LT1936 1.4A, 500kHz Step-Down Switching Regulator FEATURES DESCRIPTION ■ Wide Input Range: 3.6V to 36V The LT®1936 is a current mode PWM step-down DC/DC ■ Short-Circuit Protected Over Full Input Range converter with an internal 1.9A power switch, packaged ■ 1.9A Guaranteed Minimum Switch Current in a tiny, thermally enhanced 8-lead MSOP. The wide in- ■ 5V at 1.4A from 10V to 36V Input put range of 3.6V to 36V makes the LT1936 suitable for ■ 3.3V at 1.4A from 7V to 36V Input regulating power from a wide variety of sources, including ■ 5V at 1.2A from 6.3V to 36V Input automotive batteries, 24V industrial supplies and unregu- ■ 3.3V at 1.2A from 4.5V to 36V Input lated wall adapters. Its high operating frequency allows the ■ Output Adjustable Down to 1.20V use of small, low cost inductors and ceramic capacitors, ■ 500kHz Fixed Frequency Operation resulting in low, predictable output ripple. ■ Soft-Start Cycle-by-cycle current limit, frequency foldback and ■ Uses Small Ceramic Capacitors thermal shutdown provide protection against shorted ■ Internal or External Compensation outputs, and soft-start eliminates input current surge ■ Low Shutdown Current: <2μA during start-up. Transient response can be optimized by ■ Thermally Enhanced 8-Lead MSOP Package using external compensation components, or board space can be minimized by using internal compensation. The APPLICATIONS low current (<2μA) shutdown mode enables easy power management in battery-powered systems. ■ Automotive Battery Regulation ■ Industrial Control Supplies L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. ■ Unregulated Wall Adapters TYPICAL APPLICATION 3.3V Step-Down Converter Effi ciency 95 VIN VIN = 12V 4.5V TO 36V 90 VOUT = 5V VIN BOOST 0.22μF 10μH VOUT ON OFF SHDN SW 3.3V %) 85 4.7μF LT1936 1.2A CY ( VOUT = 3.3V 17.4k N 80 E COMP FB CI FI VC GND 10k 22μF EF 75 70 1936 TA01a 65 0 0.5 1 1.5 LOAD CURRENT (A) 1936 TA01b 1936fd 1

LT1936 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) V Voltage .................................................–0.4V to 36V IN BOOST Voltage .........................................................43V TOP VIEW BOOST 1 8COMP BOOST Above SW Voltage ........................................20V VIN 2 9 7VC SHDN Voltage ............................................–0.4V to 36V SW 3 6FB GND 4 5SHDN FB, V , COMP Voltage .................................................6V C MS8E PACKAGE Operating Temperature Range (Note 2) 8-LEAD PLASTIC MSOP LT1936E ...............................................–40°C to 85°C θJA = 40°C/W, θJC = 10°C/W EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB LT1936I ..............................................–40°C to 125°C LT1936H ............................................–40°C to 150°C Maximum Junction Temperature LT1936E, LT1936I .............................................125°C LT1936H ...........................................................150°C Storage Temperature Range ...................–65°C to 150°C Lead Temperature (Soldering, 10 sec) ..................300°C ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LT1936EMS8E#PBF LT1936EMS8E#TRPBF LTBMT 8-Lead Plastic MSOP –40°C to 85°C LT1936IMS8E#PBF LT1936IMS8E#TRPBF LTBRV 8-Lead Plastic MSOP –40°C to 125°C LT1936HMS8E#PBF LT1936HMS8E#TRPBF LTBWB 8-Lead Plastic MSOP –40°C to 150°C LEAD BASED FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LT1936EMS8E LT1936EMS8E#TR LTBMT 8-Lead Plastic MSOP –40°C to 85°C LT1936IMS8E LT1936IMS8E#TR LTBRV 8-Lead Plastic MSOP –40°C to 125°C LT1936HMS8E LT1936HMS8E#TR LTBWB 8-Lead Plastic MSOP –40°C to 150°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/ 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 Undervoltage Lockout 3.45 3.6 V Quiescent Current V = 1.5V 1.8 2.5 mA FB Quiescent Current in Shutdown V = 0V 0.1 2 μA SHDN FB Voltage ● 1.175 1.200 1.215 V FB Pin Bias Current (Note 4) V = 1.20V, E and I Grades ● 50 200 nA FB H Grade ● 50 300 nA FB Voltage Line Regulation V = 5V to 36V 0.01 %/V IN Error Amp gm V = 0.5V, I = ±5μA 250 μS C VC Error Amp Voltage Gain V = 0.8V, 1.2V 150 C 1936fd 2

LT1936 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 V Clamp 1.8 V C V Switch Threshold 0.7 V C Internal Compensation R 50 kΩ Internal Compensation C V = 1V 150 pF COMP COMP Pin Leakage V = 1.8V, E and I Grades ● 1 μA COMP H Grade ● 2 μA Switching Frequency V = 1.1V 400 500 600 kHz FB V = 0V 40 kHz FB Maximum Duty Cycle ● 87 92 % Switch Current Limit (Note 3) 1.9 2.2 2.6 A Switch V I = 1.2A 410 520 mV CESAT SW Switch Leakage Current 2 μA Minimum BOOST Voltage Above SW I = 1.2A 2 2.2 V SW BOOST Pin Current I = 1.2A 28 50 mA SW BOOST Pin Leakage V = 0V 0.1 1 μA SW SHDN Input Voltage High 2.3 V SHDN Input Voltage Low 0.3 V SHDN Pin Current V = 2.3V (Note 5) 34 50 μA SHDN V = 12V (Note 5) 140 240 μA SHDN V = 0V 0.01 0.1 μA SHDN Note 1: Stresses beyond those listed under Absolute Maximum Ratings with statistical process controls. The LT1936I specifi cations are may cause permanent damage to the device. Exposure to any Absolute guaranteed over the –40°C to 125°C temperature range. The LT1936H Maximum Rating condition for extended periods may affect device specifi cations are guaranteed over the –40°C to 150°C temperature range. reliability and lifetime. Note 3: Current limit guaranteed by design and/or correlation to static test. Note 2: The LT1936E is guaranteed to meet performance specifi cations Slope compensation reduces current limit at higher duty cycle. from 0°C to 70°C. Specifi cations over the –40°C to 85°C operating Note 4: Current fl ows out of pin. temperature range are assured by design, characterization and correlation Note 5: Current fl ows into pin. TYPICAL PERFORMANCE CHARACTERISTICS Effi ciency, V = 5V Effi ciency, V = 3.3V Switch Current Limit OUT OUT 100 100 3.0 2.5 90 VIN = 12V 90 VIN = 5V CIENCY (%) 80 VIN = 24V CIENCY (%) 80 VVIINN == 2142VV ENT LIMIT (A) 12..50 MIN TYP FFI FFI RR E E U 1.0 C 70 VOUT = 5V 70 VOUT = 3.3V TA = 25°C TA = 25°C 0.5 D1 = DFLS140L D1 = DFLS140L L1 = 15μH, TOKO D63CB L1 = 10μH, TOKO D63CB 60 60 0 0 0.5 1.0 1.5 0 0.5 1.0 1.5 0 20 40 60 80 100 LOAD CURRENT (A) LOAD CURRENT (A) DUTY CYCLE (%) 1936 G01 1936 G02 1936 G03 1936fd 3

LT1936 TYPICAL PERFORMANCE CHARACTERISTICS Maximum Load Current Maximum Load Current Switch Voltage Drop 1.8 1.8 600 VOUT = 5V VOUT = 3.3V 500 1.6 1.6 mV) TA = 85°C T (A) L = 15μH T (A) L = 10μH ROP (400 TA = 25°C N N D OAD CURRE 1.4 L = 10μH OAD CURRE 1.4 L = 6.8μH H VOLTAGE 230000 TA = –45°C L L C 1.2 1.2 WIT S100 1.0 1.0 0 0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 0.5 1.0 1.5 INPUT VOLTAGE (V) INPUT VOLTAGE (V) SWITCH CURRENT (A) 1936 G04 1936 G05 1936 G06 Feedback Voltage Undervoltage Lockout Switching Frequency 1.210 3.8 600 E (V)1.205 3.6 CY (kHz)550 G N K VOLTA1.200 VLO (V) 3.4 FREQUE500 DBAC1.195 U HING E C FE 3.2 WIT450 1.190 S 1.185 3.0 400 –50 –25 0 25 50 75 100 125 150 –50 –25 0 25 50 75 100 125 150 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) TEMPERATURE (°C) TEMPERATURE (°C) 1936 G08 1936 G09 1936 G07 Frequency Foldback Soft-Start SHDN Pin Current 700 3.0 200 TA = 25°C TA = 25°C TA = 25°C DC = 30% 600 WITCHING FREQUENCY (kHz) 543200000000 WITCH CURRENT LIMIT (A) 1122....0505 SHDN PIN CURRENT (μA)11550000 S 100 S 0.5 0 0 0 0 0.5 1.0 1.5 0 1 2 3 4 0 4 8 12 16 FB PIN VOLTAGE (V) SHDN PIN VOLTAGE (V) SHDN PIN VOLTAGE (V) 1936 G10 1936 G11 1936 G12 1936fd 4

LT1936 TYPICAL PERFORMANCE CHARACTERISTICS Minimum Input Voltage Minimum Input Voltage Switch Current Limit 7.0 5.0 3.0 VOUT = 5V VOUT = 3.3V TA = 25°C TA = 25°C 6.5 L = 15μH L = 10μH 2.5 A) E (V) 6.0 E (V) 4.5 LIMIT ( 2.0 INPUT VOLTAG 55..05 INPUT VOLTAG 34..50 WITCH CURRENT 11..05 S 4.5 0.5 4.0 3.0 0 1 10 100 1000 1 10 100 1000 –50 –25 0 25 50 75 100 125 150 LOAD CURRENT (mA) LOAD CURRENT (mA) TEMPERATURE (°C) 1936 G13 1936 G14 1936 G15 Switching Waveforms, Switching Waveforms Discontinuous Mode VSW VSW 10V/DIV 10V/DIV IL IL 500mA/DIV 500mA/DIV VOUT VOUT 20mV/DIV 20mV/DIV VIN = 12V 1μs/DIV 1936 G16 VIN = 12V 1μs/DIV 1936 G17 VOUT = 3.3V VOUT = 3.3V IOUT = 1A IOUT = 50mA L = 10μH L = 10μH COUT = 22μF COUT = 22μF V Voltages Error Amp Output Current C 2.5 60 TA = 25°C VC = 0.5V 40 2.0 A) E (V) 1.5 CURRENT LIMIT CLAMP NT (μ 20 G E A R T R 0 L U O C V VC1.0 SWITCHING THRESHOLD PIN C–20 V 0.5 –40 0 –60 –50 –25 0 25 50 75 100 125 150 0 1 2 TEMPERATURE (°C) FB PIN VOLTAGE (V) 1936 G18 1936 G19 1936fd 5

LT1936 PIN FUNCTIONS BOOST (Pin 1): The BOOST pin is used to provide a drive FB (Pin 6): The LT1936 regulates its feedback pin to voltage, higher than the input voltage, to the internal bipolar 1.200V. Connect the feedback resistor divider tap to this NPN power switch. pin. Set the output voltage according to V = 1.200V OUT (1 + R1/R2). A good value for R2 is 10k. V (Pin 2): The V pin supplies current to the LT1936’s IN IN internal regulator and to the internal power switch. This V (Pin 7): The V pin is used to compensate the LT1936 C C pin must be locally bypassed. control loop by tying an external RC network from this pin to ground. The COMP pin provides access to an internal SW (Pin 3): The SW pin is the output of the internal power RC network that can be used instead of the external switch. Connect this pin to the inductor, catch diode and components. boost capacitor. COMP (Pin 8): To use the internal compensation network, GND (Pin 4): Tie the GND pin to a local ground plane tie the COMP pin to the V pin. Otherwise, tie COMP to below the LT1936 and the circuit components. Return the C ground or leave it fl oating. feedback divider to this pin. Exposed Pad (Pin 9): The Exposed Pad must be soldered SHDN (Pin 5): The SHDN pin is used to put the LT1936 in to the PCB and electrically connected to ground. Use a shutdown mode. Tie to ground to shut down the LT1936. large ground plane and thermal vias to optimize thermal Tie to 2.3V or more for normal operation. If the shutdown performance. feature is not used, tie this pin to the V pin. SHDN also IN provides a soft-start function; see the Applications Infor- mation. Do not drive SHDN more than 5V above V . IN BLOCK DIAGRAM VIN VIN 2 C2 INT REG AND UVLO D2 ∑ BOOST ON OFF 1 SLOPE R3 COMP R Q 5 SHDN C3 S Q DRIVER Q1 C4 L1 OSC SW 3 VOUT D1 C1 FREQUENCY FOLDBACK R1 FB 6 VC gm R2 CC RC 150pF 1.200V 50k 7 VC 8 COMP 4 GND 1936 BD R4 C5 1936fd 6

LT1936 OPERATION (Refer to Block Diagram) The LT1936 is a constant frequency, current mode step- An internal regulator provides power to the control circuitry. down regulator. A 500kHz oscillator enables an RS fl ip-fl op, This regulator includes an undervoltage lockout to prevent turning on the internal 1.9A power switch Q1. An ampli- switching when V is less than ~3.45V. The SHDN pin is IN fi er and comparator monitor the current fl owing between used to place the LT1936 in shutdown, disconnecting the the V and SW pins, turning the switch off when this output and reducing the input current to less than 2μA. IN current reaches a level determined by the voltage at V . C The switch driver operates from either the input or from An error amplifi er measures the output voltage through the BOOST pin. An external capacitor and diode are used an external resistor divider tied to the FB pin and servos to generate a voltage at the BOOST pin that is higher than the V pin. If the error amplifi er’s output increases, more C the input supply. This allows the driver to fully saturate current is delivered to the output; if it decreases, less the internal bipolar NPN power switch for effi cient opera- current is delivered. An active clamp (not shown) on the tion. V pin provides current limit. The V pin is also clamped C C to the voltage on the SHDN pin; soft-start is implemented The oscillator reduces the LT1936’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 startup and overload. 1936fd 7

LT1936 APPLICATIONS INFORMATION FB Resistor Network Inductor Selection and Maximum Output Current The output voltage is programmed with a resistor divider A good fi rst choice for the inductor value is between the output and the FB pin. Choose the 1% resis- L = 2.2 (V + V ) OUT D tors according to: where V is the voltage drop of the catch diode (~0.4V) D (cid:1) V (cid:3) R1=R2(cid:5) OUT –1(cid:6) and L is in μH. With this value the maximum output cur- (cid:2)1.200 (cid:4) rent will be above 1.2A at all duty cycles and greater than 1.4A for duty cycles less than 50% (V > 2 V ). The IN OUT R2 should be 20k or less to avoid bias current errors. inductor’s RMS current rating must be greater than the Reference designators refer to the Block Diagram. maximum load current and its saturation current should be about 30% higher. For robust operation in fault conditions Input Voltage Range (start-up or short circuit) and high input voltage (>30V), The input voltage range for LT1936 applications depends the saturation current should be above 2.6A. To keep the on the output voltage and the Absolute Maximum Ratings effi ciency high, the series resistance (DCR) should be less of the V and BOOST pins. than 0.1Ω, and the core material should be intended for IN high frequency applications. Table 1 lists several vendors The minimum input voltage is determined by either the and suitable types. LT1936’s minimum operating voltage of ~3.45V or by its maximum duty cycle. The duty cycle is the fraction of Table 1. Inductor Vendors time that the internal switch is on and is determined by VENDOR URL PART SERIES TYPE the input and output voltages: Murata www.murata.com LQH55D Open V +V TDK www.component.tdk.com SLF7045 Shielded DC= OUT D SLF10145 Shielded V –V +V IN SW D Toko www.toko.com D62CB Shielded D63CB Shielded where V is the forward voltage drop of the catch diode D75C Shielded D D75F Open (~0.5V) and V is the voltage drop of the internal switch SW (~0.5V at maximum load). This leads to a minimum input Sumida www.sumida.com CR54 Open CDRH74 Shielded voltage of: CDRH6D38 Shielded CR75 Open V +V V = OUT D –V +V IN(MIN) D SW DC MAX Of course, such a simple design guide will not always result in the optimum inductor for your application. A with DC = 0.87. MAX larger value provides a slightly higher maximum load The maximum input voltage is determined by the absolute current and will reduce the output voltage ripple. If your maximum ratings of the V and BOOST pins and by the load is lower than 1.2A, then you can decrease the value IN minimum duty cycle DC = 0.08: of the inductor and operate with higher ripple current. This MIN allows you to use a physically smaller inductor, or one V +V V = OUT D –V +V with a lower DCR resulting in higher effi ciency. Be aware IN(MAX) D SW DC MIN that if the inductance differs from the simple rule above, then the maximum load current will depend on input volt- Note that this is a restriction on the operating input voltage; age. There are several graphs in the Typical Performance the circuit will tolerate transient inputs up to the absolute Characteristics section of this data sheet that show the maximum ratings of the V and BOOST pins. IN maximum load current as a function of input voltage and inductor value for several popular output voltages. Low 1936fd 8

LT1936 APPLICATIONS INFORMATION inductance may result in discontinuous mode operation, combined with trace or cable inductance forms a high which is okay but further reduces maximum load current. quality (under damped) tank circuit. If the LT1936 circuit For details of maximum output current and discontinuous is plugged into a live supply, the input voltage can ring to mode operation, see Linear Technology Application Note twice its nominal value, possibly exceeding the LT1936’s 44. Finally, for duty cycles greater than 50% (V /V voltage rating. This situation is easily avoided; see the Hot OUT IN > 0.5), there is a minimum inductance required to avoid Plugging Safety section. subharmonic oscillations. Choosing L greater than 1.6 For space sensitive applications, a 2.2μF ceramic capaci- (V + V ) μH prevents subharmonic oscillations at all OUT D tor can be used for local bypassing of the LT1936 input. duty cycles. However, the lower input capacitance will result in in- creased input current ripple and input voltage ripple, and Catch Diode may couple noise into other circuitry. Also, the increased A 1A Schottky diode is recommended for the catch diode, voltage ripple will raise the minimum operating voltage D1. The diode must have a reverse voltage rating equal of the LT1936 to ~3.7V. to or greater than the maximum input voltage. The ON Semiconductor MBRM140 is a good choice. It is rated Output Capacitor for 1A DC at a case temperature of 110°C and 1.5A at a The output capacitor has two essential functions. Along case temperature of 95°C. Diode Incorporated’s DFLS140L with the inductor, it fi lters the square wave generated is rated for 1.1A average current; the DFLS240L is rated by the LT1936 to produce the DC output. In this role it for 2A average current. The average diode current in an determines the output ripple, and low impedance at the LT1936 application is approximately I (1 – DC). OUT switching frequency is important. The second function is to store energy in order to satisfy transient loads and Input Capacitor stabilize the LT1936’s control loop. Bypass the input of the LT1936 circuit with a 4.7μF or Ceramic capacitors have very low equivalent series re- higher value ceramic capacitor of X7R or X5R type. Y5V sistance (ESR) and provide the best ripple performance. types have poor performance over temperature and ap- A good value is: plied voltage, and should not be used. A 4.7μF ceramic is adequate to bypass the LT1936 and will easily handle 150 C = the ripple current. However, if the input power source has OUT V OUT high impedance, or there is signifi cant inductance due to long wires or cables, additional bulk capacitance may be where C is in μF. Use X5R or X7R types. This choice OUT necessary. This can be provided with a low performance will provide low output ripple and good transient response. electrolytic capacitor. Transient performance can be improved with a high value capacitor if the compensation network is also adjusted to Step-down regulators draw current from the input sup- maintain the loop bandwidth. ply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage A lower value of output capacitor can be used, but transient ripple at the LT1936 and to force this very high frequency performance will suffer. With an external compensation switching current into a tight local loop, minimizing EMI. network, the loop gain can be lowered to compensate for the A 4.7μF capacitor is capable of this task, but only if it is lower capacitor value. When using the internal compensa- placed close to the LT1936 and the catch diode; see the tion network, the lowest value for stable operation is: PCB Layout section. A second precaution regarding the 66 ceramic input capacitor concerns the maximum input C > OUT V voltage rating of the LT1936. A ceramic input capacitor OUT 1936fd 9

LT1936 APPLICATIONS INFORMATION 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 This is the minimum output capacitance required, not This capacitor (C ) is not part of the loop compensation F the nominal capacitor value. For example, a 3.3V output but is used to fi lter noise at the switching frequency, and requires 20μF of output capacitance. If a small 22μF, 6.3V is required only if a phase-lead capacitor is used or if the ceramic capacitor is used, the circuit may be unstable be- output capacitor has high ESR. An alternative to using cause the effective capacitance is lower than the nominal external compensation components is to use the internal capacitance when biased at 3.3V. Look carefully at the RC network by tying the COMP pin to the V pin. This re- C capacitor’s data sheet to fi nd out what the actual capaci- duces component count but does not provide the optimum tance is under operating conditions (applied voltage and transient response when the output capacitor value is high, temperature). A physically larger capacitor, or one with a and the circuit may not be stable when the output capacitor higher voltage rating, may be required. value is low. If the internal compensation network is not used, tie COMP to ground or leave it fl oating. High performance electrolytic capacitors can be used for the output capacitor. Low ESR is important, so choose one Loop compensation determines the stability and transient that is intended for use in switching regulators. The ESR performance. Designing the compensation network is a bit should be specifi ed by the supplier, and should be 0.05Ω or less. Such a capacitor will be larger than a ceramic capacitor and will have a larger capacitance, because the LT1936 capacitor must be large to achieve low ESR. Table 2 lists CURRENT MODE SW POWER STAGE OUTPUT several capacitor vendors. gm = 2mho ERROR AMPLIFIER R1 CPL FB Frequency Compensation – gm = 250μmho The LT1936 uses current mode control to regulate the + 1.2V ESR+ C1 output. This simplifi es loop compensation. In particular, the 600k C1 LT1936 does not require the ESR of the output capacitor 150pF 50k POLYMER CERAMIC for stability, so you are free to use ceramic capacitors to OR VC COMP GND TANTALUM achieve low output ripple and small circuit size. Frequency compensation is provided by the components CF RC R2 tied to the V pin, as shown in Figure 1. Generally a capaci- CC C tor (C ) and a resistor (R ) in series to ground are used. C C 1936 F01 In addition, there may be lower value capacitor in parallel. Figure 1. Model for Loop Response 1936fd 10

LT1936 APPLICATIONS INFORMATION complicated and the best values depend on the application current proportional to the voltage at the V pin. Note that C and in particular the type of output capacitor. A practical the output capacitor integrates this current, and that the approach is to start with one of the circuits in this data capacitor on the V pin (C ) integrates the error amplifi er C C sheet that is similar to your application and tune the com- output current, resulting in two poles in the loop. In most pensation network to optimize the performance. Stability cases a zero is required and comes from either the output should then be checked across all operating conditions, capacitor ESR or from a resistor R in series with C . C C including load current, input voltage and temperature. The This simple model works well as long as the value of the LT1375 data sheet contains a more thorough discussion of inductor is not too high and the loop crossover frequency loop compensation and describes how to test the stability is much lower than the switching frequency. A phase lead using a transient load. capacitor (C ) across the feedback divider may improve PL the transient response. Figure 1 shows an equivalent circuit for the LT1936 control loop. The error amplifi er is a transconductance amplifi er Figure 2 compares the transient response across several with fi nite output impedance. The power section, consisting output capacitor choices and compensation schemes. of the modulator, power switch and inductor, is modeled In each case the load current is stepped from 200mA to as a transconductance amplifi er generating an output 800mA and back to 200mA. COUT = 22μF (AVX 1210ZD226MAT) (2a) COMP VOUT 100mV/DIV VC COUT = 22μF ×2 (2b) COMP VOUT 100mV/DIV VC COUT = 150μF (4TPC150M) VOUT (2c) COMP 100mV/DIV VC COUT = 150μF (4TPC150M) VOUT 100mV/DIV (2d) COMP VC 800mA 220k IOUT 500mA/DIV 200mA 100pF 50μs/DIV 1936 F02 Figure 2. Transient Load Response of the LT1936 with Different Output Capacitors as the Load Current is Stepped from 200mA to 800mA. V = 3.3V OUT 1936fd 11

LT1936 APPLICATIONS INFORMATION BOOST Pin Considerations circuit by using a 1μF boost capacitor and a good, low drop Schottky diode (such as the ON Semi MBR0540). Because Capacitor C3 and diode D2 are used to generate a boost the required boost voltage increases at low temperatures, voltage that is higher than the input voltage. In most cases the circuit will supply only 1A of output current when the a 0.22μF capacitor and fast switching diode (such as the ambient temperature is –45°C, increasing to 1.2A at 0°C. 1N4148 or 1N914) will work well. Figure 3 shows two Also, the minimum input voltage to start the boost circuit ways to arrange the boost circuit. The BOOST pin must is higher at low temperature. See the Typical Applications be at least 2.3V above the SW pin for best effi ciency. For section for a 2.5V schematic and performance curves. outputs of 3V and above, the standard circuit (Figure 3a) is best. For outputs between 2.8V and 3V, use a 0.47μF The minimum operating voltage of an LT1936 application capacitor and a Schottky diode. For lower output voltages is limited by the undervoltage lockout (~3.45V) and by the boost diode can be tied to the input (Figure 3b), or to the maximum duty cycle as outlined above. For proper another supply greater than 2.8V. The circuit in Figure 3a is start-up, the minimum input voltage is also limited by the more effi cient because the BOOST pin current comes from boost circuit. If the input voltage is ramped slowly, or the a lower voltage. You must also be sure that the maximum LT1936 is turned on with its SHDN pin when the output voltage rating of the BOOST pin is not exceeded. is already in regulation, then the boost capacitor may not be fully charged. Because the boost capacitor is charged A 2.5V output presents a special case. This is a popular with the energy stored in the inductor, the circuit will rely output voltage, and the advantage of connecting the on some minimum load current to get the boost circuit boost circuit to the output is that the circuit will accept a running properly. This minimum load will depend on input 36V maximum input voltage rather than 20V (due to the and output voltages, and on the arrangement of the boost BOOST pin rating). However, 2.5V is marginally adequate circuit. The minimum load generally goes to zero once the to support the boosted drive stage at low ambient tem- circuit has started. Figure 4 shows a plot of minimum load peratures. Therefore, special care and some restrictions to start and to run as a function of input voltage. In many on operation are necessary when powering the BOOST pin cases the discharged output capacitor will present a load from a 2.5V output. Minimize the voltage loss in the boost to the switcher, which will allow it to start. The plots show the worst-case situation where V is ramping very slowly. IN D2 For lower start-up voltage, the boost diode can be tied to V ; however, this restricts the input range to one-half of IN BOOST C3 the absolute maximum rating of the BOOST pin. LT1936 VIN VIN SW VOUT At light loads, the inductor current becomes discontinu- ous and the effective duty cycle can be very high. This GND reduces the minimum input voltage to approximately VBOOST – VSW≅ VOUT 300mV above VOUT. At higher load currents, the inductor MAX VBOOST≅ VIN + VOUT current is continuous and the duty cycle is limited by the (3a) D2 maximum duty cycle of the LT1936, requiring a higher input voltage to maintain regulation. BOOST C3 LT1936 Soft-Start VIN VIN SW VOUT The SHDN pin can be used to soft-start the LT1936, reducing GND the maximum input current during start-up. The SHDN pin 1933 F03 VBOOST – VSW≅ VIN is driven through an external RC fi lter to create a voltage MAX VBOOST≅ 2VIN ramp at this pin. Figure 5 shows the start-up waveforms (3b) with and without the soft-start circuit. By choosing a large Figure 3. Two Circuits for Generating the Boost Voltage 1936fd 12

LT1936 APPLICATIONS INFORMATION Minimum Input Voltage V = 3.3V Minimum Input Voltage V = 5V OUT OUT 6.0 8 VOUT = 3.3V VOUT = 5V TA = 25°C TA = 25°C 5.5 L = 10μH L = 15μH 7 TO START E (V) 5.0 E (V) TO START G G A A OLT 4.5 OLT 6 V V T T U U NP 4.0 NP TO RUN I I 5 TO RUN 3.5 3.0 4 0 10 100 1000 1 10 100 1000 LOAD CURRENT (mA) LOAD CURRENT (mA) 1936 F04a 1936 F04b Figure 4. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit RUN 5V/DIV RUN SHDN IIN 500mA/DIV GND VOUT 5V/DIV 50μs/DIV 1936 F05a RUN RUN 5V/DIV 15k SHDN IIN 500mA/DIV 0.22μF GND VOUT 5V/DIV 0.5ms/DIV 1936 F05b Figure 5. To Soft-Start the LT1936, Add a Resistor and Capacitor to the SHDN Pin. V = 12V, V = 3.3V, C = 2 × 22μF, R = 3.3Ω IN OUT OUT LOAD RC time constant, the peak start-up current can be reduced where the output will be held high when the input to the to the current that is required to regulate the output, with LT1936 is absent. This may occur in battery charging ap- no overshoot. Choose the value of the resistor so that it plications or in battery backup systems where a battery can supply 60μA when the SHDN pin reaches 2.3V. or some other supply is diode OR-ed with the LT1936’s output. If the V pin is allowed to fl oat and the SHDN pin IN Shorted and Reversed Input Protection is held high (either by a logic signal or because it is tied to V ), then the LT1936’s internal circuitry will pull its If the inductor is chosen so that it won’t saturate exces- IN quiescent current through its SW pin. This is fi ne if your sively, an LT1936 buck regulator will tolerate a shorted system can tolerate a few mA in this state. If you ground output. There is another situation to consider in systems 1936fd 13

LT1936 APPLICATIONS INFORMATION the SHDN pin, the SW pin current will drop to essentially IN GND zero. However, if the V pin is grounded while the output MINIMIZE IN LT1936 C2 is held high, then parasitic diodes inside the LT1936 can C2, D1 LOOP R4 pull large currents from the output through the SW pin D2 and the V pin. Figure 6 shows a circuit that will run only IN when the input voltage is present and that protects against C3 a shorted or reversed input. D1 R2 D4 R1 MBRS140 L1 C1 VIN VIN BOOST LT1936 SHDN SW VOUT GND OUT VC COMP GND FB VIAS 1936 F07 Figure 7. A Good PCB Layout Ensures Low EMI Operation BACKUP High Temperature Considerations 1936 F06 The die temperature of the LT1936 must be lower than the Figure 6. Diode D4 Prevents a Shorted Input from Discharging maximum rating of 125°C (150°C for the H grade). This is a Backup Battery Tied to the Output; It Also Protects the Circuit generally not a concern unless the ambient temperature from a Reversed Input. The LT1936 Runs Only When the Input is Present is above 85°C. For higher temperatures, care should be taken in the layout of the circuit to ensure good heat sink- PCB Layout ing of the LT1936. The maximum load current should be For proper operation and minimum EMI, care must be derated as the ambient temperature approaches 125°C taken during printed circuit board layout. Figure 7 shows (150°C for the H grade). the recommended component placement with trace, The die temperature is calculated by multiplying the LT1936 ground plane and via locations. Note that large, switched power dissipation by the thermal resistance from junction currents fl ow in the LT1936’s V and SW pins, the catch IN to ambient. Power dissipation within the LT1936 can be diode (D1) and the input capacitor (C2). The loop formed estimated by calculating the total power loss from an by these components should be as small as possible. These effi ciency measurement and subtracting the catch diode components, along with the inductor and output capacitor, loss. The resulting temperature rise at full load is nearly should be placed on the same side of the circuit board, independent of input voltage. Thermal resistance depends and their connections should be made on that layer. Place on the layout of the circuit board, but values from 40°C/W a local, unbroken ground plane below these components. to 60°C/W are typical. The SW and BOOST nodes should be as small as possible. Finally, keep the FB and V nodes small so that the ground Die temperature rise was measured on a 4-layer, 5cm × C traces will shield them from the SW and BOOST nodes. 6.5cm circuit board in still air at a load current of 1.4A. The Exposed Pad on the bottom of the package must be For 12V input to 3.3V output the die temperature elevation soldered to ground so that the pad acts as a heat sink. To above ambient was 26°C; for 24V in to 3.3V out the rise keep thermal resistance low, extend the ground plane as was 31°C; for 12V in to 5V the rise was 31°C and for 24V much as possible, and add thermal vias under and near in to 5V the rise was 34°C. the LT1936 to additional ground planes within the circuit board and on the bottom side. 1936fd 14

LT1936 APPLICATIONS INFORMATION Hot Plugging Safely input voltage, possibly exceeding the LT1936’s rating and damaging the part. If the input supply is poorly controlled The small size, robustness and low impedance of ceramic or the user will be plugging the LT1936 into an energized capacitors make them an attractive option for the input supply, the input network should be designed to prevent bypass capacitor of LT1936 circuits. However, these capaci- this overshoot. tors can cause problems if the LT1936 is plugged into a live supply (see Linear Technology Application Note 88 for Figure 8 shows the waveforms that result when an LT1936 a complete discussion). The low loss ceramic capacitor circuit is connected to a 24V supply through six feet of combined with stray inductance in series with the power 24-gauge twisted pair. The fi rst plot is the response with source forms an under damped tank circuit, and the voltage a 4.7μF ceramic capacitor at the input. The input voltage at the V pin of the LT1936 can ring to twice the nominal rings as high as 50V and the input current peaks at 26A. IN CLOSING SWITCH DANGER SIMULATES HOT PLUG VIN IIN VIN 20V/DIV LT1936 RINGING VIN MAY EXCEED + ABSOLUTE MAXIMUM RATING OF THE LT1936 4.7μF LOW STRAY IIN 10A/DIV IMPEDANCE INDUCTANCE ENERGIZED DUE TO 6 FEET 24V SUPPLY (2 METERS) OF 20μs/DIV TWISTED PAIR (8a) VIN LT1936 20V/DIV + 22μF + 35V 4.7μF AI.EI. IIN 10A/DIV (8b) 20μs/DIV 0.7Ω VIN LT1936 20V/DIV + 0.1μF 4.7μF IIN 10A/DIV (8c) 20μs/DIV 1936 F08 Figure 8. A Well Chosen Input Network Prevents Input Voltage Overshoot and Ensures Reliable Operation When the LT1936 is Connected to a Live Supply 1936fd 15

LT1936 APPLICATIONS INFORMATION One method of damping the tank circuit is to add another Other Linear Technology Publications capacitor with a series resistor to the circuit. In Figure 8b Application Notes 19, 35 and 44 contain more detailed an aluminum electrolytic capacitor has been added. This descriptions and design information for buck regulators capacitor’s high equivalent series resistance damps the and other switching regulators. The LT1376 data sheet circuit and eliminates the voltage overshoot. The extra has a more extensive discussion of output ripple, loop capacitor improves low frequency ripple fi ltering and compensation and stability testing. Design Note 100 can slightly improve the effi ciency of the circuit, though shows how to generate a bipolar output supply using a it is likely to be the largest component in the circuit. An buck regulator. alternative solution is shown in Figure 8c. A 0.7Ω resistor is added in series with the input to eliminate the voltage Outputs Greater Than 6V overshoot (it also reduces the peak input current). A 0.1μF For outputs greater than 6V, add a resistor of 1k to 2.5k capacitor improves high frequency fi ltering. This solution across the inductor to damp the discontinuous ringing is smaller and less expensive than the electrolytic capacitor. of the SW node, preventing unintended SW current. The For high input voltages its impact on effi ciency is minor, 12V Step-Down Converter circuit in the Typical Applica- reducing effi ciency by one percent for a 5V output at full tions section shows the location of this resistor. Also note load operating from 24V. that for outputs above 6V, the input voltage range will be limited by the maximum rating of the BOOST pin. The 12V circuit shows how to overcome this limitation using an additional Zener diode. TYPICAL APPLICATIONS 3.3V Step-Down Converter D2 VIN 4.5V TO 36V C3 L1 VIN BOOST 0.22μF 10μH VOUT ON OFF SHDN SW 3.3V C1 1.2A LT1936 D1 R1 4.7μF 17.4k COMP FB VC GND R2 C2 10k 47μF 1936 TA03 1936fd 16

LT1936 TYPICAL APPLICATIONS 5V Step-Down Converter D2 VIN 6.3V TO 36V C3 L1 VIN BOOST 0.22μF 15μH VOUT ON OFF SHDN SW 5V C1 LT1936 D1 R1 1.2A 4.7μF 31.6k COMP FB VC GND R2 C2 10k 22μF 1936 TA04 1.8V Step-Down Converter Effi ciency, 1.8V Output D2 90 2.0 VIN VOUT = 1.8V 3.6V TO 20V TA = 25°C C3 L1 VIN BOOST 0.22μF 4.7μH VOUT 80 VIN = 5V 1.5 ON OFF C4.17μF SHDLNT1936SW D1 1R01k 11..83VA NCY (%) 70 VIN = 12V 1.0 POWER L E O COVMCP GNDFB R2 C472μF EFFICI SS (W) 20k ×2 60 0.5 POWER LOSS D1: DFLS140L 1936 TA05a D2: 1N4148 L1: TOKO D63CB 50 0 0 0.5 1 1.5 LOAD CURRENT (A) 1936 TA05b 1.2V Step-Down Converter Effi ciency, 1.2V Output D2 80 2.0 VIN VOUT = 1.2V 3.6V TO 20V TA = 25°C C3 L1 75 VIN BOOST 0.22μF 3.3μH VOUT VIN = 5V 1.5 ON OFF C4.17μF SHDLNT1936SW D1 11..23VA NCY (%) 6750 VIN = 12V 1.0 POWER L COVMCP GNDFB 100k C472μF EFFICIE 60 OSS (W) ×2 0.5 55 POWER LOSS D1: DFLS140L 1936 TA06a D2: 1N4148 L1: TOKO D63CB 50 0 0 0.5 1 1.5 LOAD CURRENT (A) 1936 TA06b 1936fd 17

LT1936 TYPICAL APPLICATIONS 2.5V Step-Down Converter D2 VIN 3.6V TO 36V C3 L1 VIN BOOST 1μF 6.2μH VOUT 2.5V ON OFF SHDN SW 1.2A C1 LT1936 D1 R1 TA > 0°C 4.7μF 11k COMP FB VC GND R2 C2 10k 47μF D1: DFLS140L 1936 TA07a D2: MBRO540 L1: TOKO D63CB Effi ciency, 2.5V Output Minimum Input Voltage 100 5.5 VOUT = 2.5V VOUT = 2.5V TA = 25°C 5.0 90 TO START Y (%) VIN = 5V AGE (V) 4.5 TA = –45°C EFFICIENC 80 VIN = 12V NPUT VOLT 4.0 TO RUN TTOA =S T2A5R°CT 70 I TA = –45°C 3.5 TO RUN TA = 25°C 60 3.0 0 0.5 1.0 1.5 1 10 100 1000 LOAD CURRENT (A) LOAD CURRENT (mA) 1936 TA07b 1936 TA07c 12V Step-Down Converter D3 D2 6.8V VIN 14.5V TO 36V C3 L1 VIN BOOST 0.22μF 22μH ON OFF SHDN SW C1 LT1936 D1 1.8k V12OVUT 2.2μF 1.2A COMP FB R1 VC GND R2 182k C2 20k 22μF D1: MBRM140 D2: 1N4148 1936 TA08 D3: CMDZ5235B 1936fd 18

LT1936 PACKAGE DESCRIPTION MS8E Package 8-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1662 Rev E) BOTTOM VIEW OF EXPOSED PAD OPTION 2.06± 0.102 1 (.081± .004) 0.29 1.83± 0.102 REF 2.794± 0.102 0.889± 0.127 (.072± .004) (.110± .004) (.035± .005) 0.05 REF DETAIL “B” 5.23 (.206) 2.083± 0.102 3.20 – 3.45 CORNER TAIL IS PART OF MIN (.082± .004) (.126 – .136) DETAIL “B” THE LEADFRAME FEATURE. FOR REFERENCE ONLY 8 NO MEASUREMENT PURPOSE 3.00± 0.102 0.42± 0.038 0.65 (.118± .004) 0.52 (.0165± .0015) (.0256) (NOTE 3) 8 7 6 5 (.0205) TYP BSC REF RECOMMENDED SOLDER PAD LAYOUT 3.00± 0.102 4.90± 0.152 DETAIL “A” (.118± .004) 0.254 (.193± .006) (NOTE 4) (.010) 0° – 6° TYP GAUGE PLANE 1 2 3 4 0.53± 0.152 (.021± .006) 1.10 0.86 (.043) (.034) DETAIL “A” MAX REF 0.18 (.007) SEATING PLANE 0.22 – 0.38 0.1016± 0.0508 (.009 – .015) (.004± .002) 0.65 TYP MSOP (MS8E) 0908 REV E (.0256) NOTE: BSC 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 1936fd 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 representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

LT1936 TYPICAL APPLICATION 2.5V Step-Down Converter Minimum Input Voltage D2 5.5 VIN VOUT = 2.5V 3.6V TO 20V C3 VIN BOOST 0.22μF 8.L21μH VOUT 5.0 CINOPNUNTE LCOTWINEGR TSH TEH BEO MOISNTI MCIURMC UINITP TUOT THE ON OFF SHDN SW 2.5V V) VOLTAGE TO RUN AND TO START TO LESS C4.17μF LT1936 D1 1R11k 1.3A LTAGE ( 4.5 THAN 3.7V AT ALL LOADS O COMP FB V UT 4.0 VC GND R2 C2 NP 10k 47μF I 3.5 D1: DFLS140L 1936 TA09a D2: 1N4148 L1: TOKO D63CB 3.0 1 10 100 1000 LOAD CURRENT (mA) 1936 TA09b RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1676 60V, 440mA (I ), 100kHz, High Effi ciency Step-Down V : 7.4V to 60V, V = 1.24V, I = 3.2mA, I = 2.5μA, OUT IN OUT(MIN) Q SD DC/DC Converter SO-8 Package LT1765 25V, 2.75A (I ), 1.25MHz, High Effi ciency Step-Down V : 3V to 25V, V = 1.20V, I = 1mA, I = 15μA, OUT IN OUT(MIN) Q SD DC/DC Converter SO-8 and 16-Lead TSSOPE Packages LT1766 60V, 1.2A (I ), 200kHz, High Effi ciency Step-Down V : 5.5V to 60V, V = 1.20V, I = 2.5mA, I = 25μA, OUT IN OUT(MIN) Q SD DC/DC Converter 16-Lead TSSOP/TSSOPE 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 MS8/MS8E Packages LT1776 40V, 550mA (I ), 200kHz, High Effi ciency Step-Down V : 7.4V to 40V, V = 1.24V, I = 3.2mA, I = 30μA, OUT IN OUT(MIN) Q SD DC/DC Converter N8/SO-8 Packages LT1933 600mA, 500kHz, Step-Down Switching Regulator in SOT-23 V : 3.6V to 36V, V = 1.25V, I = 1.6mA, I < 1μA, IN OUT(MIN) Q SD ThinSOT™ Package LT1940 25V, Dual 1.4A (I ), 1.1MHz, High Effi ciency Step-Down V : 3V to 25V, V = 1.2V, I = 3.8mA, I < 1μA, OUT IN OUT(MIN) Q SD DC/DC Converter 16-Lead TSSOPE Package LT1956 60V, 1.2A (I ), 500kHz, High Effi ciency Step-Down V : 5.5V to 60V, V = 1.20V, I = 2.5mA, I = 25μA, OUT IN OUT(MIN) Q SD DC/DC Converter 16-Lead TSSOP/TSSOPE Packages LT1976 60V, 1.2A (I ), 200kHz, High Effi ciency Step-Down V : 3.3V to 60V, V = 1.20V, I = 100μA, I < 1μA, OUT IN OUT(MIN) Q SD DC/DC Converter with Burst Mode® Operation 16-Lead TSSOPE Package LT3010 80V, 50mA, Low Noise Linear Regulator V : 1.5V to 80V, V = 1.28V, I = 30μA, I < 1μA, IN OUT(MIN) Q SD MS8E Package LTC®3407 Dual 600mA (I ), 1.5MHz, Synchronous Step-Down V : 2.5V to 5.5V, V = 0.6V, I = 40μA, I < 1μA, OUT IN OUT(MIN) Q SD DC/DC Converter 10-Lead MSE Package LTC3412 2.5A (I ), 4MHz, Synchronous Step-Down DC/DC V : 2.5V to 5.5V, V = 0.8V, I = 60μA, I < 1μA, OUT IN OUT(MIN) Q SD Converter 16-Lead TSSOPE Package LTC3414 4A (I ), 4MHz, Synchronous Step-Down DC/DC Converter V : 2.3V to 5.5V, V = 0.8V, I = 64μA, I < 1μA, OUT IN OUT(MIN) Q SD 20-Lead TSSOPE Package LT3430/LT3431 60V, 2.75A (I ), 200kHz/500kHz, High Effi ciency V : 5.5V to 60V, V = 1.20V, I = 2.5mA, I = 30μA, OUT IN OUT(MIN) Q SD Step-Down DC/DC Converters 16-Lead TSSOPE Package Burst Mode is a registered trademark of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation. 1936fd 20 Linear Technology Corporation LT 1108 REV D • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2006