. This minimum load will depend to the current that is required to regulate the output, with on the input and output voltages and on the arrangement no overshoot. Choose the value of the resistor so that it of the boost circuit. The minimum load generally goes to can supply 20µA when the SHDN pin reaches 2.3V. zero once the circuit has started. Figure 7 shows a plot of minimum load to start and to run as a function of input Shorted and Reversed Input Protection voltage. In many cases the discharged output capacitor If the inductor is chosen so that it won’t saturate exces- will present a load to the switcher which will allow it to sively, an LT3505 buck regulator will tolerate a shorted start. The plots show the worst-case situation where V IN output. There is another situation to consider in systems is ramping verly slowly. For lower start-up voltage, the where the output will be held high when the input to the boost diode can be tied to V ; however this restricts the IN LT3505 is absent. This may occur in battery charging ap- input range to one-half of the absolute maximum rating 7.2 5.5 TA = 25°C TA = 25°C 7.0 5.3 TO START TO START 6.8 5.1 V) 6.6 V) 4.9 GE ( 6.4 GE ( 4.7 A A T T L 6.2 L 4.5 O O V V T 6.0 T 4.3 U U INP 5.8 TO RUN INP 4.1 TO RUN 5.6 3.9 5.4 3.7 5.2 3.5 1 10 100 1000 1 10 100 1000 LOAD CURRENT (mA) LOAD CURRENT (mA) 3505 G15 (7a) Typical Minimum Input Voltage, V = 5V, f = 750kHz (7b) Typical Minimum Input Voltage, V = 3.3V, f = 750kHz OUT SW OUT SW Figure 7 3505fc 16

LT3505 APPLICATIONS INFORMATION plications or in battery backup systems where a battery or some other supply is diode OR-ed with the LT3505’s RUN SHDN output. If the V pin is allowed to fl oat and the SHDN pin IN GND is held high (either by a logic signal or because it is tied to V ), then the LT3505’s internal circuitry will pull its IN quiescent current through its SW pin. This is fi ne if your system can tolerate a few mA in this state. If you ground VSW the SHDN pin, the SW pin current will drop to essentially 5V/DIV zero. However, if the V pin is grounded while the output IN is held high, then parasitic diodes inside the LT3505 can IL pull large currents from the output through the SW pin 1A/DIV and the V pin. Figure 9 shows a circuit that will run only IN when the input voltage is present and that protects against VOUT 2V/DIV a shorted or reversed input. VIN = 12V 10 s/DIV 3505 F08a VOUT = 3.3V L = 2.5 H COUT = 10 F D4 RT = 20.0k VIN VIN BOOST SW VOUT LT3505 RUN SHDN FB 15k RT GND VC BACKUP SHDN 0.068 F GND 3505 F09 Figure 9. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output; It Also Protects the Circuit VSW from a Reversed Input. The LT3505 Runs Only When the Input 5V/DIV is Present Hot Plugging Safely IL 1A/DIV The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input VOUT 2V/DIV bypass capacitor of LT3505 circuits. However, these ca- VIN = 12V 10 s/DIV 3505 F08b pacitors can cause problems if the LT3505 is plugged into VOUT = 3.3V L = 2.5 H a live supply (see Linear Technology Application Note 88 CROT U=T 2=0 1.00kF for a complete discussion). The low loss ceramic capacitor combined with stray inductance in series with the power Figure 8. To Soft-Start the LT3505, Add a Resistor and Capacitor source forms an underdamped tank circuit and the voltage to the SHDN pin. V = 12V, V = 3.3V, C = 10µF, R = IN OUT OUT LOAD at the V pin of the LT3505 can ring to twice the nominal IN 5Ω, R = 20.0k, L = 2.5µH T input voltage, possibly exceeding the LT3505’s rating and 3505fc 17

LT3505 APPLICATIONS INFORMATION damaging the part. If the input supply is poorly controlled alternative solution is shown in Figure 9c. A 1Ω resistor or the user will be plugging the LT3505 into an energized is added in series with the input to eliminate the voltage supply, the input network should be designed to prevent overshoot (it also reduces the peak input current). A 0.1µF this overshoot. capacitor improves high frequency fi ltering. This solution is smaller and less expensive than the electrolytic capacitor. Figure 10 shows the waveforms that result when an LT3505 For high input voltages its impact on effi ciency is minor, circuit is connected to a 24V supply through six feet of reducing effi ciency only one percent for a 5V output at full 24-gauge twisted pair. The fi rst plot is the response with load operating from 24V. a 2.2µF ceramic capacitor at the input. The input voltage rings as high as 35V and the input current peaks at 20A. Frequency Compensation One method of damping the tank circuit is to add another capacitor with a series resistor to the circuit. In Figure 9b The LT3505 uses current mode control to regulate the an aluminum electrolytic capacitor has been added. This output. This simplifi es loop compensation. In particular, capacitor’s high equivalent series resistance damps the the LT3505 does not require the ESR of the output capaci- circuit and eliminates the voltage overshoot. The extra tor for stability allowing the use of ceramic capacitors to capacitor improves low frequency ripple fi ltering and achieve low output ripple and small circuit size. can slightly improve the effi ciency of the circuit, though Frequency compensation is provided by the components it is likely to be the largest component in the circuit. An tied to the V pin, as shown in Figure 10. Generally a C CLOSING SWITCH SIMULATES HOT PLUG IIN VIN DANGER! LT3505 VIN + 20V/DIV RINGING VIN MAY EXCEED 2.2µF ABSOLUTE MAXIMUM RATING OF THE LT3505 IIN 5A/DIV LOW STRAY IMPEDANCE INDUCTANCE 20µs/DIV ENERGIZED DUE TO 6 FEET 24V SUPPLY (2 METERS) OF TWISTED PAIR (9a) LT3505 VIN + 10µF + 20V/DIV 35V 2.2µF AI.EI. IIN 5A/DIV (9b) 20µs/DIV 1Ω LT3505 VIN + 20V/DIV 0.1µF 2.2µF IIN 5A/DIV (9c) 20µs/DIV 3505 F10 Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and Ensures Reliable Operation When the LT3505 is Connected to a Live Supply 3505fc 18

LT3505 APPLICATIONS INFORMATION capacitor (C ) and a resistor (R ) in series to ground are C C LT3505 CURRENT MODE used. In addition, a lower value fi lter capacitor (CF) may be 0.8V – POWER STAGE added in parallel. The fi lter capacitor is not a part of the loop gm = SW OUT 1.1A/V + compensation but is used to fi lter noise at the switching R1 CPL frequency, and is required only if a phase-lead capacitor – FB is used or if the output capacitor has high ESR. VC gm = 200µA/V ESR Loop compensation determines the stability and transient ERROR + 780mV + C1 AMPLIFIER C1 performance. Designing the compensation network is a bit 2M complicated and the best values depend on the application R2 and in particular the type of output capacitor. A practical approach is to start with one of the circuits in this data RC sheet that is similar to your application and tune the com- CF CC pensation network to optimize the performance. Stability should then be checked across all operating conditions, 3505 F11 including load current, input voltage and temperature. The Figure 11. Model for Loop Response LT1375 data sheet contains a more thorough discussion of loop compensation and describes how to test the stability PCB Layout using a transient load. For proper operation and minimum EMI, care must be taken Figure 11 shows an equivalent circuit for the LT3505 control during printed circuit board layout. Figure 12 shows the loop. The error amp is a transconductance amplifi er with recommended component placement with trace, ground fi nite output impedance. The power section, consisting of plane and via locations. Note that large, switched currents the modulator, power switch and inductor, is modeled as fl ow in the LT3505’s V and SW pins, the catch diode (D1) a transconductance amplifi er generating an output cur- IN and the input capacitor (C2). The loop formed by these rent proportional to the voltage at the V node. Note that C components should be as small as possible and tied to the output capacitor integrates this current and that the capacitor on the V node (C ) integrates the error ampli- C C fi er output current, resulting in two poles in the loop. R SYSTEM C GROUND VOUT provides a zero. With the recommended output capacitor, : VIAS TO LOCAL GROUND PLANE the loop crossover occurs above the R C zero. This simple : OUTLINE OF LOCAL GROUND PLANE C C model works well as long as the value of the inductor is C1 not too high and the loop crossover frequency is much VOUT lower than the switching frequency. With a larger ceramic capacitor (very low ESR), crossover may be lower and a BOOST SW phase lead capacitor (C ) across the feedback divider may PL improve the phase margin and transient response. Large 1 8 VC FB 2 7 electrolytic capacitors may have an ESR large enough to 3 6 RT create an additional zero and the phase lead may not be D1 4 5 necessary. C2 POWER SIGNAL If the output capacitor is different than the recommended GROUND GROUND capacitor, stability should be checked across all operat- 3505 F12 ing conditions, including load current, input voltage and temperature. VIN SHUTDOWN Figure 12. A Good PCB Layout Ensures Proper, Low EMI Operation 3505fc 19

LT3505 APPLICATIONS INFORMATION system ground in only one place. These components, along ing the total power loss from an effi ciency measurement with the inductor and output capacitor, should be placed and subtracting the catch diode loss. Thermal resistance on the same side of the circuit board and their connections depends on the layout of the circuit board, but 43°C/W is should be made on that layer. Place a local, unbroken ground typical for the (3mm × 3mm) DFN (DD) package. plane below these components and tie this ground plane to system ground at one location, ideally at the ground Outputs Greater Than 6V terminal of the output capacitor C1. The SW and BOOST For outputs greater than 6V, add a 1k to 2.5k resistor nodes should be as small as possible. Finally, keep the across the inductor to damp the discontinuous ringing FB node small so that the ground pin and ground traces of the SW node, preventing unintended SW current. The will shield it from the SW and BOOST nodes. Include vias 12V Step-Down Converter circuit in the Typical Applica- near the exposed GND pad of the LT3505 to help remove tions section shows the location of this resistor. Also note heat from the LT3505 to the ground plane. that for outputs above 10V, the input voltage range will be limited by the maximum rating of the BOOST pin. The High Temperature Considerations 12V circuit shows how to overcome this limitation using The die temperature of the LT3505 must be lower than the an additional zener diode. maximum rating of 125°C. This is generally not a concern unless the ambient temperature is above 85°C. For higher Other Linear Technology Publications temperatures, care should be taken in the layout of the Application notes AN19, AN35 and AN44 contain more circuit to ensure good heat sinking of the LT3505. The detailed descriptions and design information for Buck maximum load current should be derated as the ambient regulators and other switching regulators. The LT1376 temperature approaches 125°C. The die temperature is data sheet has a more extensive discussion of output calculated by multiplying the LT3505 power dissipation ripple, loop compensation and stability testing. Design by the thermal resistance from junction to ambient. Power Note DN100 shows how to generate a bipolar output dissipation within the LT3505 can be estimated by calculat- supply using a Buck regulator. 3505fc 20

LT3505 TYPICAL APPLICATIONS 2.2MHz, 3.3V Step-Down Converter 1N4148 6V TO 3V6IVN VIN BOOST V3.O3UVT 22..5205 Switching 0.1µF 3.3µH A] Frequency ON OFF SHDN SW nt [2.00 Maximum LT3505 36.5k urre1.75 Load Current CMPZ521400VB FB 22pF oad C1.50 L 698k RT GND VC MBRM140 11.3k 10µF MHz] / 11.2.05 cy [0.75 n 20.0k 100k que0.50 e Fr 1µF 22pF 0.25 0.00 5 10 15 20 25 30 35 40 3505 TA02 Input Voltage [V] LTC3505 • TA02b 1.2MHz, 1.8V Step-Down Converter BAT54 1.60 3.6V TO 2V5IVN VIN BOOST A]1.40 0.1µF 4.7µH VOUT ent [1.20 ON OFF SHDN SW 1.8V urr LT3505 26.1k 68pF 1.2A d C1.00 a 12V o CMPZ5242B FB Hz] / L0.80 1.5M RT GND VC MBRM140 20.0k 22µF ncy [M0.60 Switching ue0.40 Frequency q 44.2k 60.4k Fre0.20 Maximum Load Current 2.2µF 120pF 0.00 0 5 10 15 20 25 INPUT VOLTAGE (V) 3505 TA03 LT3505 • TA03b 3505fc 21

LT3505 TYPICAL APPLICATIONS 750kHz, 3.3V Step-Down Converter 4.2V TO 3V6IVN VIN BOOST 1N4148 V3.O3UVT 0.1µF 1.1A, VIN > 5V 10µH 1.2A, VIN > 8V ON OFF SHDN SW LT3505 36.5k 68pF FB RT GND VC 11.3k 10µF MBRM140 75.0k 69.8k 1µF 70pF 3505 TA04 1MHz, 12V Step-Down Converter CMDZ5235B 6V 1N4148 0.1µF 1k* 0.25W BOOST 13.5V TO 3V6IVN VIN SW 15µH V112AOV,U VTIN > 16.5V LT3505 71.5k 22pF 1.1A, VIN > 20.5V ON OFF SHDN FB RT GND VC MBRM140 4.99k 10µF 54.9k 100k 3.3µF 22pF *FOR CONTINUOUS OPERATION ABOVE 30V, 3505 TA05 USE TWO 2k, 0.25W RESISTORS IN PARALLEL 3505fc 22

LT3505 PACKAGE DESCRIPTION DD Package 8-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1698) R = 0.115 0.38 ± 0.10 TYP 5 8 0.675 ±0.05 3.5 ±0.05 1.65 ±0.05 3.00 ±0.10 1.65 ± 0.10 2.15 ±0.05 (2 SIDES) (4 SIDES) (2 SIDES) PIN 1 PACKAGE TOP MARK OUTLINE (NOTE 6) (DD) DFN 1203 4 1 0.25 ± 0.05 0.200 REF 0.75 ±0.05 0.25 ± 0.05 0.50 0.50 BSC BSC 2.38 ±0.10 2.38 ±0.05 0.00 – 0.05 (2 SIDES) (2 SIDES) BOTTOM VIEW—EXPOSED PAD RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE OUTLINE M0-229 VARIATION OF (WEED-1) MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 2.DRAWING NOT TO SCALE 5. EXPOSED PAD SHALL BE SOLDER PLATED 3. ALL DIMENSIONS ARE IN MILLIMETERS 6.SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON TOP AND BOTTOM OF PACKAGE MS8E Package 8-Lead Plastic MSOP (Reference LTC DWG # 05-08-1662) 2.794 ± 0.102 0.889 ± 0.127 BOTTOM VIEW OF (.110 ± .004) (.035 ± .005) EXPOSED PAD OPTION 3.00 ± 0.102 2.06 ± 0.102 (.118 ± .004) 0.52 1 (.081 ± .004) (NOTE 3) 8 7 6 5 (.0205) REF 5.23 1.83 ± 0.102 (.206) 2.083 ± 0.102 3.20 – 3.45 (.072 ± .004) MIN (.082 ± .004) (.126 – .136) 3.00 ± 0.102 4.90 ± 0.152 (.118 ± .004) (.193 ± .006) (NOTE 4) 0.42 ± 0.038 0.65 (.0165 ± .0015) (.0256) TYP BSC 8 1 2 3 4 RECOMMENDED SOLDER PAD LAYOUT 1.10 0.86 (.043) (.034) DETAIL “A” DETAIL “A” MAX REF 0.254 0.18 (.010) 0° – 6° TYP (.007) SEATING GAUGE PLANE PLANE 0.22 – 0.38 0.127 ± 0.076 (.009 – .015) (.005 ± .003) 0.53 ± 0.152 TYP 0.65 MSOP (MS8E) 0603 (.021 ± .006) (.0256) BSC NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX 3505fc Information furnished by Linear Technology Corporation is believed to be accurate and reliable. 23 However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

LT3505 TYPICAL APPLICATIONS 300kHz, 3.3V Step-Down Converter 4V TO 3V6IVN VIN BOOST 1N4148 V3.O3UVT 0.47µF22µH 11A.2,A V, IVNI N> 5>V 8.5V ON OFF SHDN SW LT3505 36.5k 100pF FB 68µF RT GND VC 11.3k KEMET A700D686M010ATE015 MBRM140 226k 100k 2.2µF 150pF 3505 TA06 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1766 60V, 1.2A (I ), 200kHz, High Effi ciency Step-Down V : 5.5V to 60V, V = 1.2V, I = 2.5mA, I < 25µA, OUT IN OUT(MIN) Q SD DC/DC Converter TSSOP16/TSSOP16E Packages LT1767 25V, 1.2A (I ), 1.25MHz, High Effi ciency Step-Down V : 3V to 25V, V = 1.20V, I = 1mA, I < 6µA, OUT IN OUT(MIN) Q SD DC/DC Converter MS8E Package LT1933 500mA (I ), 500kHz, Step-Down Switching Regulator in V : 3.6V to 36V, V = 1.25V, I = 1.6mA, I < 1µA, OUT IN OUT(MIN) Q SD SOT-23 TSSOP16/TSSOP16E Packages LT1936 36V, 1.4A (I ), 500kHz, High Effi ciency Step-Down V : 3.6V to 36V, V = 1.20V, I = 1.9mA, I < 1µA, OUT IN OUT(MIN) Q SD DC/DC Converter MS8E Package LT1940 Dual 25V, 1.4A (I ), 1.1MHz, High Effi ciency Step-Down V : 3.6V to 25V, V = 1.25V, I = 3.8mA, I < 30µA, OUT IN OUT(MIN) Q SD DC/DC Converter TSSOP16E Package LT1976/LT1977 60V, 1.2A (I ), 200kHz/500kHz, High Effi ciency Step- V : 3.3V to 60V, V = 1.25V, I = 100µA, I < 1µA, OUT IN OUT(MIN) Q SD Down DC/DC Converters with Burst Mode® Operation TSSOP16E Package LT3434/LT3435 60V, 2.4A (I ), 200kHz/500kHz, High Effi ciency Step- V : 3.3V to 60V, V = 1.25V, I = 100µA, I < 1µA, OUT IN OUT(MIN) Q SD Down DC/DC Converters with Burst Mode Operation TSSOP16E Package LT3437 60V, 400mA (I ), Micropower Step-Down DC/DC V : 3.3V to 60V, V = 1.25V, I = 100µA, I = <1µA, OUT IN OUT(MIN) Q SD Converter with Burst Mode Operation DFN Package LT3493 36V, 1.2A (I ), 750kHz, High Effi ciency Step-Down V : 3.6V to 36V, V = 0.78V, I = 1.9mA, I < 2µA, OUT IN OUT(MIN) Q SD DC/DC Converter DFN Package Burst Mode is a registered trademark of Linear Technology Corporation. 3505fc 24 Linear Technology Corporation LT 0807 REV C • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 " FAX: (408) 434-0507 #"# www.linear.com  LINEAR TECHNOLOGY CORPORATION 2006