LT3503 1A, 2.2MHz Step-Down Switching Regulator in 2mm × 3mm DFN FEATURES DESCRIPTIOU ■ Wide Input Range: 3.6V to 20V The LT®3503 is a current mode PWM step-down DC/DC ■ 5V at 1.2A from 11V to 18V Input converter with an internal 1.45A power switch. The wide ■ 5V at 1A from 7.2V to 18V Input operating input range of 3.6V to 20V makes the LT3503 ■ 3.3V at 1.2A from 8.5V to 12V Input ideal for regulating power from a wide variety of sources. ■ 3.3V at 1A from 5.5V to 12V Input Its high operating frequency allows the use of tiny, low ■ Fixed Frequency Operation: 2.2MHz cost inductors and ceramic capacitors, resulting in low, ■ Output Adjustable Down to 780mV predictable output ripple. ■ Short-Circuit Robust Cycle-by-cycle current limit provides protection against ■ Uses Tiny Capacitors and Inductors shorted outputs and soft-start eliminates input current ■ Soft-Start surge during start-up. The low current (<2µA) shutdown ■ Internally Compensated mode provides output disconnect, enabling easy power ■ Low Shutdown Current: <2µA management in battery-powered systems. ■ Low V Switch: 400mV at 1A CESAT ■ Thermally Enhanced, 2mm × 3mm 6-Pin DFN , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Low Profile Package APPLICATIOUS ■ Automotive Battery Regulation ■ Industrial Control Supplies ■ Wall Transformer Regulation ■ Distributed Supply Regulation ■ Battery-Powered Equipment TYPICAL APPLICATIOU 3.3V Step-Down Converter Efficiency 85 VIN V3.O3UVT 80 VIN = 12V 4.5V20 TVO M 1A2VX VIN BOOST 1A, VIN > 5.5V LT3503 0.1µF 2.5µH 1.2A, VIN > 8.5V 75 ON OFF SHDN SW %) 37.4k 120pF Y ( 70 C N GND FB 10µF CIE 65 1µF 11.5k EFFI 60 3503 TA01a 55 50 0 0.2 0.4 0.6 0.8 1.0 1.2 LOAD CURRENT (A) 3503 TA01b 3503f 1

LT3503 ABSOLUTE W AXIW UW RATIU GS PACKAGE/ORDER IU FORW ATIOU (Note 1) Input Voltage (VIN) .................................................. 20V TOP VIEW BOOST Pin Voltage.................................................. 40V FB 1 6 SHDN BOOST Pin Above SW Pin ....................................... 20V SHDN Pin ................................................................ 20V GND 2 7 5 VIN FB Voltage ................................................................. 6V BOOST 3 4 SW Operating Temperature Range (Note 2)...–40°C to 85°C DCB PACKAGE Maximum Junction Temperature..........................125°C 6-LEAD (2mm (cid:0) 3mm) PLASTIC DFN Storage Temperature Range................. –65°C to 150°C TJMAX = 125°C, θJA = 64°C/W EXPOSED PAD (PIN 7) IS GND, MUST BE SOLDERED TO PCB ORDER PART NUMBER DCB PART MARKING LT3503EDCB LCGW Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. V = 12V, V = 17V, unless otherwise noted. (Note 2) IN BOOST PARAMETER CONDITIONS MIN TYP MAX UNITS V Operating Range 3.6 20 V IN Undervoltage Lockout 3.0 3.4 3.6 V Feedback Voltage ● 765 780 795 mV FB Pin Bias Current V = Measured V (Note 4) ● 50 150 nA FB REF Quiescent Current Not Switching 1.9 2.6 mA Quiescent Current in Shutdown VSHDN = 0V 0.01 2 µA Reference Line Regulation V = 5V to 20V 0.007 %/V IN Switching Frequency V = 0.7V 2.0 2.2 2.4 MHz FB V = 0V 36 kHz FB Maximum Duty Cycle ● 76 81 % 3503f 2

LT3503 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. V = 12V, V = 17V, unless otherwise noted. (Note 2) IN BOOST PARAMETER CONDITIONS MIN TYP MAX UNITS Switch Current Limit (Note 3) 1.45 1.75 2.2 A Switch V I = 1A 400 mV CESAT SW Switch Leakage Current 2 µA Minimum Boost Voltage Above Switch I = 1A 2 2.3 V SW BOOST Pin Current I = 1A 25 50 mA SW SHDN Input Voltage High 2.3 V SHDN Input Voltage Low 0.3 V SHDN Bias Current VSHDN = 2.3V (Note 5) 6 15 µA VSHDN = 0V 0.01 0.1 µA 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 flows out of pin. reliability and lifetime. Note 5: Current flows into pin. Note 2: The LT3503E is guaranteed to meet performance specifications from 0°C to 85°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. TYPICAL PERFORW AU CE CHARACTERISTICS TA = 25°C unless otherwise noted. Efficiency (V = 3.3V) Efficiency (V = 5V) OUT OUT 90 90 L = 2.5µH L = 3.3µH 88 88 VIN = 12V 86 86 VIN = 8V 84 84 Y (%) 82 VIN = 12V Y (%) 82 VIN = 16V C C N 80 N 80 EFFICIE 78 EFFICIE 78 76 76 74 74 72 72 70 70 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) 3503 G01 3503 G02 3503f 3

LT3503 TYPICAL PERFORW AU CE CHARACTERISTICS TA = 25°C unless otherwise noted. Maximum Load Current Maximum Load Current Switch Voltage Drop 1.8 1.8 500 VOUT = 3.3V VOUT = 5V 1.6 L = 2.2µH 1.6 L = 3.3µH 450 TYPICAL TYPICAL UTPUT CURRENT (A) 11100.....02486 MINIMUM UTPUT CURRENT (A) 00111.....68024 MINIMUM V (mV)CE(SWITCH)342231000555000000 TA = 25°TCA = 85T°AC = –45°C O O 0.4 0.4 100 0.2 0.2 50 0 0 0 4 5 6 7 8 9 10 11 12 6 7 8 9 10 11 12 13 14 15 0 300 600 900 1200 1500 INPUT VOLTAGE (V) INPUT VOLTAGE (V) SWITCH CURRENT (mA) 3503 G03 3503 G04 3503 G07 Undervoltage Lockout Switching Frequency 4.00 2.75 3.90 3.80 Hz) M2.50 3.70 Y ( C N V)3.60 UE LO (3.50 REQ2.25 UV3.40 G F N HI 3.30 C T2.00 WI 3.20 S 3.10 3.00 1.75 –45 –25 –5 15 35 55 75 –45 –25 –5 15 35 55 75 TEMPERATURE (°C) TEMPERATURE (°C) 3503 G08 3503 G09 Frequency Foldback Soft-Start 2.5 2.0 1.8 CHING FREQUENCY (MHz) 211...005 TCH CURRENT LIMIT (A) 001111......680462 WIT 0.5 SWI 0.4 S 0.2 0 0 0 100 200 300 400 500 600 700 800 0 0.25 0.50 0.75 1 1.25 1.50 1.75 2 FEEDBACK VOLTAGE (mV) SHDN PIN VOLTAGE (V) 3503 G10 3503 G11 3503f 4

LT3503 TYPICAL PERFORW AU CE CHARACTERISTICS TA = 25°C unless otherwise noted. Typical Minimum Input Voltage Typical Minimum Input Voltage SHDN Pin Current (V = 3.3V) (V = 5V) OUT OUT 50 5.5 7.7 45 40 7.2 5.0 35 V) V) A) 30 GE ( GE ( 6.7 µ A A (DN 25 OLT 4.5 OLT H V V IS 20 UT UT 6.2 P P 15 N N I I 4.0 10 5.7 5 0 3.5 5.2 0 2 4 6 8 10 12 14 16 18 20 1 10 1OO 1000 1 10 100 1000 VSHDN (V) LOAD CURRENT (mA) LOAD CURRENT (mA) 3503 G12 3503 G13 3503 G14 Switch Current Limit Switch Current Limit 2.0 1.8 1.9 1.7 MIT (A) 11..78 MIT (A) 1.6 TYPICAL NT LI 1.6 NT LI 1.5 RE 1.5 RE 1.4 R R H CU 1.4 H CU 1.3 TC 1.3 TC MINIMUM WI WI 1.2 S 1.2 S 1.1 1.1 1.0 1.0 –45 –25 -5 15 35 55 75 0 20 40 60 80 100 TEMPERATURE (°C) DUTY CYCLE (%) (cid:0)(cid:1)(cid:2)(cid:0)(cid:4)(cid:5)(cid:1) 3503 G16 3503f 5

LT3503 PIU FUUCTIOUS FB (Pin 1): The LT3503 regulates its feedback pin to V (Pin 5): The V pin supplies current to the LT3503’s IN IN 780mV. Connect the feedback resistor divider tap to this internal regulator and to the internal power switch. This pin. Set the output voltage according to: pin must be locally bypassed. SHDN (Pin 6): The SHDN pin is used to put the LT3503 in ⎛ VOUT ⎞ R1=R2⎜ –1⎟ shutdown mode. Tie to ground to shut down the LT3503. ⎝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- GND (Pin 2): Tie the GND pin to a local ground plane below mation section. the LT3503 and the circuit components. Return the feed- Exposed Pad (Pin 7): The Exposed Pad must be soldered back divider to this pin. to the PCB and electrically connected to ground. Use a BOOST (Pin 3): The BOOST pin is used to provide a drive large ground plane and thermal vias to optimize thermal voltage, higher than the input voltage, to the internal performance. bipolar NPN power switch. SW (Pin 4): The SW pin is the output of the internal power switch. Connect this pin to the inductor, catch diode and boost capacitor. BLOCK DIAGRAW VIN VIN 5 C2 INT REG AND UVLO D2 Σ BOOST ON OFF 3 SLOPE R3 COMP R Q SHDN C3 6 S Q DRIVER Q1 C4 L1 OSC SW 4 VOUT D1 C1 FREQUENCY FOLDBACK VC gm 780mV GND FB 2 1 R2 R1 3503 BD 3503f 6

LT3503 OPERATIOU (Refer to Block Diagram) The LT3503 is a constant frequency, current mode step- An internal regulator provides power to the control cir- down regulator. A 2.2MHz oscillator enables an RS flip- cuitry. This regulator includes an undervoltage lockout to flop, turning on the internal 1.75A power switch Q1. An prevent switching when V is less than ~3.4V. The SHDN IN amplifier and comparator monitor the current flowing pin is used to place the LT3503 in shutdown, disconnect- between the V and SW pins, turning the switch off when ing the output and reducing the input current to less than IN this current reaches a level determined by the voltage at 2µA. V . An error amplifier measures the output voltage through C The switch driver operates from either the input or from an external resistor divider tied to the FB pin and servos the the BOOST pin. An external capacitor and diode are used V node. If the error amplifier’s output increases, more C to generate a voltage at the BOOST pin that is higher than current is delivered to the output; if it decreases, less the input supply. This allows the driver to fully saturate the current is delivered. An active clamp (not shown) on the V C internal bipolar NPN power switch for efficient operation. node provides current limit. The V node is also clamped C to the voltage on the SHDN pin; soft-start is implemented The oscillator reduces the LT3503’s operating frequency by generating a voltage ramp at the SHDN pin using an when the voltage at the FB pin is low. This frequency external resistor and capacitor. foldback helps to control the output current during start- up and overload. 3503f 7

LT3503 APPLICATIOUS IUFORWATIOU FB Resistor Network In pulse-skipping mode the part skips pulses to control the inductor current and regulate the output voltage, possibly The output voltage is programmed with a resistor divider producing a spectrum of frequencies below 2.2MHz. between the output and the FB pin. Choose the 1% resistors according to: Note that this is a restriction on the operating input voltage to remain in constant-frequency operation; the circuit will R1=R2⎛⎜ VOUT –1⎞⎟ tolerate transient inputs up to the absolute maximum ⎝0.78V ⎠ ratings of the V and BOOST pins when the output is in IN regulation. The input voltage should be limited to V R2 should be 20.0k or less to avoid bias current errors. IN(PS) during overload conditions (short-circuit or start-up). Reference designators refer to the Block Diagram. An optional phase lead capacitor of 22pF between VOUT Minimum On Time and FB reduces light-load output ripple. The part will still regulate the output at input voltages that exceed V (up to 20V), but the output voltage ripple Input Voltage Range IN(PS) increases. Figure1 illustrates switching waveforms in The input voltage range for LT3503 applications depends continuous mode for a 0.78V output application near on the output voltage and on the absolute maximum V = 6V. IN(PS) ratings of the V and BOOST pins. IN As the input voltage is increased, the part is required to The minimum input voltage is determined by either the switch for shorter periods of time. Delays associated with LT3503’s minimum operating voltage of 3.6V, or by its turning off the power switch dictate the minimum on time maximum duty cycle. The duty cycle is the fraction of time of the part. The minimum on time for the LT3503 is that the internal switch is on and is determined by the input ~130ns. Figure 2 illustrates the switching waveforms and output voltages: when the input voltage is increased to V = 14V. IN V +V OUT D DC= VIN–VSW +VD VSW 10V/DIV where VD is the forward voltage drop of the catch diode IL 1A/DIV (~0.4V) and V is the voltage drop of the internal switch SW (~0.4V at maximum load). This leads to a minimum input VOUT 20mV/DIV voltage of: COUT = 47µF 1µs/DIV 3503 F01 V +V VOUT = 0.78V V = OUT D –V +V VIN = 7V IN(MIN) DC D SW ILOAD = 1.1A MAX L = 1.1µH Figure 1 with DC = 0.81 (0.76 over temperature). MAX The maximum input voltage is determined by the absolute maximum ratings of the V and BOOST pins. For con- VSW IN 10V/DIV stant-frequency operation the maximum input voltage is IL determined by the minimum duty cycle, DC = 0.29. If 1A/DIV MIN the duty cycle requirement is less than DCMIN, the part will VOUT 20mV/DIV enter pulse-skipping mode. The onset of pulse-skipping occurs at: COUT = 47µF 1µs/DIV 3503 F02 VOUT = 0.78V VIN(PS) = VOUT +VD –VD+VSW VILL IO=NA 1=D. 11=µ4 1VH.1A DC MIN Figure 2 3503f 8

LT3503 APPLICATIOUS IUFORWATIOU Now the required on time has decreased below the mini- mum on time of 130ns. Instead of the switch pulse width VSW 10V/DIV becoming narrower to accommodate the lower duty cycle IL requirement, the switch pulse width remains fixed at 1A/DIV 130ns. In Figure 2 the inductor current ramps up to a value VOUT 20mV/DIV exceeding the load current and the output ripple increases to ~40mV. The part then remains off until the output COUT = 47µF 1µs/DIV 3503 F03 VOUT = 0.78V voltage dips below 100% of the programmed value before VIN = 20V it begins switching again. ILOAD = 1.1A L = 1.1µH Figure 3 Provided that the output remains in regulation and that the inductor does not saturate, operation above V is safe IN(PS) and will not damage the part. Figure 3 illustrates the VSW 10V/DIV switching waveforms when the input voltage is increased IL to its absolute maximum rating of 20V. 1A/DIV The part is robust enough to survive prolonged operation VOUT 20mV/DIV under these conditions as long as the peak inductor current does not exceed 2.2A. In Figure 3 the peak inductor COUT = 2 × 47µF 1µs/DIV 3503 F04 VOUT = 0.78V current of 2A suggests that the saturation current rating of VIN = 20V ILOAD = 1.1A the inductor should be ~2.6A, which may require an L = 2.7µH Figure 4 inductor of large physical size. The peak inductor current value can be reduced by simultaneously increasing the where V is the voltage drop of the catch diode (~0.4V) and inductance and output capacitance. In Figure 4 the peak D L is in µH. With this value there will be no subharmonic inductor current is reduced to 1.3A by doubling the output oscillation for applications with 50% or greater duty cycle. capacitor and inductor values. Now the required inductor The inductor’s RMS current rating must be greater than current saturation rating is ~1.7A, so that even though the your maximum load current and its saturation current inductance value has increased, it may be possible to should be about 30% higher. For robust operation in fault achieve a physically smaller inductor size. conditions, the saturation current should be above 2.2A. Note that inductor current saturation ratings often de- To keep efficiency high, the series resistance (DCR) should crease with temperature and that inductor current satura- be less than 0.1Ω. Table 1 lists several vendors and types tion may further limit performance in this operating regime. that are suitable. Of course, such a simple design guide will not always Inductor Selection and Maximum Output Current result in the optimum inductor for your application. A A good first choice for the inductor value is: larger value provides a higher maximum load current and L = 0.6 (V + V ) reduces output voltage ripple at the expense of slower OUT D 3503f 9

LT3503 APPLICATIOUS IUFORWATIOU Table 1. 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 CDRH8D28 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 transient response. If your load is lower than 1A, then you impedance, or there is significant inductance due to long can decrease the value of the inductor and operate with wires or cables, additional bulk capacitance may be nec- higher ripple current. This allows you to use a physically essary. This can be provided with a low performance smaller inductor, or one with a lower DCR resulting in electrolytic capacitor. higher efficiency. There are several graphs in the Typical Step-down regulators draw current from the input supply Performance Characteristics section of this data sheet that in pulses with very fast rise and fall times. The input show the maximum load current as a function of input capacitor is required to reduce the resulting voltage ripple voltage and inductor value for several popular output at the LT3503 and to force this very high frequency voltages. Low inductance may result in discontinuous switching current into a tight local loop, minimizing EMI. mode operation, which is okay, but further reduces maxi- A 1µF capacitor is capable of this task, but only if it is mum load current. For details of the maximum output placed close to the LT3503 and the catch diode; see the current and discontinuous mode operation, see Linear PCB Layout section. A second precaution regarding the Technology Application Note 44. ceramic input capacitor concerns the maximum input voltage rating of the LT3503. A ceramic input capacitor Catch Diode combined with trace or cable inductance forms a high Depending on load current, a 1A to 2A Schottky diode is quality (underdamped) tank circuit. If the LT3503 circuit is recommended for the catch diode, D1. The diode must plugged into a live supply, the input voltage can ring to have a reverse voltage rating equal to or greater than the twice its nominal value, possibly exceeding the LT3503’s maximum input voltage. The ON Semiconductor MBRM140 voltage rating. This situation is easily avoided; see the Hot is a good choice; it is rated for 1A continuous forward Plugging Safely section. current and a maximum reverse voltage of 40V. Output Capacitor Input Capacitor The output capacitor has two essential functions. Along Bypass the input of the LT3503 circuit with a 1µF or higher with the inductor, it filters the square wave generated by value ceramic capacitor of X7R or X5R type. Y5V types the LT3503 to produce the DC output. In this role it have poor performance over temperature and applied determines the output ripple so low impedance at the voltage and should not be used. A 1µF ceramic is adequate switching frequency is important. The second function is to bypass the LT3503 and will easily handle the ripple to store energy in order to satisfy transient loads and current. However, if the input power source has high stabilize the LT3503’s control loop. 3503f 10

LT3503 APPLICATIOUS IUFORWATIOU Ceramic capacitors have very low equivalent series resis- Figure 5 shows the transient response of the LT3503 with tance (ESR) and provide the best ripple performance. A a few output capacitor choices. The output is 3.3V. The good value is: load current is stepped from 0.5A to 1.1A and back to 0.5A, and the oscilloscope traces show the output voltage. The C = 24/V OUT OUT upper photo shows the recommended value. The second where COUT is in µF. Use X5R or X7R types and keep in photo shows the improved response (less voltage drop) mind that a ceramic capacitor biased with VOUT will have resulting from a phase lead capacitor. The last photo less than its nominal capacitance. This choice will provide shows the response to a high performance electrolytic low output ripple and good transient response. Transient capacitor. Transient performance is improved due to the performance can be improved with a high value capacitor, large output capacitance. but a phase lead capacitor across the feedback resistor R1 may be required to get the full benefit (see the Compen- BOOST Pin Considerations sation section). Using a small output capacitor results in Capacitor C3 and diode D2 are used to generate a boost an increased loop crossover frequency and increased voltage that is higher than the input voltage. In most cases sensitivity to noise. A 22pF capacitor connected between a 0.1µF capacitor and fast switching diode (such as the V and the FB pin is required to filter noise at the FB pin OUT 1N4148 or 1N914) will work well. Figure 6 shows two and ensure stability. ways to arrange the boost circuit. The BOOST pin must be High performance electrolytic capacitors can be used for at least 2.3V above the SW pin for best efficiency. For the output capacitor. Low ESR is important, so choose outputs of 3.3V and above, the standard circuit (Figure 6a) one that is intended for use in switching regulators. The is best. For outputs between 3V and 3.3V, use a 0.22µF ESR should be specified by the supplier and should be capacitor. For outputs between 2.5V and 3V, use a 0.47µF 0.1Ω or less. Such a capacitor will be larger than a capacitor and a small Schottky diode (such as the BAT-54). ceramic capacitor and will have a larger capacitance, be- For lower output voltages tie a Schottky diode to the input cause the capacitor must be large to achieve low ESR. (Figure 6b). The circuit in Figure 6a is more efficient Table 2 lists several capacitor vendors. because the BOOST pin current comes from a lower voltage source. You must also be sure that the maximum voltage rating of the BOOST pin is not exceeded. Table 2. Capacitor Vendors Vendor Phone URL Part Series Comments Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, EEF Series Tantalum Kemet (864) 963-6300 www.kemet.com Ceramic, Tantalum T494, T495 Sanyo (408) 749-9714 www.sanyovideo.com Ceramic, Polymer, POSCAP Tantalum Murata (404) 436-1300 www.murata.com Ceramic AVX www.avxcorp.com Ceramic, Tantalum TPS Series Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic 3503f 11

LT3503 APPLICATIOUS IUFORWATIOU ILOAD 1A/DIV VOUT 36.5k FB 10µF VOUT 11.3k 50mV/DIV 40µs/DIV 3503 F05a ILOAD VOUT 1A/DIV 36.5k 1nF FB 10µF 11.3k VOUT 50mV/DIV 40µs/DIV 3503 F05b VOUT 1AIL/ODAIVD 36.5k 1nF + FB 100µF 11.3k KEMET VOUT A700D686M010ATE015 50mV/DIV 40µs/DIV 3503 F05c Figure 5. Transient Load Response of the LT3503 with Different Output Capacitors as the Load Current is Stepped from 0.5A to 1.1A. VIN = 12V, VOUT = 3.3V, L = 3.3µH 3503f 12

LT3503 APPLICATIOUS IUFORWATIOU D2 D2 BOOST C3 BOOST C3 LT3503 LT3503 VIN VIN SW VOUT VIN VIN SW VOUT GND GND 3503 F06a 3503 F06b VBOOST – VSW ≅ VOUT VBOOST – VSW ≅ VIN MAX VBOOST ≅ VIN + VOUT MAX VBOOST ≅ 2VIN (6a) (6b) Figure 6. Two Circuits for Generating the Boost Voltage 7.7 5.5 7.2 TO START 5.0 V) V) GE ( 6.7 TO START GE ( A A LT LT 4.5 VO TO RUN VO TO RUN UT 6.2 UT P P N N I I 4.0 5.7 5.2 3.5 1 10 100 1000 1 10 1OO 1000 LOAD CURRENT (mA) LOAD CURRENT (mA) 3503 F07a 3503 F07b (7a) Typical Minimum Input Voltage, (7b) Typical Minimum Input Voltage, V = 5V V = 3.3V OUT OUT Figure 7 The minimum operating voltage of an LT3503 application to start and to run as a function of input voltage. In many is limited by the undervoltage lockout (3.6V) and by the cases the discharged output capacitor will present a load maximum duty cycle as outlined above. For proper start- to the switcher which will allow it to start. The plots show up, the minimum input voltage is also limited by the boost the worst-case situation where V is ramping verly slowly. IN circuit. If the input voltage is ramped slowly, or the LT3503 For lower start-up voltage, the boost diode can be tied to is turned on with its SHDN pin when the output is already V ; however this restricts the input range to one-half of IN in regulation, then the boost capacitor may not be fully the absolute maximum rating of the BOOST pin. charged. Because the boost capacitor is charged with the At light loads, the inductor current becomes discontinu- energy stored in the inductor, the circuit will rely on some ous and the effective duty cycle can be very high. This minimum load current to get the boost circuit running reduces the minimum input voltage to approximately properly. This minimum load will depend on the input and 400mV above V . At higher load currents, the inductor OUT output voltages, and on the arrangement of the boost current is continuous and the duty cycle is limited by the circuit. The minimum load generally goes to zero once the maximum duty cycle of the LT3503, requiring a higher circuit has started. Figure 7 shows a plot of minimum load input voltage to maintain regulation. 3503f 13

LT3503 APPLICATIOUS IUFORWATIOU Soft-Start where the output will be held high when the input to the LT3503 is absent. This may occur in battery charging The SHDN pin can be used to soft-start the LT3503, applications or in battery backup systems where a battery reducing the maximum input current during start-up. The or some other supply is diode OR-ed with the LT3503’s SHDN pin is driven through an external RC filter to create output. If the V pin is allowed to float and the SHDN pin a voltage ramp at this pin. Figure 8 shows the start-up IN is held high (either by a logic signal or because it is tied to waveforms with and without the soft-start circuit. By V ), then the LT3503’s internal circuitry will pull its choosing a large RC time constant, the peak start up IN quiescent current through its SW pin. This is fine if your current can be reduced to the current that is required to system can tolerate a few mA in this state. If you ground regulate the output, with no overshoot. Choose the value the SHDN pin, the SW pin current will drop to essentially of the resistor so that it can supply 20µA when the SHDN zero. However, if the V pin is grounded while the output pin reaches 2.3V. IN is held high, then parasitic diodes inside the LT3503 can pull large currents from the output through the SW pin and Shorted and Reversed Input Protection the V pin. Figure 9 shows a circuit that will run only when IN If the inductor is chosen so that it won’t saturate exces- the input voltage is present and that protects against a sively, an LT3503 buck regulator will tolerate a shorted shorted or reversed input. output. There is another situation to consider in systems VSW 5V/DIV RUN SHDN IL GND 1A/DIV VOUT 2V/DIV VIN = 12V 10µs/DIV 3503 F08a VOUT = 3.3V L = 3.3µH COUT = 10µF RUN VSW 5V/DIV 15k SHDN 0.068µF GND IL 1A/DIV VOUT 2V/DIV VIN = 12V 10µs/DIV 3503 F08b VOUT = 3.3V L = 3.3µH COUT = 10µF Figure 8. To Soft-Start the LT3503, Add a Resistor and Capacitor to the SHDN Pin. VIN = 12V, VOUT = 3.3V, COUT = 10µF, RLOAD = 5Ω 3503f 14

LT3503 APPLICATIOUS IUFORWATIOU D4 a live supply (see Linear Technology Application Note 88 VIN VIN BOOST for a complete discussion). The low loss ceramic capaci- LT3503 tor combined with stray inductance in series with the SHDN SW VOUT power source forms an underdamped tank circuit, and the GND FB voltage at the V pin of the LT3503 can ring to twice the IN nominal input voltage, possibly exceeding the LT3503’s BACKUP rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LT3503 into an 3503 F09 energized supply, the input network should be designed to prevent this overshoot. Figure 9. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output; It Also Protects the Circuit Figure 10 shows the waveforms that result when an from a Reversed Input. The LT3503 Runs Only When the Input LT3503 circuit is connected to a 20V supply through six is Present feet of 24-gauge twisted pair. The first plot is the response with a 2.2µF ceramic capacitor at the input. The input Hot Plugging Safely voltage rings as high as 35V and the input current peaks The small size, robustness and low impedance of ceramic at 20A. One method of damping the tank circuit is to add capacitors make them an attractive option for the input another capacitor with a series resistor to the circuit. In bypass capacitor of LT3503 circuits. However, these ca- Figure 10b an aluminum electrolytic capacitor has been pacitors can cause problems if the LT3503 is plugged into added. This capacitor’s high equivalent series resistance CLOSING SWITCH SIMULATES HOT PLUG IIN VIN DANGER! LT3503 VIN + 20V/DIV RINGING VIN MAY EXCEED 2.2µF ABSOLUTE MAXIMUM RATING OF THE LT3503 IIN 5A/DIV LOW STRAY IMPEDANCE INDUCTANCE 20µs/DIV ENERGIZED DUE TO 6 FEET 20V SUPPLY (2 METERS) OF TWISTED PAIR (10a) LT3503 VIN + 10µF + 20V/DIV 35V 2.2µF AI.EI. IIN 5A/DIV (10b) 20µs/DIV 1Ω LT3503 VIN + 20V/DIV 0.1µF 2.2µF IIN 5A/DIV (10c) 20µs/DIV 3503 F10 Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and Ensures Reliable Operation When the LT3503 is Connected to a Live Supply 3503f 15

LT3503 APPLICATIOUS IUFORWATIOU damps the circuit and eliminates the voltage overshoot. capacitor, the loop crossover occurs above the R C zero. C C The extra capacitor improves low frequency ripple filtering This simple model works well as long as the value of the and can slightly improve the efficiency of the circuit, inductor is not too high and the loop crossover frequency though it is likely to be the largest component in the circuit. is much lower than the switching frequency. With a larger An alternative solution is shown in Figure 10c. A 1Ω ceramic capacitor (very low ESR), crossover may be resistor is added in series with the input to eliminate the lower and a phase lead capacitor (C ) across the feed- PL voltage overshoot (it also reduces the peak input current). back divider may improve the phase margin and transient A 0.1µF capacitor improves high frequency filtering. This response. Large electrolytic capacitors may have an ESR solution is smaller and less expensive than the electrolytic large enough to create an additional zero, and the phase capacitor. For high input voltages its impact on efficiency lead may not be necessary. is minor, reducing efficiency less than one half percent for If the output capacitor is different than the recommended a 5V output at full load operating from 20V. capacitor, stability should be checked across all operating conditions, including load current, input voltage and tem- Frequency Compensation perature. The LT1375 data sheet contains a more thor- The LT3503 uses current mode control to regulate the ough discussion of loop compensation and describes how output. This simplifies loop compensation. In particular, to test the stability using a transient load. the LT3503 does not require the ESR of the output capacitor for stability allowing the use of ceramic capaci- PCB Layout tors to achieve low output ripple and small circuit size. For proper operation and minimum EMI, care must be Figure 11 shows an equivalent circuit for the LT3503 taken during printed circuit board layout. Figure 12 shows control loop. The error amp is a transconductance ampli- the recommended component placement with trace, fier with finite output impedance. The power section, ground plane and via locations. Note that large, switched consisting of the modulator, power switch and inductor, currents flow in the LT3503’s V and SW pins, the catch IN is modeled as a transconductance amplifier generating an diode (D1) and the input capacitor (C2). The loop formed output current proportional to the voltage at the V node. by these components should be as small as possible and C Note that the output capacitor integrates this current, and tied to system ground in only one place. These compo- that the capacitor on the V node (C ) integrates the error nents, along with the inductor and output capacitor, C C amplifier output current, resulting in two poles in the loop. should be placed on the same side of the circuit board, and R provides a zero. With the recommended output their connections should be made on that layer. Place a C LT3503 CURRENT MODE POWER STAGE 0.8V – gm = SW OUT 1.1A/V + R1 CPL FB – VC gm = 200µA/V ESR 2R4Ck ERROR + 780mV + C1 100CpCF 2MAMPLIFIER C1 R2 GND 3503 F11 Figure 11. Model for Loop Response 3503f 16

LT3503 APPLICATIOUS IUFORWATIOU SYSTEM GROUND C2 D1 C1 VOUT VIN SHUTDOWN 3503 F12 : VIAS TO LOCAL GROUND PLANE : OUTLINE OF LOCAL GROUND PLANE Figure 12. A Good PCB Layout Ensures Proper, Low EMI Operation local, unbroken ground plane below these components, efficiency measurement and subtracting the catch diode and tie this ground plane to system ground at one location, loss. The resulting temperature rise at full load is nearly ideally at the ground terminal of the output capacitor C1. independent of input voltage. Thermal resistance depends The SW and BOOST nodes should be as small as possible. on the layout of the circuit board, but 64°C/W is typical for Finally, keep the FB node small so that the ground pin and the (2mm × 3mm) DFN (DCB) package. ground traces will shield it from the SW and BOOST nodes. Include vias near the exposed GND pad of the LT3503 to Outputs Greater Than 6V help remove heat from the LT3503 to the ground plane. For outputs greater than 6V, add a resistor of 1k to 2.5k across the inductor to damp the discontinuous ringing of High Temperature Considerations the SW node, preventing unintended SW current. The die temperature of the LT3503 must be lower than the maximum junction of 125°C. This is generally not a Other Linear Technology Publications concern unless the ambient temperature is above 85°C. Application notes AN19, AN35 and AN44 contain more For higher temperatures, care should be taken in the layout detailed descriptions and design information for Buck of the circuit to ensure good heat sinking of the LT3503. regulators and other switching regulators. The LT1376 The maximum load current should be derated as the data sheet has a more extensive discussion of output ambient temperature approaches 125°C. The die tem- ripple, loop compensation and stability testing. Design perature is calculated by multiplying the LT3503 power Note DN100 shows how to generate a bipolar output dissipation by the thermal resistance from junction to supply using a Buck regulator. ambient. Power dissipation within the LT3503 can be estimated by calculating the total power loss from an 3503f 17

LT3503 TYPICAL APPLICATIOUS 0.78V Step-Down Converter BAT54 VIN 3.6V TO 4V VIN BOOST 20V MAX LT3503 0.1µF 1.0µH VOUT ON OFF SHDN SW 0.78V 1.2A MBRM140 GND FB 47µF 1µF 3503 TA02 1.8V Step-Down Converter BAT54 VIN 3.6V TO 7.6V VIN BOOST 20V MAX LT3503 0.1µF 1.5µH VOUT ON OFF SHDN SW 1.8V 1.2A MBRM140 26.1k 120pF GND FB 22µF 1µF 20k 3503 TA03 2.5V Step-Down Converter VIN BAT54 V2.O5UVT 3.6V20 TVO M 1A0VX VIN BOOST 1A, VIN > 4.5V LT3503 0.47µF1.8µH 1.2A, VIN > 8.1V ON OFF SHDN SW MBRM140 22.1k 22pF GND FB 10µF 1µF 10k 3503 TA04 3503f 18

LT3503 PACKAGE DESCRIPTIOUN DCB Package 6-Lead Plastic DFN (2mm × 3mm) (Reference LTC DWG # 05-08-1715) 0.70 ±0.05 1.65 ±0.05 3.55 ±0.05 (2 SIDES) 2.15 ±0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 1.35 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 2.00 ±0.10 R = 0.115 0.40 ± 0.10 (2 SIDES) TYP R = 0.05 4 6 TYP 3.00 ±0.10 1.65 ± 0.10 (2 SIDES) (2 SIDES) PIN 1 BAR PIN 1 NOTCH TOP MARK R0.20 OR 0.25 (SEE NOTE 6) × 45° CHAMFER 3 1 (DCB6) DFN 0405 0.25 ± 0.05 0.200 REF 0.75 ±0.05 0.50 BSC 1.35 ±0.10 (2 SIDES) 0.00 – 0.05 BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (TBD) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 3503f Information furnished by Linear Technology Corporation is believed to be accurate and reliable. 19 However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

LT3503 TYPICAL APPLICATIOU 5V Step-Down Converter VIN 1N4148 V5VOUT 6.7V20 TVO M 1A8VX VIN BOOST 1A, VIN > 7.2V LT3503 0.1µF 3.3µH 1.2A, VIN > 11V ON OFF SHDN SW MBRM140 61.9k 120pF GND FB 10µF 1µF 11.3k 3503 TA05 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1766 60V, 1.2A IOUT, 200kHz, High Efficiency Step-Down VIN: 5.5V to 60V, VOUT(MIN) = 1.2V, IQ = 2.5mA, ISD = 25µA, DC/DC Converter TSSOP16/TSSOP16E Packages LT1767 25V, 1.2A IOUT, 1.25MHz, High Efficiency Step-Down VIN: 3V to 25V, VOUT(MIN) = 1.20V, IQ = 1mA, ISD < 6µA, DC/DC Converter MS8E Package LT1933 500mA IOUT, 500kHz, Step-Down Switching Regulator VIN: 3.6V to 36V, VOUT(MIN) = 1.25V, IQ = 1.6mA, ISD < 1µA, in SOT-23 ThinSOTTM Package LT1936 36V, 1.4A IOUT, 500kHz, High Efficiency Step-Down VIN: 3.6V to 36V, VOUT(MIN) = 1.20V, IQ = 1.9mA, ISD < 1µA, DC/DC Converter MS8E Package LT1940 Dual 25V, Dual 1.4A IOUT, 1.1MHz, High Efficiency Step-Down VIN: 3.6V to 25V, VOUT(MIN) = 1.25V, IQ = 3.5mA, ISD = <30µA, DC/DC Converter TSSOP16E Package LT1976/LT1977 60V, 1.2A IOUT, 200kHz/500kHz, High Efficiency Step-Down VIN: 3.3V to 60V, VOUT(MIN) = 1.25V, IQ = 100µA, ISD < 1µA, DC/DC Converter with Burst Mode® Operation TSSOP16E Package LT3434/LT3435 60V, 2.4A IOUT, 200kHz/500kHz, High Efficiency Step-Down VIN: 3.3V to 60V, VOUT(MIN) = 1.25V, IQ = 100µA, ISD < 1µA, DC/DC Converter with Burst Mode Operation TSSOP16E Package LT3437 60V, 400mA IOUT, Micropower Step-Down DC/DC Converter VIN: 3.3V to 60V, VOUT(MIN) = 1.25V, IQ = 100µA, ISD < 1µA, with Burst Mode Operation 3mm × 3mm DFN10 and TSSOP16E Packages LT3481 34V, 2A IOUT, 2.8MHz Micropower Step-Down DC/DC ConverterVIN: 3.3V to 34V, VOUT(MIN) = 1.265V, IQ = 50µA, ISD < 1µA, with IQ = 50µA 3mm × 3mm DFN10 and MS10E Packages LT3493 36V, 1.2A IOUT, 750kHz High Efficiency DC/DC Converter VIN: 3.6V to 36V, VOUT(MIN) = 0.78V, IQ = 1.9mA, ISD < 2µA, 2mm × 3mm DFN8 Package LT3505 36V, 1.2A IOUT, 750kHz High Efficiency Step-Down VIN: 3.6V to 36V, VOUT(MIN) = 0.78V, IQ = 1.9mA, ISD < 2µA, DC/DC Converter 3mm × 3mm DFN8 and MS8E Packages LT3506/LT3506A Dual 25V, 1.6A IOUT, 575kHz/1.1MHz High Efficiency VIN: 3.6V to 25V, VOUT(MIN) = 0.8V, IQ = 3.8mA, ISD < 30µA, Step-Down DC/DC Converter 5mm × 4mm DFN16 Package Burst Mode is a registered trademark of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation. 3503f 20 Linear Technology Corporation LT 1006 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2006