LTC1871-1

FEATURES n n n n n n n n n n n n n n n LTC1871-1 Wide Input Range, No RSENSE™ Current Mode Boost, Flyback and SEPIC Controller DESCRIPTION The LTC®1871-1 is a wide input range, current mode, boost, flyback or SEPIC controller that drives an N-channel power MOSFET and requires very few external components. It eliminates the need for a current sense resistor by utilizing the power MOSFET’s on-resistance, thereby maximizing efficiency. Higher output voltage applications are possible with the LTC1871-1 by connecting the SENSE pin to a resistor in the source of the power MOSFET. The IC’s operating frequency can be set with an external resistor over a 50kHz to 1MHz range, and can be synchronized to an external clock using the MODE/SYNC pin. The LTC1871-1 differs from the LTC1871 by having a lower pulse skip threshold, making it ideal for applications requiring constant frequency operation at light loads. The lower pulse skip threshold also helps maintain constant frequency operation in applications with a wide input voltage range. For applications requiring primaryto-secondary side isolation, please refer to the LTC1871 datasheet. The LTC1871-1 is available in the 10-lead MSOP package. L, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation. No RSENSE is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. High Efficiency (No Sense Resistor Required) Wide Input Voltage Range: 2.5V to 36V Current Mode Control Provides Excellent Transient Response High Maximum Duty Cycle (92% Typ) ± 2% RUN Pin Threshold with 100mV Hysteresis ±1% Internal Voltage Reference Ultra Low Pulse Skip Threshold for Wide Input Range Applications Micropower Shutdown: IQ = 10μA Programmable Operating Frequency (50kHz to 1MHz) with One External Resistor Synchronizable to an External Clock Up to 1.3 × fOSC User-Controlled Pulse Skip or Burst Mode® Operation Internal 5.2V Low Dropout Voltage Regulator Output Overvoltage Protection Capable of Operating with a Sense Resistor for High Output Voltage Applications Small 10-Lead MSOP Package APPLICATIONS n n Telecom Power Supplies Portable Electronic Equipment TYPICAL APPLICATION VIN 3.3V L1 1μH D1 RUN ITH RC 22k CC1 6.8nF CC2 47pF R2 37.4k 1% R1 12.1k 1% FB FREQ RT 80.6k 1% MODE/SYNC LTC1871-1 INTVCC GATE GND CVCC 4.7μF X5R SENSE VIN VOUT 5V 7A (10A PEAK) COUT2 22μF 6.3V X5R ×2 GND 100 90 80 EFFICIENCY (%) 70 60 50 40 30 0.001 Burst Mode OPERATION Efficiency of Figure 1 + M1 COUT1 150μF 6.3V ×4 PULSE-SKIP MODE + CIN 22μF 6.3V ×2 18711 F01a CIN: TAIYO YUDEN JMK325BJ226MM COUT1: PANASONIC EEFUEOJ151R COUT2: TAIYO YUDEN JMK325BJ226MM D1: MBRB2515L L1: SUMIDA CEP125-H 1R0MH M1: FAIRCHILD FDS7760A 0.01 0.1 1 OUTPUT CURRENT (A) 10 18711 F01b Figure 1. High Efficiency 3.3V Input, 5V Output Boost Converter (Bootstrapped) 18711fb 1 LTC1871-1 ABSOLUTE MAXIMUM RATINGS (Note 1) PIN CONFIGURATION TOP VIEW RUN ITH FB FREQ MODE/ SYNC 1 2 3 4 5 10 9 8 7 6 SENSE VIN INTVCC GATE GND VIN Voltage ............................................... – 0.3V to 36V INTVCC Voltage............................................ –0.3V to 7V INTVCC Output Current .......................................... 50mA GATE Voltage ............................ –0.3V to VINTVCC + 0.3V ITH, FB Voltages ....................................... –0.3V to 2.7V RUN, MODE/SYNC Voltages ....................... –0.3V to 7V FREQ Voltage ............................................ –0.3V to 1.5V SENSE Pin Voltage .................................... –0.3V to 36V Operating Junction Temperature Range (Note 2) LTC1871E-1 ......................................... –40°C to 85°C LTC1871I-1 ........................................ –40°C to 125°C Junction Temperature (Note 3) ............................ 125°C Storage Temperature Range................... –65°C to 150°C Lead Temperature (Soldering, 10 sec) .................. 300°C MS PACKAGE 10-LEAD PLASTIC MSOP TJMAX = 125°C, θJA = 120°C/W ORDER INFORMATION LEAD FREE FINISH LTC1871EMS-1#PBF LTC1871IMS-1#PBF LEAD BASED FINISH LTC1871EMS-1 LTC1871IMS-1 TAPE AND REEL LTC1871EMS-1#TRPBF LTC1871IMS-1#TRPBF TAPE AND REEL LTC1871EMS-1#TR LTC1871IMS-1#TR PART MARKING LTCTV LTCTV PART MARKING LTCTV LTCTV PACKAGE DESCRIPTION 10-Lead Plastic MSOP 10-Lead Plastic MSOP PACKAGE DESCRIPTION 10-Lead Plastic MSOP 10-Lead Plastic MSOP TEMPERATURE RANGE –40°C to 85°C –40°C to 125°C TEMPERATURE RANGE –40°C to 85°C –40°C to 125°C Consult LTC Marketing for parts specified 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 specifications, go to: http://www.linear.com/tapeandreel/ The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 5V, VRUN = 1.5V, RFREQ = 80k, VMODE/SYNC = 0V, unless otherwise specified. SYMBOL VIN(MIN) IQ PARAMETER Minimum Input Voltage I-Grade (Note 2) Input Voltage Supply Current Continuous Mode (Note 4) VMODE/SYNC = 5V, VFB = 1.4V, VITH = 0.75V VMODE/SYNC = 5V, VFB = 1.4V, VITH = 0.75V, I-Grade (Note 2) Burst Mode Operation, No Load VMODE/SYNC = 0V, VITH = 0V (Note 5) VMODE/SYNC = 0V, VITH = 0V (Note 5), I-Grade (Note 2) Shutdown Mode VRUN = 0V VRUN = 0V, I-Grade (Note 2) ● ● ● ● ELECTRICAL CHARACTERISTICS CONDITIONS MIN 2.5 2.5 TYP MAX UNITS V V Main Control Loop 550 550 250 250 10 10 1000 1000 500 500 20 20 μA μA μA μA μA μA 18711fb 2 LTC1871-1 ELECTRICAL CHARACTERISTICS SYMBOL VRUN+ VRUN– VRUN(HYST) IRUN VFB PARAMETER Rising RUN Input Threshold Voltage Falling RUN Input Threshold Voltage ● The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 5V, VRUN = 1.5V, RFREQ = 80k, VMODE/SYNC = 0V, unless otherwise specified. CONDITIONS MIN 1.223 1.198 50 I-Grade (Note 2) RUN Input Current Feedback Voltage VITH = 0.4V (Note 5) VITH = 0.4V (Note 5), I-Grade (Note 2) ● ● ● TYP 1.348 1.248 100 100 1 1.230 MAX 1.273 1.298 150 175 60 1.242 1.248 1.255 60 0.02 0.03 UNITS V V V mV mV nA V V V nA %/V %/V % % RUN Pin Input Threshold Hysteresis 35 1.218 1.212 1.205 IFB ΔVFB ΔVIN ΔVFB ΔVITH ΔVFB(OV) gm VITH(BURST) VSENSE(MAX) ISENSE(ON) ISENSE(OFF) Oscillator fOSC FB Pin Input Current Line Regulation Load Regulation VITH = 0.4V (Note 5) 2.5V ≤ VIN ≤ 30V 2.5V ≤ VIN ≤ 30V, I-Grade (Note 2) VMODE/SYNC = 0V, VITH = 0.5V to 0.9V (Note 5) VMODE/SYNC = 0V, VITH = 0.5V to 0.9V (Note 5) I-Grade (Note 2) ● ● ● 18 0.002 0.002 –1 –1 2.5 –0.1 –0.1 6 650 195 120 ● ΔFB Pin, Overvoltage Lockout Error Amplifier Transconductance Burst Mode Operation ITH Pin Voltage Maximum Current Sense Input Threshold SENSE Pin Current (GATE High) SENSE Pin Current (GATE Low) Oscillator Frequency Oscillator Frequency Range VFB(OV) – VFB(NOM) in Percent ITH Pin Load = ±5μA (Note 5) Falling ITH Voltage (Note 5) Duty Cycle < 20% Duty Cycle < 20%, I-Grade (Note 2) VSENSE = 0V VSENSE = 30V RFREQ = 80k RFREQ = 80k, I-Grade (Note 2) I-Grade (Note 2) ● ● ● ● 10 % μmho mV 150 35 0.1 180 200 50 5 350 350 1000 1000 mV mV μA μA kHz kHz kHz kHz % % 100 250 250 50 50 87 300 300 DMAX fSYNC/fOSC tSYNC(MIN) tSYNC(MAX) VIL(MODE) VIH(MODE) RMODE/SYNC VFREQ Maximum Duty Cycle I-Grade (Note 2) Recommended Maximum Synchronized Frequency Ratio MODE/SYNC Minimum Input Pulse Width MODE/SYNC Maximum Input Pulse Width Low Level MODE/SYNC Input Voltage I-Grade (Note 2) High Level MODE/SYNC Input Voltage I-Grade (Note 2) MODE/SYNC Input Pull-Down Resistance Nominal FREQ Pin Voltage ● ● 92 92 1.25 1.25 25 0.8/fOSC 97 97 1.30 1.30 87 fOSC = 300kHz (Note 6) fOSC = 300kHz (Note 6), I-Grade (Note 2) VSYNC = 0V to 5V VSYNC = 0V to 5V ns ns 0.3 0.3 V V V V 1.2 1.2 50 0.62 kΩ V 18711fb 3 LTC1871-1 ELECTRICAL CHARACTERISTICS SYMBOL VINTVCC ΔVINTVCC ΔVIN1 ΔVINTVCC ΔVIN2 VLDO(LOAD) VDROPOUT IINTVCC GATE Driver tr tf GATE Driver Output Rise Time GATE Driver Output Fall Time CL = 3300pF (Note 7) CL = 3300pF (Note 7) 17 8 100 100 ns ns INTVCC Load Regulation INTVCC Regulator Dropout Voltage Bootstrap Mode INTVCC Supply 0 ≤ IINTVCC ≤ 20mA, VIN = 7.5V INTVCC Load = 20mA RUN = 0V, SENSE = 5V I-Grade (Note 2) ● The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 5V, VRUN = 1.5V, RFREQ = 80k, VMODE/SYNC = 0V, unless otherwise specified. PARAMETER INTVCC Regulator Output Voltage INTVCC Regulator Line Regulation INTVCC Regulator Line Regulation CONDITIONS VIN = 7.5V VIN = 7.5V, I-Grade (Note 2) 7.5V ≤ VIN ≤ 15V 15V ≤ VIN ≤ 30V –2 ● MIN 5.0 5.0 TYP 5.2 5.2 8 70 –0.2 280 10 MAX 5.4 5.4 25 200 UNITS V V mV mV % mV Low Dropout Regulator 20 30 μA μA Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC1871E-1 is guaranteed to meet performance specifications from 0°C to 85°C junction temperature. Specifications over the – 40°C to 85°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LTC1871I-1 is guaranteed over the full –40°C to 125°C operating junction temperature range. Note 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: TJ = TA + (PD • 110°C/W) Note 4: The dynamic input supply current is higher due to power MOSFET gate charging (QG • fOSC). See Applications Information. Note 5: The LTC1871-1 is tested in a feedback loop which servos VFB to the reference voltage with the ITH pin forced to the midpoint of its voltage range (0.3V ≤ VITH ≤ 1.2V, midpoint = 0.75V). Note 6: In a synchronized application, the internal slope compensation gain is increased by 25%. Synchronizing to a significantly higher ratio will reduce the effective amount of slope compensation, which could result in subharmonic oscillation for duty cycles greater than 50%. Note 7: Rise and fall times are measured at 10% and 90% levels. TYPICAL PERFORMANCE CHARACTERISTICS FB Voltage vs Temp 1.25 1.231 FB Voltage Line Regulation 60 50 FB PIN CURRENT (nA) FB Pin Current vs Temperature 1.24 FB VOLTAGE (V) FB VOLTAGE (V) 40 30 20 10 1.23 1.230 1.22 1.21 –50 –25 1.229 0 25 50 75 100 125 150 TEMPERATURE (°C) 18711 G01 0 5 10 15 20 VIN (V) 25 30 35 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 18711 G03 18711 G02 18711fb 4 LTC1871-1 TYPICAL PERFORMANCE CHARACTERISTICS Shutdown Mode IQ vs VIN 30 20 Shutdown Mode IQ vs Temperature VIN = 5V 600 500 Burst Mode IQ vs VIN SHUTDOWN MODE IQ (μA) SHUTDOWN MODE IQ (μA) 15 Burst Mode IQ (μA) 0 25 50 75 100 125 150 TEMPERATURE (°C) 18711 G05 20 400 300 200 100 10 10 5 0 0 10 20 VIN (V) 30 40 18711 G04 0 –50 –25 0 0 10 20 VIN (V) 30 40 18711 G06 Burst Mode IQ vs Temperature 500 18 16 400 Burst Mode IQ (μA) 14 12 Dynamic IQ vs Frequency CL = 3300pF IQ(TOT) = 550μA + Qg • f 60 50 40 TIME (ns) Gate Drive Rise and Fall Time vs CL IQ (mA) 300 10 8 6 RISE TIME 30 20 FALL TIME 10 200 100 4 2 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 18711 G07 0 0 200 400 800 600 FREQUENCY (kHz) 1000 1200 0 0 2000 4000 6000 8000 CL (pF) 10000 12000 18711 G09 18711 G08 RUN Thresholds vs VIN 1.5 1.40 RUN Thresholds vs Temperature 1000 RT vs Frequency RUN THRESHOLDS (V) 1.4 RUN THRESHOLDS (V) 1.35 RT (kΩ) 0 25 50 75 100 125 150 TEMPERATURE (°C) 18711 G11 1.30 100 1.3 1.25 1.2 0 10 20 VIN (V) 30 40 18711 G10 1.20 –50 –25 10 0 100 200 300 400 500 600 700 800 900 1000 FREQUENCY (kHz) 18711 G12 18711fb 5 LTC1871-1 TYPICAL PERFORMANCE CHARACTERISTICS Frequency vs Temperature 325 320 GATE FREQUENCY (kHz) 155 SENSE PIN CURRENT (μA) 315 310 305 300 295 290 285 280 275 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 18711 G13 Maximum Sense Threshold vs Temperature 160 MAX SENSE THRESHOLD (mV) 35 SENSE Pin Current vs Temperature GATE HIGH VSENSE = 0V 150 30 145 140 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 18711 G14 25 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 18711 G15 INTVCC Load Regulation VIN = 7.5V 5.2 INTVCC VOLTAGE (V) INTVCC VOLTAGE (V) 5.3 5.4 INTVCC Line Regulation 500 450 DROPOUT VOLTAGE (mV) 400 350 300 250 200 150 100 50 INTVCC Dropout Voltage vs Current, Temperature 150°C 125°C 75°C 25°C 5.1 5.2 0°C –50°C 5.0 0 10 20 30 40 50 60 INTVCC LOAD (mA) 70 80 5.1 0 5 10 15 20 25 VIN (V) 30 0 35 40 0 5 10 15 INTVCC LOAD (mA) 20 18711 G18 18711 G16 18711 G17 PIN FUNCTIONS RUN (Pin 1): The RUN pin provides the user with an accurate means for sensing the input voltage and programming the start-up threshold for the converter. The falling RUN pin threshold is nominally 1.248V and the comparator has 100mV of hysteresis for noise immunity. When the RUN pin is below this input threshold, the IC is shut down and the VIN supply current is kept to a low value (typ 10μA). The Absolute Maximum Rating for the voltage on this pin is 7V. ITH (Pin 2): Error Amplifier Compensation Pin. The current comparator input threshold increases with this control voltage. Nominal voltage range for this pin is 0V to 1.40V. FB (Pin 3): Receives the feedback voltage from the external resistor divider across the output. Nominal voltage for this pin in regulation is 1.230V. FREQ (Pin 4): A resistor from the FREQ pin to ground programs the operating frequency of the chip. The nominal voltage at the FREQ pin is 0.6V. 18711fb 6 LTC1871-1 PIN FUNCTIONS MODE/SYNC (Pin 5): This input controls the operating mode of the converter and allows for synchronizing the operating frequency to an external clock. If the MODE/ SYNC pin is connected to ground, Burst Mode operation is enabled. If the MODE/SYNC pin is connected to INTVCC, or if an external logic-level synchronization signal is applied to this input, Burst Mode operation is disabled and the IC operates in a continuous mode. GND (Pin 6): Ground Pin. GATE (Pin 7): Gate Driver Output. INTVCC (Pin 8): The Internal 5.20V Regulator Output. The gate driver and control circuits are powered from this voltage. Decouple this pin locally to the IC ground with a minimum of 4.7μF low ESR tantalum or ceramic capacitor. VIN (Pin 9): Main Supply Pin. Must be closely decoupled to ground. SENSE (Pin 10): The Current Sense Input for the Control Loop. Connect this pin to the drain of the power MOSFET for VDS sensing and highest efficiency. Alternatively, the SENSE pin may be connected to a resistor in the source of the power MOSFET. Internal leading edge blanking is provided for both sensing methods. BLOCK DIAGRAM RUN SLOPE COMPENSATION BIAS AND START-UP CONTROL + C2 1 – 1.248V VIN FREQ 4 0.6V MODE/SYNC 5 85mV 1.230V 50k OV IOSC PWM LATCH LOGIC S Q R BURST COMPARATOR CURRENT COMPARATOR C1 GND INTVCC V-TO-I OSC 9 GATE 7 0.175V FB 3 EA gm 1.230V ITH 2 INTVCC 8 5.2V LDO 1.230V SLOPE 1.230V UV TO START-UP CONTROL GND BIAS VREF 6 18711 BD 2.00V + – + + + – – – + SENSE + – 10 V-TO-I ILOOP RLOOP VIN 18711fb 7 LTC1871-1 OPERATION Main Control Loop The LTC1871-1 is a constant frequency, current mode controller for DC/DC boost, SEPIC and flyback converter applications. The LTC1871-1 is distinguished from conventional current mode controllers because the current control loop can be closed by sensing the voltage drop across the power MOSFET switch instead of across a discrete sense resistor, as shown in Figure 2. This sensing technique improves efficiency, increases power density, and reduces the cost of the overall solution. L VIN VIN SENSE VSW GATE GND GND D VOUT which causes the current comparator C1 to trip at a higher peak inductor current value. The average inductor current will therefore rise until it equals the load current, thereby maintaining output regulation. The nominal operating frequency of the LTC1871-1 is programmed using a resistor from the FREQ pin to ground and can be controlled over a 50kHz to 1000kHz range. In addition, the internal oscillator can be synchronized to an external clock applied to the MODE/SYNC pin and can be locked to a frequency between 100% and 130% of its nominal value. When the MODE/SYNC pin is left open, it is pulled low by an internal 50k resistor and Burst Mode operation is enabled. If this pin is taken above 2V or an external clock is applied, Burst Mode operation is disabled and the IC operates in continuous mode. With no load (or an extremely light load), the controller will skip pulses in order to maintain regulation and prevent excessive output ripple. The RUN pin controls whether the IC is enabled or is in a low current shutdown state. A micropower 1.248V reference and comparator C2 allow the user to program the supply voltage at which the IC turns on and off (comparator C2 has 100mV of hysteresis for noise immunity). With the RUN pin below 1.248V, the chip is off and the input supply current is typically only 10μA. An overvoltage comparator OV senses when the FB pin exceeds the reference voltage by 6.5% and provides a reset pulse to the main RS latch. Because this RS latch is reset-dominant, the power MOSFET is actively held off for the duration of an output overvoltage condition. The LTC1871-1 can be used either by sensing the voltage drop across the power MOSFET or by connecting the SENSE pin to a conventional shunt resistor in the source of the power MOSFET, as shown in Figure 2. Sensing the voltage across the power MOSFET maximizes converter efficiency and minimizes the component count, but limits the output voltage to the maximum rating for this pin (36V). By connecting the SENSE pin to a resistor in the source of the power MOSFET, the user is able to program output voltages significantly greater than 36V. + COUT 2a. SENSE Pin Connection for Maximum Efficiency (VSW < 36V) L VIN VIN GATE VSW D VOUT + SENSE GND GND COUT RS 18711 F02 2b. SENSE Pin Connection for Precise Control of Peak Current or for VSW > 36V Figure 2. Using the SENSE Pin On the LTC1871-1 For circuit operation, please refer to the Block Diagram of the IC and Figure 1. In normal operation, the power MOSFET is turned on when the oscillator sets the PWM latch and is turned off when the current comparator C1 resets the latch. The divided-down output voltage is compared to an internal 1.230V reference by the error amplifier EA, which outputs an error signal at the ITH pin. The voltage on the ITH pin sets the current comparator C1 input threshold. When the load current increases, a fall in the FB voltage relative to the reference voltage causes the ITH pin to rise, 18711fb 8 LTC1871-1 OPERATION Programming the Operating Mode For applications where maximizing the efficiency at very light loads (e.g., 1μF) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with CO, causing a nearly instantaneous drop in VO. No regulator can deliver enough current to prevent this problem if the load switch resistance is low and it is driven quickly. The only solution is to limit the rise time of the switch drive in order to limit the inrush current di/dt to the load. Boost Converter Design Example The design example given here will be for the circuit shown in Figure 1. The input voltage is 3.3V, and the output is 5V at a maximum load current of 7A (10A peak). 1. The duty cycle is: D= VO + VD – VIN 5 + 0.4 – 3.3 = = 38.9% 5 + 0.4 VO + VD 2. Pulse-skip operation is chosen so the MODE/SYNC pin is shorted to INTVCC. 3. The operating frequency is chosen to be 300kHz to reduce the size of the inductor. From Figure 5, the resistor from the FREQ pin to ground is 80k. 4. An inductor ripple current of 40% of the maximum load current is chosen, so the peak input current (which is also the minimum saturation current) is: IO(MAX) 7 IIN(PEAK) = 1+ = 1.2 • = 13.8A • 2 1– DMAX 1– 0. 39 18711fb 20 LTC1871-1 APPLICATIONS INFORMATION The inductor ripple current is: IO(MAX) 7 IL = • = 0.4 • = 4.6A 1– DMAX 1– 0.39 And so the inductor value is: VIN(MIN) 3.3V L= • DMAX = • 0.39 = 0.93μH IL • f 4.6A • 300kHz The component chosen is a 1μH inductor made by Sumida (part number CEP125-H 1ROMH) which has a saturation current of greater than 20A. 5. With the input voltage to the IC bootstrapped to the output of the power supply (5V), a logic-level MOSFET can be used. Because the duty cycle is 39%, the maximum SENSE pin threshold voltage is reduced from its low duty cycle typical value of 150mV to approximately 140mV. Assuming a MOSFET junction temperature of 125°C, the room temperature MOSFET RDS(ON) should be less than: 1– DMAX RDS(ON) VSENSE(MAX) • 1+ •I • 2 O(MAX) T 1– 0.39 = 0.140V • = 6.8m 0.4 1+ • 7A • 1.5 2 The MOSFET used was the Fairchild FDS7760A, which has a maximum RDS(ON) of 8mΩ at 4.5V VGS, a BVDSS of greater than 30V, and a gate charge of 37nC at 5V VGS. 6. The diode for this design must handle a maximum DC output current of 10A and be rated for a minimum reverse voltage of VOUT, or 5V. A 25A, 15V diode from On Semiconductor (MBRB2515L) was chosen for its high power dissipation capability. 7. The output capacitor usually consists of a high valued bulk C connected in parallel with a lower valued, low ESR ceramic. Based on a maximum output ripple voltage of 1%, or 50mV, the bulk C needs to be greater than: COUT IOUT(MAX) 0.01• VOUT • f = 7A = 466μF 0.01• 5V • 300kHz The RMS ripple current rating for this capacitor needs to exceed: VO – VIN(MIN) IRMS(COUT) IO(MAX) • = VIN(MIN) 7A • 5V – 3.3V = 5A 3.3V To satisfy this high RMS current demand, four 150μF Panasonic capacitors (EEFUEOJ151R) are required. In parallel with these bulk capacitors, two 22μF low , ESR (X5R) Taiyo Yuden ceramic capacitors (JMK325BJ226MM) are added for HF noise reduction. Check the output ripple with a single oscilloscope probe connected directly across the output capacitor terminals, where the HF switching currents flow. 8. The choice of an input capacitor for a boost converter depends on the impedance of the source supply and the amount of input ripple the converter will safely tolerate. For this particular design and lab setup a 100μF Sanyo Poscap (6TPC 100M), in parallel with two 22μF Taiyo Yuden ceramic capacitors (JMK325BJ226MM) is required (the input and return lead lengths are kept to a few inches, but the peak input current is close to 20A!). As with the output node, check the input ripple with a single oscilloscope probe connected across the input capacitor terminals. 18711fb 21 LTC1871-1 APPLICATIONS INFORMATION PC Board Layout Checklist 1. In order to minimize switching noise and improve output load regulation, the GND pin of the LTC1871-1 should be connected directly to 1) the negative terminal of the INTVCC decoupling capacitor, 2) the negative terminal of the output decoupling capacitors, 3) the source of the power MOSFET or the bottom terminal of the sense resistor, 4) the negative terminal of the input capacitor and 5) at least one via to the ground plane immediately adjacent to Pin 6. The ground trace on the top layer of the PC board should be as wide and short as possible to minimize series resistance and inductance. VIN L1 JUMPER R3 RC R2 R1 RT CC R4 J1 PIN 1 LTC1871-1 CIN CVCC M1 SWITCH NODE IS ALSO THE HEAT SPREADER FOR L1, M1, D1 PSEUDO-KELVIN SIGNAL GROUND CONNECTION COUT COUT D1 VIAS TO GROUND PLANE TRUE REMOTE OUTPUT SENSING VOUT BULK C LOW ESR CERAMIC 18711 F14 Figure 14. LTC1871-1 Boost Converter Suggested Layout R3 VIN R4 1 10 9 J1 L1 SENSE VIN SWITCH NODE CC RC RUN ITH LTC1871-1 2 R1 R2 RT 3 4 5 FB FREQ MODE/ SYNC INTVCC GATE GND 8 7 6 CVCC M1 D1 + CIN GND BOLD LINES INDICATE HIGH CURRENT PATHS Figure 15. LTC1871-1 Boost Converter Layout Diagram 18711fb 22 + PSEUDO-KELVIN GROUND CONNECTION COUT VOUT 18711 F15 LTC1871-1 APPLICATIONS INFORMATION 2. Beware of ground loops in multiple layer PC boards. Try to maintain one central ground node on the board and use the input capacitor to avoid excess input ripple for high output current power supplies. If the ground plane is to be used for high DC currents, choose a path away from the small-signal components. 3. Place the CVCC capacitor immediately adjacent to the INTVCC and GND pins on the IC package. This capacitor carries high di/dt MOSFET gate drive currents. A low ESR and ESL 4.7μF ceramic capacitor works well here. 4. The high di/dt loop from the bottom terminal of the output capacitor, through the power MOSFET, through the boost diode and back through the output capacitors should be kept as tight as possible to reduce inductive ringing. Excess inductance can cause increased stress on the power MOSFET and increase HF noise on the output. If low ESR ceramic capacitors are used on the output to reduce output noise, place these capacitors close to the boost diode in order to keep the series inductance to a minimum. 5. Check the stress on the power MOSFET by measuring its drain-to-source voltage directly across the device terminals (reference the ground of a single scope probe directly to the source pad on the PC board). Beware of inductive ringing which can exceed the maximum specified voltage rating of the MOSFET. If this ringing cannot be avoided and exceeds the maximum rating of the device, either choose a higher voltage device or specify an avalanche-rated power MOSFET. Not all MOSFETs are created equal (some are more equal than others). 6. Place the small-signal components away from high frequency switching nodes. In the layout shown in Figure 14, all of the small-signal components have been placed on one side of the IC and all of the power components have been placed on the other. This also allows the use of a pseudo-Kelvin connection for the signal ground, where high di/dt gate driver currents flow out of the IC ground pin in one direction (to the bottom plate of the INTVCC decoupling capacitor) and small-signal currents flow in the other direction. 7. If a sense resistor is used in the source of the power MOSFET, minimize the capacitance between the SENSE pin trace and any high frequency switching nodes. The LTC1871-1 contains an internal leading edge blanking time of approximately 180ns, which should be adequate for most applications. 8. For optimum load regulation and true remote sensing, the top of the output resistor divider should connect independently to the top of the output capacitor (Kelvin connection), staying away from any high dV/dt traces. Place the divider resistors near the LTC1871-1 in order to keep the high impedance FB node short. 9. For applications with multiple switching power converters connected to the same input supply, make sure that the input filter capacitor for the LTC1871-1 is not shared with other converters. AC input current from another converter could cause substantial input voltage ripple, and this could interfere with the operation of the LTC1871-1. A few inches of PC trace or wire (L ≈ 100nH) between the CIN of the LTC1871-1 and the actual source VIN should be sufficient to prevent current sharing problems. L1 C1 D1 VIN SW 16a. SEPIC Topology VIN VOUT VIN 16b. Current Flow During Switch On-Time VIN D1 VIN 16c. Current Flow During Switch Off-Time Figures 16. SEPIC Topology and Current Flow 18711fb + + • + + • + L2 COUT + • VOUT + • RL + • RL VOUT + • RL 23 LTC1871-1 APPLICATIONS INFORMATION SEPIC Converter Applications The LTC1871-1 is also well suited to SEPIC (single-ended primary inductance converter) converter applications. The SEPIC converter shown in Figure 16 uses two inductors. The advantage of the SEPIC converter is the input voltage may be higher or lower than the output voltage, and the output is short-circuit protected. The first inductor, L1, together with the main switch, resembles a boost converter. The second inductor, L2, together with the output diode D1, resembles a flyback or buck-boost converter. The two inductors L1 and L2 can be IIN SW ON SW OFF independent but can also be wound on the same core since identical voltages are applied to L1 and L2 throughout the switching cycle. By making L1 = L2 and winding them on the same core the input ripple is reduced along with cost and size. All of the SEPIC applications information that follows assumes L1 = L2 = L. SEPIC Converter: Duty Cycle Considerations For a SEPIC converter operating in a continuous conduction mode (CCM), the duty cycle of the main switch is: D= VO + VD VIN + VO + VD IL1 17a. Input Inductor Current IL2 IO where VD is the forward voltage of the diode. For converters where the input voltage is close to the output voltage the duty cycle is near 50%. The maximum output voltage for a SEPIC converter is: VO(MAX) = ( VIN + VD ) DMAX 1 – VD 1– DMAX 1– DMAX 17b. Output Inductor Current IIN IC1 IO The maximum duty cycle of the LTC1871-1 is typically 92%. SEPIC Converter: The Peak and Average Input Currents The control circuit in the LTC1871-1 is measuring the input current (either using the RDS(ON) of the power MOSFET or by means of a sense resistor in the MOSFET source), so the output current needs to be reflected back to the input in order to dimension the power MOSFET properly. Based on the fact that, ideally, the output power is equal to the input power, the maximum input current for a SEPIC converter is: D IIN(MAX) = IO(MAX) • MAX 1– DMAX The peak input current is: DMAX 1– DMAX 17c. DC Coupling Capacitor Current ID1 IO 17d. Diode Current VOUT (AC) ΔVCOUT ΔVESR RINGING DUE TO TOTAL INDUCTANCE (BOARD + CAP) 17e. Output Ripple Voltage Figure 17. SEPIC Converter Switching Waveforms IIN(PEAK) = 1+ 2 • IO(MAX) • The maximum duty cycle, DMAX, should be calculated at minimum VIN. 18711fb 24 LTC1871-1 APPLICATIONS INFORMATION The constant ‘χ’ represents the fraction of ripple current in the inductor relative to its maximum value. For example, if 30% ripple current is chosen, then χ = 0.30 and the peak current is 15% greater than the average. It is worth noting here that SEPIC converters that operate at high duty cycles (i.e., that develop a high output voltage from a low input voltage) can have very high input currents, relative to the output current. Be sure to check that the maximum load current will not overload the input supply. SEPIC Converter: Inductor Selection For most SEPIC applications the equal inductor values will fall in the range of 10μH to 100μH. Higher values will reduce the input ripple voltage and reduce the core loss. Lower inductor values are chosen to reduce physical size and improve transient response. Like the boost converter, the input current of the SEPIC converter is calculated at full load current and minimum input voltage. The peak inductor current can be significantly higher than the output current, especially with smaller inductors and lighter loads. The following formulas assume CCM operation and calculate the maximum peak inductor currents at minimum VIN: IL1(PEAK) = 1+ IL2(PEAK) = 1+ 2 2 • IO(MAX) • • IO(MAX) • VO + VD VIN(MIN) VIN(MIN) + VD VIN(MIN) By making L1 = L2 and winding them on the same core, the value of inductance in the equation above is replace by 2L due to mutual inductance. Doing this maintains the same ripple current and energy storage in the inductors. For example, a Coiltronix CTX10-4 is a 10μH inductor with two windings. With the windings in parallel, 10μH inductance is obtained with a current rating of 4A (the number of turns hasn’t changed, but the wire diameter has doubled). Splitting the two windings creates two 10μH inductors with a current rating of 2A each. Therefore, substituting 2L yields the following equation for coupled inductors: VIN(MIN) L1= L2 = •D 2 • IL • f MAX Specify the maximum inductor current to safely handle IL(PK) specified in the equation above. The saturation current rating for the inductor should be checked at the minimum input voltage (which results in the highest inductor current) and maximum output current. SEPIC Converter: Power MOSFET Selection The power MOSFET serves two purposes in the LTC1871-1: it represents the main switching element in the power path, and its RDS(ON) represents the current sensing element for the control loop. Important parameters for the power MOSFET include the drain-to-source breakdown voltage (BVDSS), the threshold voltage (VGS(TH)), the on-resistance (RDS(ON)) versus gate-to-source voltage, the gate-to-source and gate-to-drain charges (QGS and QGD, respectively), the maximum drain current (ID(MAX)) and the MOSFET’s thermal resistances (RTH(JC) and RTH(JA)). The gate drive voltage is set by the 5.2V INTVCC low dropout regulator. Consequently, logic-level threshold MOSFETs should be used in most LTC1871-1 applications. If low input voltage operation is expected (e.g., supplying power from a lithium-ion battery), then sublogic-level threshold MOSFETs should be used. The maximum voltage that the MOSFET switch must sustain during the off-time in a SEPIC converter is equal to the sum of the input and output voltages (VO + VIN). As a result, careful attention must be paid to the BVDSS specifications for the MOSFETs relative to the maximum actual switch voltage in the application. Many logic-level 18711fb The ripple current in the inductor is typically 20% to 40% (i.e., a range of ‘χ’ from 0.20 to 0.40) of the maximum average input current occurring at VIN(MIN) and IO(MAX) and ΔIL1 = ΔIL2. Expressing this ripple current as a function of the output current results in the following equations for calculating the inductor value: VIN(MIN) • DMAX L= IL • f where: IL = • IO(MAX) • DMAX 1– DMAX 25 LTC1871-1 APPLICATIONS INFORMATION devices are limited to 30V or less. Check the switching waveforms directly across the drain and source terminals of the power MOSFET to ensure the VDS remains below the maximum rating for the device. During the MOSFET’s on-time, the control circuit limits the maximum voltage drop across the power MOSFET to about 150mV (at low duty cycle). The peak inductor current is therefore limited to 150mV/RDS(ON). The relationship between the maximum load current, duty cycle and the RDS(ON) of the power MOSFET is: RDS(ON) VSENSE(MAX) IO(MAX ) • 1+ 1 2 • T that the converter is capable of delivering the required load current over all operating conditions (load, line and temperature) and for the worst-case specifications for VSENSE(MAX) and the RDS(ON) of the MOSFET listed in the manufacturer’s data sheet. The power dissipated by the MOSFET in a SEPIC converter is: D PFET = IO(MAX) • MAX 1– DMAX + k • VIN(MIN) + VO 2 • RDS(ON) • DMAX • T • 1 VO + VD +1 VIN(MIN) ( D )1.85 •IO(MAX) • 1– MAX X • CRSS • f DMA The VSENSE(MAX) term is typically 150mV at low duty cycle and is reduced to about 100mV at a duty cycle of 92% due to slope compensation, as shown in Figure 8. The constant ‘χ’ in the denominator represents the ripple current in the inductors relative to their maximum current. For example, if 30% ripple current is chosen, then χ = 0.30. The ρT term accounts for the temperature coefficient of the RDS(ON) of the MOSFET, which is typically 0.4%/°C. Figure 9 illustrates the variation of normalized RDS(ON) over temperature for a typical power MOSFET. Another method of choosing which power MOSFET to use is to check what the maximum output current is for a given RDS(ON) since MOSFET on-resistances are available in discrete values. IO(MAX) VSENSE(MAX) RDS(ON) • 1+ 1 2 • T The first term in the equation above represents the I2R losses in the device and the second term, the switching losses. The constant k = 1.7 is an empirical factor inversely related to the gate drive current and has the dimension of 1/current. From a known power dissipated in the power MOSFET, its junction temperature can be obtained using the following formula: TJ = TA + PFET •RTH(JA) The RTH(JA) to be used in this equation normally includes the RTH(JC) for the device plus the thermal resistance from the board to the ambient temperature in the enclosure. This value of TJ can then be used to check the original assumption for the junction temperature in the iterative calculation process. SEPIC Converter: Output Diode Selection To maximize efficiency, a fast-switching diode with low forward drop and low reverse leakage is desired. The output diode in a SEPIC converter conducts current during the switch off-time. The peak reverse voltage that the diode must withstand is equal to VIN(MAX) + VO. The average forward current in normal operation is equal to the output current, and the peak current is equal to: ID(PEAK) = 1+ 2 • IO(MAX) • VO + VD +1 VIN(MIN) • 1 VO + VD +1 VIN(MIN) Calculating Power MOSFET Switching and Conduction Losses and Junction Temperatures In order to calculate the junction temperature of the power MOSFET, the power dissipated by the device must be known. This power dissipation is a function of the duty cycle, the load current and the junction temperature itself. As a result, some iterative calculation is normally required to determine a reasonably accurate value. Since the controller is using the MOSFET as both a switching and a sensing element, care should be taken to ensure The power dissipated by the diode is: PD = IO(MAX) • VD 18711fb 26 LTC1871-1 APPLICATIONS INFORMATION and the diode junction temperature is: TJ = TA + PD • RTH(JA) The RTH(JA) to be used in this equation normally includes the RTH(JC) for the device plus the thermal resistance from the board to the ambient temperature in the enclosure. SEPIC Converter: Output Capacitor Selection Because of the improved performance of today’s electrolytic, tantalum and ceramic capacitors, engineers need to consider the contributions of ESR (equivalent series resistance), ESL (equivalent series inductance) and the bulk capacitance when choosing the correct component for a given output ripple voltage. The effects of these three parameters (ESR, ESL, and bulk C) on the output voltage ripple waveform are illustrated in Figure 17 for a typical coupled-inductor SEPIC converter. The choice of component(s) begins with the maximum acceptable ripple voltage (expressed as a percentage of the output voltage), and how this ripple should be divided between the ESR step and the charging/discharging ΔV. For the purpose of simplicity we will choose 2% for the maximum output ripple, to be divided equally between the ESR step and the charging/discharging ΔV. This percentage ripple will change, depending on the requirements of the application, and the equations provided below can easily be modified. For a 1% contribution to the total ripple voltage, the ESR of the output capacitor can be determined using the following equation: ESRCOUT where: ID(PEAK) = 1+ 2 • IO(MAX) • VO + VD +1 VIN(MIN) 0.01• VO IIN(PEAK ) For many designs it is possible to choose a single capacitor type that satisfies both the ESR and bulk C requirements for the design. In certain demanding applications, however, the ripple voltage can be improved significantly by connecting two or more types of capacitors in parallel. For example, using a low ESR ceramic capacitor can minimize the ESR step, while an electrolytic or tantalum capacitor can be used to supply the required bulk C. Once the output capacitor ESR and bulk capacitance have been determined, the overall ripple voltage waveform should be verified on a dedicated PC board (see Board Layout section for more information on component placement). Lab breadboards generally suffer from excessive series inductance (due to inter-component wiring), and these parasitics can make the switching waveforms look significantly worse than they would be on a properly designed PC board. The output capacitor in a SEPIC regulator experiences high RMS ripple currents, as shown in Figure 17. The RMS output capacitor ripple current is: IRMS(C1) = IO(MAX) • VO + VD VIN(MIN) Note that the ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life. This makes it advisable to further derate the capacitor or to choose a capacitor rated at a higher temperature than required. Several capacitors may also be placed in parallel to meet size or height requirements in the design. Manufacturers such as Nichicon, United Chemicon and Sanyo should be considered for high performance throughhole capacitors. The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest product of ESR and size of any aluminum electrolytic, at a somewhat higher price. In surface mount applications, multiple capacitors may have to be placed in parallel in order to meet the ESR or RMS current handling requirements of the application. Aluminum electrolytic and dry tantalum capacitors are both available in surface mount packages. In the case of tantalum, it is critical that the capacitors have been surge tested for use in switching power supplies. An excellent 18711fb For the bulk C component, which also contributes 1% to the total ripple: COUT IO(MAX) 0.01• VO • f 27 LTC1871-1 APPLICATIONS INFORMATION choice is AVX TPS series of surface mount tantalum. Also, ceramic capacitors are now available with extremely low ESR, ESL and high ripple current ratings. SEPIC Converter: Input Capacitor Selection The input capacitor of a SEPIC converter is less critical than the output capacitor due to the fact that an inductor is in series with the input and the input current waveform is triangular in shape. The input voltage source impedance determines the size of the input capacitor which is typically in the range of 10μF to 100μF A low ESR capacitor . is recommended, although it is not as critical as for the output capacitor. The RMS input capacitor ripple current for a SEPIC converter is: 1 • IL IRMS(CIN) = 12 Please note that the input capacitor can see a very high surge current when a battery is suddenly connected to the input of the converter and solid tantalum capacitors can fail catastrophically under these conditions. Be sure to specify surge-tested capacitors! SEPIC Converter: Selecting the DC Coupling Capacitor The coupling capacitor C1 in Figure 16 sees nearly a rectangular current waveform as shown in Figure 17. During the switch off-time the current through C1 is IO(VO/VIN) while approximately – IO flows during the on-time. This current waveform creates a triangular ripple voltage on C1: VC1(P P) = which is typically close to VIN(MAX). The ripple current through C1 is: IRMS(C1) = IO(MAX) • VO + VD VIN(MIN) The value chosen for the DC coupling capacitor normally starts with the minimum value that will satisfy 1) the RMS current requirement and 2) the peak voltage requirement (typically close to VIN). Low ESR ceramic and tantalum capacitors work well here. SEPIC Converter Design Example The design example given here will be for the circuit shown in Figure 18. The input voltage is 5V to 15V and the output is 12V at a maximum load current of 1.5A (2A peak). 1. The duty cycle range is: D= VO + VD = 45.5% to 71.4% VIN + VO + VD 2. The operating mode chosen is pulse skipping, so the MODE/SYNC pin is shorted to INTVCC. 3. The operating frequency is chosen to be 300kHz to reduce the size of the inductors; the resistor from the FREQ pin to ground is 80k. 4. An inductor ripple current of 40% is chosen, so the peak input current (which is also the minimum saturation current) is: IL1(PEAK) = 1+ = 1+ 2 • IO(MAX) • VO + VD VIN(MIN) IO(MAX) C1• f • VO VIN + VO + VD The maximum voltage on C1 is then: VC1(MAX) = VIN + VC1(P 2 P) 0.4 12 + 0.5 • 1.5 • = 4.5A 2 5 The inductor ripple current is: IL = • IO(MAX) • DMAX 1– DMAX 0.714 = 0.4 • 1.5 • = 1.5A 1– 0.714 18711fb 28 LTC1871-1 APPLICATIONS INFORMATION And so the inductor value is: VIN(MIN) 5 L= • DMAX = • 0.714 = 4μH 2 • IL • f 2 • 1.5 • 300k The component chosen is a BH Electronics BH5101007, which has a saturation current of 8A. 5. With an minimum input voltage of 5V, only logic-level power MOSFETs should be considered. Because the maximum duty cycle is 71.4%, the maximum SENSE pin threshold voltage is reduced from its low duty cycle typical value of 150mV to approximately 120mV. Assuming a MOSFET junction temperature of 125°C, the room temperature MOSFET RDS(ON) should be less than: R3 1M 1 2 RC 33k CC1 R1 6.8nF 12.1k 1% R2 105k 1% 10 9 RDS(ON) VSENSE(MAX) IO(MAX ) • 1+ 1 2 • T • 1 VO + VD +1 VIN(MIN) = 1 1 0.12 = 12.7m • • 1.5 1.2 • 1.5 12.5 +1 5 For a SEPIC converter, the switch BVDSS rating must be greater than VIN(MAX) + VO, or 27V. This comes close to an IRF7811W, which is rated to 30V, and has a maximum room temperature RDS(ON) of 12mΩ at VGS = 4.5V. • L1* SENSE VIN LTC1871-1 CDC 10μF 25V X5R VIN 4.5V to 15V RUN ITH D1 + 8 7 6 CVCC 4.7μF X5R M1 L2* 3 4 RT 80.6k 1% 5 FB FREQ MODE/SYNC INTVCC GATE GND COUT1 47μF 20V ×2 VOUT 12V 1.5A (2A PEAK) COUT2 10μF 25V X5R ×2 GND 18711 F018a CC2 47pF + CIN 47μF • CIN, COUT1: KEMET T495X476K020AS CDC, COUT2: TAIYO YUDEN TMK432BJ106MM D1: INTERNATIONAL RECTIFIER 30BQ040 L1, L2: BH ELECTRONICS BH510-1007 (*COUPLED INDUCTORS) M1: INTERNATIONAL RECTIFIER IRF7811W Figure 18a. 4.5V to 15V Input, 12V/2A Output SEPIC Converter 100 95 90 85 EFFICIENCY (%) 80 75 70 65 60 55 50 45 0.001 VO = 12V MODE = INTVCC 0.01 0.1 1 OUTPUT CURRENT (A) 10 18711 F18b VIN = 12V VIN = 4.5V VIN = 15V Figure 18b. SEPIC Efficiency vs Output Current 18711fb 29 LTC1871-1 APPLICATIONS INFORMATION VIN = 4.5V VOUT = 12V VOUT (AC) 200mV/DIV VOUT (AC) 200mV/DIV VIN = 15V VOUT = 12V IOUT 0.5A/DIV IOUT 0.5A/DIV 50μs/DIV 50μs/DIV 18711 F19 Figure 19. LTC1871-1 SEPIC Converter Load Step Response 6. The diode for this design must handle a maximum DC output current of 2A and be rated for a minimum reverse voltage of VIN + VOUT, or 27V. A 3A, 40V diode from International Rectifier (30BQ040) is chosen for its small size, relatively low forward drop and acceptable reverse leakage at high temp. 7. The output capacitor usually consists of a high valued bulk C connected in parallel with a lower valued, low ESR ceramic. Based on a maximum output ripple voltage of 1%, or 120mV, the bulk C needs to be greater than: IOUT(MAX) COUT = 0.01• VOUT • f 1.5A = 41μF 0.01• 12V • 300kHz The RMS ripple current rating for this capacitor needs to exceed: VO IRMS(COUT) IO(MAX) • = VIN(MIN) 1.5A • 12V = 2.3A 5V with a single oscilloscope probe connected directly across the output capacitor terminals, where the HF switching currents flow. 8. The choice of an input capacitor for a SEPIC converter depends on the impedance of the source supply and the amount of input ripple the converter will safely tolerate. For this particular design and lab setup, a single 47μF Kemet tantalum capacitor (T495X476K020AS) is adequate. As with the output node, check the input ripple with a single oscilloscope probe connected across the input capacitor terminals. If any HF switching noise is observed it is a good idea to decouple the input with a low ESR, X5R ceramic capacitor as close to the VIN and GND pins as possible. 9. The DC coupling capacitor in a SEPIC converter is chosen based on its RMS current requirement and must be rated for a minimum voltage of VIN plus the AC ripple voltage. Start with the minimum value which satisfies the RMS current requirement and then check the ripple voltage to ensure that it doesn’t exceed the DC rating. IRMS(CI) IO(MAX) • = 1.5A • VO + VD VIN(MIN) 12V + 0.5V = 2.4A 5V To satisfy this high RMS current demand, two 47μF Kemet capacitors (T495X476K020AS) are required. As a result, the output ripple voltage is a low 50mV to 60mV. In parallel with these tantalums, two 10μF low ESR (X5R) , Taiyo Yuden ceramic capacitors (TMK432BJ106MM) are added for HF noise reduction. Check the output ripple For this design a single 10μF low ESR (X5R) Taiyo , Yuden ceramic capacitor (TMK432BJ106MM) is adequate. 18711fb 30 LTC1871-1 TYPICAL APPLICATIONS 2.5V to 3.3V Input, 5V/2A Output Boost Converter VIN 2.5V to 3.3V L1 1.8μH D1 1 2 RC 22k CC1 R1 6.8nF 12.1k 1% R2 37.4k 1% RUN ITH LTC1871-1 3 4 RT 80.6k 1% 5 FB FREQ MODE/SYNC INTVCC GATE GND 8 7 6 CVCC 4.7μF X5R M1 SENSE VIN 10 9 VOUT 5V 2A COUT2 10μF 6.3V X5R ×2 GND 18711 TA01a + COUT1 150μF 6.3V ×2 CC2 47pF + CIN 47μF 6.3V CIN: COUT1: COUT2: CVCC: SANYO POSCAP 6TPA47M SANYO POSCAP 6TPB150M TAIYO YUDEN JMK316BJ106ML TAIYO YUDEN LMK316BJ475ML D1: INTERNATIONAL RECTIFIER 30BQ015 L1: TOKO DS104C2 B952AS-1R8N M1: SILICONIX/VISHAY Si9426 Output Efficiency at 2.5V and 3.3V Input 100 95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 50 0.001 0.01 0.1 1 OUTPUT CURRENT (A) 10 18711 TA01b 18711fb 31 LTC1871-1 TYPICAL APPLICATIONS 18V to 27V Input, 28V Output, 400W 2-Phase, Low Ripple, Synchronized RF Base Station Power Supply (Boost) VIN 18V to 27V R2 8.45k 1% R1 93.1k 1% 1 2 CC1 47pF RUN ITH LTC1871-1 3 4 CFB1 47pF RT1 150k 5% 5 FB FREQ MODE/SYNC INTVCC GATE GND 8 7 6 CVCC1 4.7μF X5R CIN2 2.2μF 35V X5R M1 RS1 0.007Ω 1W COUT5* 330μF 50V ×4 SENSE VIN 10 9 COUT1 2.2μF 35V X5R ×3 L1 5.6μH L2 5.6μH D1 + CIN1 330μF 50V + COUT2 330μF 50V GND EXT CLOCK INPUT (200kHz) 1 2 RC 22k 10 9 L3 5.6μH L4 5.6μH D2 RUN ITH SENSE VIN LTC1871-1 CC2 47pF 3 CFB2 47pF R3 12.1k 1% 4 RT2 150k 5% 5 FB FREQ MODE/SYNC INTVCC GATE GND 8 7 6 CVCC2 4.7μF X5R CIN3 2.2μF 35V X5R M2 RS2 0.007Ω 1W R4 CC3 261k 6.8nF 1% COUT3 2.2μF + 35V X5R ×3 L5* 0.3μH COUT4 330μF 50V CIN1: CIN2, 3: COUT2, 4, 5: COUT1, 3, 6: CVCC1, 2: SANYO 50MV330AX TAIYO YUDEN GMK325BJ225MN SANYO 50MV330AX TAIYO YUDEN GMK325BJ225MN TAIYO YUDEN LMK316BJ475ML L1 TO L4: L5: D1, D2: M1, M2: SUMIDA CEP125-5R6MC-HD SUMIDA CEP125-0R3NC-ND ON SEMICONDUCTOR MBR2045CT INTERNATIONAL RECTIFIER IRLZ44NS 5V to 12V Input, ±12V/0.2A Output SEPIC Converter with Undervoltage Lockout • L1* 1 2 RC 22k CC1 R4 6.8nF 127Ω 1% R3 1.10k 1% RUN ITH LTC1871-1 3 4 RT 60.4k 1% 5 FB FREQ MODE/SYNC INTVCC GATE GND 8 7 6 CVCC 4.7μF 10V X5R CIN1 1μF 16V X5R M1 L2* SENSE VIN 10 9 COUT1 4.7μF 16V X5R ×3 CDC1 4.7μF 16V X5R VIN 5V to 12V R2 54.9k 1% R1 127k 1% D1 CC2 100pF + CIN2 47μF 16V AVX • RS 0.02Ω CDC2 4.7μF 16V X5R NOTE: 1. VIN UVLO+ = 4.47V VIN UVLO– = 4.14V D1, D2: MBS120T3 L1 TO L3: COILTRONICS VP1-0076 (*COUPLED INDUCTORS) M1: SILICONIX/VISHAY Si4840 D2 L3* • GND COUT2 4.7μF 16V X5R VOUT2 ×3 –12V 18711 TA03 0.4A 32 + 18711 TA04 COUT6* 2.2μF 35V X5R VOUT 28V 14A *L5, COUT5 AND COUT6 ARE AN OPTIONAL SECONDARY FILTER TO REDUCE OUTPUT RIPPLE FROM