LTC3864 60V Low IQ Step-Down DC/DC Controller with 100% Duty Cycle Capability Description Features n n n n n n n n n n n n n n n n Wide Operating VIN Range: 3.5V to 60V Wide VOUT Range: 0.8V to VIN Low Operating IQ = 40µA Very Low Dropout Operation: 100% Duty Cycle Strong High Voltage MOSFET Gate Driver Constant Frequency Current Mode Architecture Verified FMEA for Adjacent Pin Open/Short Selectable High Efficiency Burst Mode® Operation or Pulse-Skipping Mode at Light Loads Programmable Fixed Frequency: 50kHz to 850kHz Phase-Lockable Frequency: 75kHz to 750kHz Accurate Current Limit Programmable Soft-Start or Voltage Tracking Internal Soft-Start Guarantees Smooth Start-Up Power Good Output Voltage Monitor Low Shutdown IQ = 7µA Available in Small 12-Pin Thermally Enhanced MSOP and DFN Packages The LTC®3864 is a robust, high voltage step-down DC/DC controller optimized for automotive and industrial applications. It drives a P-channel power MOSFET switch allowing 100% duty cycle operation. The wide input and output voltage ranges cover a multitude of applications. This device has been verified with the failure mode and effects analysis (FMEA) procedure for operation during failure conditions. The LTC3864 offers excellent light load efficiency, drawing only 40µA quiescent current in a user programmable Burst Mode operation. Its peak current mode, constant frequency PWM architecture provides for good control of switching frequency and output current limit. The switching frequency can be programmed from 50kHz to 850kHz with an external resistor and can be synchronized to an external clock from 75kHz to 750kHz. The LTC3864 offers programmable soft-start or output tracking. Safety features include overvoltage protection, overcurrent and short-circuit protection including frequency foldback and a power good output signal. Applications n n n The LTC3864 is available in thermally enhanced 12-Pin MSOP and 3mm × 4mm DFN packages. Industrial and Automotive Power Supplies Telecom Power Supplies Distributed Power Systems L, LT, LTC, LTM, OPTI-LOOP, Linear Technology, Burst Mode and the Linear logo are registered trademarks and Hot Swap is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents including 5731694. Typical Application 5.2V to 60V Input, 5V/2A Output, 350kHz Step-Down Converter 10µF 0.47µF RUN 3.3nF 9.09k 90 VIN 25mΩ PLLIN/MODE SS 100 SENSE LTC3864 GATE ITH FREQ 10µH VOUT* 5V 2A 100k 422k SGND 47µF ×2 PGOOD PGND *VOUT FOLLOWS VIN WHEN 3.5V ≤ VIN ≤ 5.2V EFFICIENCY (%) 350kHz CAP Efficiency VIN* 5.2V TO 60V 80 PULSE-SKIPPING 70 60 50 0.01 VFB 80.6k Burst Mode OPERATION VIN = 12V VOUT = 5V 0.1 LOAD CURRENT (A) 1 3864 TA01b 3864 TA01a 3864fa For more information www.linear.com/LTC3864 1 LTC3864 Absolute Maximum Ratings (Note 1) Input Supply Voltage (VIN).......................... –0.3V to 65V VIN-VSENSE Voltage....................................... –0.3V to 6V VIN-VCAP Voltage......................................... –0.3V to 10V RUN Voltage............................................... –0.3V to 65V PGOOD, PLLIN/MODE Voltages.................... –0.3V to 6V SS, ITH, FREQ, VFB Voltages......................... –0.3V to 5V Operating Junction Temperature Range (Notes 2, 3, 4) LTC3864E,I ........................................ –40°C to 125°C LTC3864H........................................... –40°C to 150°C LTC3864MP........................................ –55°C to 150°C Storage Temperature Range................... –65°C to 150°C Lead Temperature (Soldering, 10 sec) MSOP Package.................................................. 300°C Pin Configuration TOP VIEW PLLIN/MODE 1 12 GATE TOP VIEW FREQ 2 11 VIN SGND 3 10 SENSE SS 4 VFB ITH 13 PGND 9 CAP 5 8 RUN 6 7 PGOOD DE PACKAGE 12-LEAD (4mm × 3mm) PLASTIC DFN TJMAX = 150°C, θJA = 43°C/W, θJC = 5.5°C/W EXPOSED PAD (PIN 13) IS PGND, MUST BE SOLDERED TO PCB FOR OPTIMAL THERMAL PERFORMANCE PLLIN/MODE FREQ SGND SS VFB ITH 1 2 3 4 5 6 13 PGND 12 11 10 9 8 7 GATE VIN SENSE CAP RUN PGOOD MSE PACKAGE 12-LEAD PLASTIC MSOP TJMAX = 150°C, θJA = 40°C/W, θJC = 10°C/W EXPOSED PAD (PIN 13) IS PGND, MUST BE SOLDERED TO PCB FOR OPTIMAL THERMAL PERFORMANCE Order Information LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3864EMSE#PBF LTC3864EMSE#TRPBF 3864 12-Lead Plastic MSOP –40°C to 125°C LTC3864IMSE#PBF LTC3864IMSE#TRPBF 3864 12-Lead Plastic MSOP –40°C to 125°C LTC3864HMSE#PBF LTC3864HMSE#TRPBF 3864 12-Lead Plastic MSOP –40°C to 150°C LTC3864MPMSE#PBF LTC3864MPMSE#TRPBF 3864 12-Lead Plastic MSOP –55°C to 150°C LTC3864EDE#PBF LTC3864EDE#TRPBF 3864 12-Lead (4mm × 3mm) Plastic DFN –40°C to 125°C LTC3864IDE#PBF LTC3864IDE#TRPBF 3864 12-Lead (4mm × 3mm) Plastic DFN –40°C to 125°C LTC3864HDE#PBF LTC3864HDE#TRPBF 3864 12-Lead (4mm × 3mm) Plastic DFN –40°C to 150°C LTC3864MPDE#PBF LTC3864MPDE#TRPBF 3864 12-Lead (4mm × 3mm) Plastic DFN –55°C to 150°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on non-standard lead based finish parts. 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/ 3864fa 2 For more information www.linear.com/LTC3864 LTC3864 Electrical Characteristics The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4) VIN = 12V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Input Supply VIN Input Voltage Operating Range VUVLO Undervoltage Lockout IQ Input DC Supply Current 3.5 (VIN-VCAP) Ramping Up Threshold (VIN-VCAP) Ramping Down Threshold Hysteresis l l 3.25 3.00 60 V 3.50 3.25 0.25 3.8 3.50 V V V 0.77 1.2 mA Pulse-Skipping Mode PLLIN/MODE = 0V, FREQ = 0V, VFB = 0.83V (No Load) Burst Mode Operation PLLIN/MODE = Open, FREQ = 0V, VFB = 0.83V (No Load) 40 60 µA Shutdown Supply Current RUN = 0V 7 12 µA 0.800 0.809 V 0.005 %/V Output Sensing VREG Regulated Feedback Voltage VITH = 1.2V (Note 5) ∆VREG ∆VIN Feedback Voltage Line Regulation VIN = 3.8V to 60V (Note 5) ∆VREG ∆VITH Feedback Voltage Load Regulation VITH = 0.6V to 1.8V (Note 5) gm(EA) Error Amplifier Transconductance VITH = 1.2V, ∆IITH = ±5µA (Note 5) IFB Feedback Input Bias Current l 0.792 –0.005 –0.1 –0.015 0.1 1.8 % mS –50 –10 50 nA 85 95 103 mV 0.1 2 µA 1.26 1.32 V Current Sensing VILIM Current Limit Threshold (VIN-VSENSE) VFB = 0.77V ISENSE SENSE Pin Input Current VSENSE = VIN l Start-Up and Shutdown VRUN RUN Pin Enable Threshold VRUNHYS RUN Pin Hysteresis ISS Soft-Start Pin Charging Current VRUN Rising l 1.22 VSS = 0V 150 mV 10 µA Switching Frequency and Clock Synchronization f Programmable Switching Frequency RFREQ = 24.9kΩ RFREQ = 64.9kΩ RFREQ = 105kΩ 375 105 440 810 505 kHz kHz kHz fLO Low Switching Frequency FREQ = 0V 320 350 380 kHz fHI High Switching Frequency FREQ = Open 485 535 585 kHz 750 kHz fSYNC Synchronization Frequency l VCLK(IH) Clock Input High Level into PLLIN/MODE l VCLK(LO) Clock Input Low Level into PLLIN/MODE l fFOLD Foldback Frequency as Percentage of Programmable Frequency tON(MIN) Minimum On-Time 75 2 V 0.5 VFB = 0V, FREQ = 0V V 18 % 220 ns Gate Driver VCAP Gate Bias LDO Output Voltage (VIN-VCAP) IGATE = 0mA VCAPDROP Gate Bias LDO Dropout Voltage VIN = 5V, IGATE = 15mA ∆VCAP(LINE) Gate Bias LDO Line Regulation 9V ≤ VIN ≤ 60V, IGATE = 0mA ∆VCAP(LOAD) Gate Bias LDO Load Regulation Load = 0mA to 20mA l 7.6 –3.5 8.0 8.5 V 0.2 0.5 V 0.002 0.03 %/V % 3864fa For more information www.linear.com/LTC3864 3 LTC3864 Electrical Characteristics The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4) VIN = 12V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS RUP Gate Pull-Up Resistance Gate High 2 Ω RDN Gate Pull-Down Resistance Gate Low 0.9 Ω 0.2 PGOOD and Overvoltage VPGL PGOOD Voltage Low IPGOOD = 2mA IPG PGOOD Leakage Current VPGOOD = 5V %PGD PGOOD Trip Level VFB Ramping Negative with Respect to VREG Hysteresis –13 VFB Ramping Positive with Respect to VREG Hysteresis 7 0.4 V 1 µA –10 2.5 –7 % % 10 2.5 13 % % tPGDLY PGOOD Delay PGOOD Going High to Low PGOOD Going Low to High 100 100 µs µs VFBOV VFB Overvoltage Lockout Threshold GATE Going High without Delay, VFB(OV)-VFB(NOM) in Percent 10 % 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: Continuous operation above the specified maximum operating junction temperature may impair device reliability or permanently damage the device. Note 3: The junction temperature (TJ in °C) is calculated from the ambient temperature (TA in °C) and power dissipation (PD in Watts) as follows: TJ = TA + (PD • θJA) where θJA (in °C/W) is the package thermal impedance provided in the Pin Configuration section for the corresponding package. Note 4: The LTC3864 is tested under pulsed loading conditions such that TJ ≈ TA. The LTC3864E is guaranteed to meet performance specifications from 0°C to 85°C operating junction temperature range. The LTC3864E specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LTC3864I is guaranteed to meet performance specifications over the –40°C to 125°C operating junction temperature range, the LTC3864H is guaranteed over the –40°C to 150°C operating junction temperature range, and the LTC3864MP is guaranteed and tested over the full –55°C to 150°C operating junction temperature range. High junction temperatures degrade operating lifetimes; operating lifetime is derated for junction temperatures greater than 125°C. The maximum ambient temperature consistent with these specifications is determined by specific operating conditions in conjunction with board layout, the rated package thermal impedance and other environmental factors. Note 5: The LTC3864 is tested in a feedback loop that adjust VFB to achieve a specified error amplifier output voltage (on ITH pin). 3864fa 4 For more information www.linear.com/LTC3864 LTC3864 Typical Performance Characteristics Pulse-Skipping Mode Operation Waveforms TA = 25°C, unless otherwise noted. Burst Mode Operation Waveforms VOUT 50mV/DIV VOUT 50mV/DIV VSW 10V/DIV VSW 10V/DIV Transient Response: Pulse-Skipping Mode Operation ILOAD 2A/DIV VOUT 500mV/DIV IL 500mA/DIV IL 500mA/DIV IL 2A/DIV 2µs/DIV VIN = 12V VOUT = 5V ILOAD = 100mA FIGURE 8 CIRCUIT 10µs/DIV 100µs/DIV VIN = 12V VOUT = 5V TRANSIENT = 100mA TO 2A FIGURE 8 CIRCUIT 3864 G02 VIN = 12V VOUT = 5V ILOAD = 100mA FIGURE 8 CIRCUIT 3864 G01 Transient Response: Burst Mode Operation Dropout Behavior (100% Duty Cycle) 3864 G03 Low VIN Operation VIN 2V/DIV ILOAD 2A/DIV VOUT 500mV/DIV VIN 1V/DIV VOUT 2V/DIV GATE 10V/DIV IL 2A/DIV 100µs/DIV VIN = 12V VOUT = 5V TRANSIENT = 100mA TO 2A FIGURE 8 CIRCUIT VOUT 1V/DIV VOUT = VIN IN DROPOUT SW 5V/DIV 3864 G05 50ms/DIV VIN TRANSIENT: 12V TO 4V AND BACK TO 12V VOUT = 5V, ILOAD = 100mA, FIGURE 8 CIRCUIT 3864 G04 20ms/DIV 3864 G06 VIN = 0V TO 3.8V THEN BACK TO 0V ILOAD = 100mA FIGURE 8 CIRCUIT Soft Start-Up into a Prebiased Output Normal Soft Start-Up VOUT PROGRAMMED TO 5V, BUT STARTS UP IN DROPOUT SINCE VIN < 5V Output Tracking RUN 5V/DIV VIN 5V/DIV VOUT PREBIASED TO 2.9V SS 200mV/DIV VOUT 1V/DIV SS 200mV/DIV SS 200mV/DIV VOUT 1V/DIV 1ms/DIV VIN = 12V, VOUT = 5V FIGURE 8 CIRCUIT 3864 G07 VOUT 2V/DIV 1ms/DIV VIN = 12V, VOUT = 5V ILOAD = 0.5mA FIGURE 8 CIRCUIT 3864 G08 20ms/DIV VIN = 12V, VOUT = 5V ILOAD = 100mA FIGURE 8 CIRCUIT 3864 G09 3864fa For more information www.linear.com/LTC3864 5 LTC3864 Typical Performance Characteristics Overcurrent Protection SHORTCIRCUIT TRIGGER 1A 1A SHORT-CIRCUIT REGION VIN 20V/DIV VOUT 5V/DIV SOFT RECOVERY FROM SHORT IL 1A/DIV VOUT 500mV/DIV IL 2A/DIV VOUT DROOPS DUE TO REACHING CURRENT LIMIT 20ms/DIV VIN = 12V, VOUT = 5V FIGURE 8 CIRCUIT Pulse-Skipping Mode Input Current Over Input Voltage (No Load) 950 VIN = 12V, VOUT = 5V ILOAD = 0A FIGURE 8 CIRCUIT 65 2ms/DIV VIN = 12V, SURGE TO 48V VOUT = 5V ILOAD = 200mA, FIGURE 8 CIRCUIT 3864 G11 IVIN (µA) 55 50 45 40 25 VIN = 12V, VOUT = 5V ILOAD = 0A FIGURE 8 CIRCUIT 900 60 3864 G12 Shutdown Current Over Input Voltage FIGURE 8 CIRCUIT 20 850 15 IVIN (µA) 70 GATE 20V/DIV VOUT 50mV/DIV 500µs/DIV VIN = 12V, VOUT = 5V FIGURE 8 CIRCUIT 3864 G10 Burst Mode Input Current Over Input Voltage (No Load) IVIN (µA) VIN Line Transient Behavior Short-Ciruit Protection 3.2A ILOAD 1A/DIV TA = 25°C, unless otherwise noted. 800 10 750 5 35 30 0 10 20 30 VIN(V) 40 50 700 60 0 10 20 30 VIN (V) 40 50 Output Regulation Over Input Voltage 0.010 0 –0.010 1.0 VIN = 12V, VOUT = 5V ILOAD NORMALIZED AT ILOAD = 1A FIGURE 8 CIRCUIT 0.8 0.005 0 Burst Mode OPERATION PULSE-SKIPPING 0 10 20 30 VIN (V) 40 50 60 3864 G16 30 VIN (V) 40 50 –0.010 –0.5 60 0.6 VIN = 12V, VOUT = 5V ILOAD = 200mA VOUT NORMALIZED TO TA = 25°C FIGURE 8 CIRCUIT 0.4 0.2 0 –0.2 –0.4 –0.005 –0.005 20 Output Regulation Over Temperature NORMALIZED ∆VOUT (%) NORMALIZED ∆VOUT (%) NORMALIZED ∆VOUT (%) 0.005 10 3864 G15 Output Regulation Over Load Current VOUT = 5V ILOAD = 200mA VOUT NORMALIZED AT VIN = 12V FIGURE 8 CIRCUIT 0 3864 G14 3864 G13 0.010 0 60 –0.6 Burst Mode OPERATION PULSE-SKIPPING 0 0.5 1 1.5 ILOAD (A) 2 2.5 3864 G17 Burst Mode OPERATION PULSE-SKIPPING –0.8 –1.0 –75 –25 25 75 125 TEMPERATURE (°C) 175 3864 G18 3864fa 6 For more information www.linear.com/LTC3864 LTC3864 Typical Performance Characteristics Free Running Frequency Over Input Voltage Free Running Frequency Over Temperature FREQ = OPEN 550 100 500 f (kHz) 500 f (kHz) 120 FREQUENCY FOLDBACK % FREQ = OPEN 550 450 450 400 400 FREQ = 0V 350 0 20 10 30 VIN (V) FREQ = 0V 350 40 50 300 –75 60 25 75 125 TEMPERATURE (°C) –25 0.1 –0.3 –0.4 –3.0 5 10 IGATE (mA) 15 –0.5 20 0 5 10 IGATE (mA) 15 20 92 25 75 125 TEMPERATURE (°C) 70 60 50 40 30 20 10 175 3864 G25 0 0.4 0.8 1.2 ITH VOLTAGE (V) RUN Pin Pull-Up Current Over Temperature 0.65 12 10 8 6 –75 2 1.6 3864 G24 RUN PULL-UP CURRENT (µA) 94 –25 80 –10 14 SS PULL-UP CURRENT (µV) CURRENT LIMIT SENSE VOLTAGE (mV) 100 800 Burst Mode OPERATION PULSE-SKIPPING 90 SS Pin Pull-Up Current Over Temperature 96 600 3864 G23 Current Sense Voltage Over Temperature 98 400 VFB (mV) 0 3864 G22 90 –75 100 CURRENT SENSE VOLTAGE (mV) –0.2 0 200 3864 G21 VIN = 5V –0.1 –2.5 0 Current Sense Voltage Over ITH Voltage 0.0 –2.0 0 175 (VIN - VCAP) DROPOUT (V) (VIN - VCAP) REGULATION (%) 0.0 –1.5 40 GATE Bias LDO (VIN - VCAP) Dropout Behavior 0.5 –1.0 60 3864 G20 GATE Bias LDO (VIN - VCAP) Load Regulation –0.5 80 20 3864 G19 –3.5 Frequency Foldback % Over Feedback Voltage 600 600 300 TA = 25°C, unless otherwise noted. –25 25 75 125 TEMPERATURE (°C) 175 3864 G26 0.55 0.45 0.35 0.25 –75 –25 25 75 125 TEMPERATURE (°C) 175 3864 G27 3864fa For more information www.linear.com/LTC3864 7 LTC3864 Pin Functions PLLIN/MODE (Pin 1): External Reference Clock Input and Burst Mode Enable/Disable. When an external clock is applied to this pin, the internal phase-locked loop will synchronize the turn-on edge of the gate drive signal with the rising edge of the external clock. When no external clock is applied, this input determines the operation during light loading. Floating this pin selects low IQ (40μA) Burst Mode operation. Pulling to ground selects pulse-skipping mode operation. FREQ (Pin 2): Switching Frequency Set Point Input. The switching frequency is programmed by an external setpoint resistor RFREQ connected between the FREQ pin and signal ground. An internal 20µA current source creates a voltage across the external setpoint resistor to set the internal oscillator frequency. Alternatively, this pin can be driven directly by a DC voltage to set the oscillator frequency. Grounding selects a fixed operating frequency of 350kHz. Floating selects a fixed operating frequency of 535kHz. SGND (Pin 3): Ground Reference for Small Signal Analog Component (Signal Ground). Signal ground should be used as the common ground for all small signal analog inputs and compensation components. Connect signal ground to power ground (ground reference for power components) only at one point using a single PCB trace. SS (Pin 4): Soft-Start and External Tracking Input. The LTC3864 regulates the feedback voltage to the smaller of 0.8V or the voltage on the SS pin. An internal 10μA pull-up current source is connected to this pin. A capacitor to ground at this pin sets the ramp time to the final regulated output voltage. Alternatively, another voltage supply connected through a resistor divider to this pin allows the output to track the other supply during start-up. VFB (Pin 5): Output Feedback Sense. A resistor divider from the regulated output point to this pin sets the output voltage. The LTC3864 will nominally regulate VFB to the internal reference value of 0.8V. If VFB is less than 0.4V, the switching frequency will linearly decrease and fold back to about one-fifth of the internal oscillator frequency to reduce the minimum duty cycle. ITH (Pin 6): Current Control Threshold and Controller Compensation Point. This pin is the output of the error amplifier and the switching regulator’s compensation 8 point. The voltage ranges from 0V to 2.9V, with 0.8V corresponding to zero sense voltage (zero current). PGOOD (Pin 7): Power Good Indicator Output. This open drain logic output is pulled to ground when the output voltage is outside of a ±10% window around the regulation point. The PGOOD switches states only after a 100µs delay. RUN (Pin 8): Digital Run Control Input. A RUN voltage above the 1.26V threshold enables normal operation, while a voltage below the threshold shuts down the controller. An internal 0.4µA current source pulls the RUN pin up to about 3.3V. The RUN pin can be connected to an external power supply up to 60V. CAP (Pin 9): Gate Driver (–) Supply. A low ESR ceramic bypass capacitor of at least 0.47µF or 10X the effective CMILLER of the P-channel power MOSFET, is required from VIN to this pin to serve as a bypass capacitor for the internal regulator. To insure stable low noise operation, the bypass capacitor should be placed adjacent to the VIN and CAP pins and connected using the same PCB metal layer. SENSE (Pin 10): Current Sense Input. A sense resistor RSENSE from VIN pin to the SENSE pin sets the maximum current limit. The peak inductor current limit is equal to 95mV/RSENSE. For accuracy, it is important that the VIN pin and the SENSE pin route directly to the current sense resistor and make a Kelvin (4-wire) connection. VIN (Pin 11): Chip Power Supply. A minimum bypass capacitor of 0.1µF is required from the VIN pin to power ground. For best performance use a low ESR ceramic capacitor placed near the VIN pin. GATE (Pin 12): Gate Drive Output for External P-Channel MOSFET. The gate driver bias supply voltage (VIN-VCAP) is regulated to 8V when VIN is greater than 8V. The gate driver is disabled when (VIN-VCAP) is less than 3.5V (typical), 3.8V maximum in startup and 3.25V (typical) 3.5V maximum in normal operation. PGND (Exposed Pad Pin 13): Ground Reference for Power Components (Power Ground). The PGND exposed pad must be soldered to the circuit board for electrical contact and for rated thermal performance of the package. Connect signal ground to power ground only at one point using a single PCB trace. For more information www.linear.com/LTC3864 3864fa LTC3864 FUNCTIONAL Diagram VIN UVLO + – 0.4µA CIN VIN 3.25V RSENSE SENSE RUN RUN 1.26V + – – LOGIC CONTROL DRV GATE MP L Q S R CLOCK PLLIN/MODE 20µA FREQ MODE/CLOCK DETECT + PLL SYSTEM – VCO ICMP VOUT COUT IN Burst Mode OPERATION LDO OUT VIN – 8V O.425V – 10µA + + – SGND VOUT SLOPE COMPENSATION RPGD OV PGOOD + UV PGND SS 0.8V CSS EA (Gm = 1.8mS) O.88V – DELAY 100µs RFB2 VFB + – D1 + + RFREQ CCAP CAP O.72V RFB1 ITH 3864 FD RITH CITH1 3864fa For more information www.linear.com/LTC3864 9 LTC3864 Operation Main Control Loop (Refer to Functional Diagram) Shutdown and Soft-Start The LTC3864 uses a peak current-mode control architecture to regulate the output in an asynchronous step-down DC/DC switching regulator. The VFB input is compared to an internal reference by a transconductance error amplifier (EA). The internal reference can be either a fixed 0.8V reference VREF or the voltage input on the SS pin. In normal operation VFB regulates to the internal 0.8V reference voltage. In soft-start or tracking mode, when the SS pin voltage is less than the internal 0.8V reference voltage, VFB will regulate to the SS pin voltage. The error amplifier output connects to the ITH (current [I] threshold [TH]) pin. The voltage level on the ITH pin is then summed with a slope compensation ramp to create the peak inductor current set point. When the RUN pin is below 0.7V, the controller and most internal circuits are disabled. In this micropower shutdown state, the LTC3864 draws only 7µA. Releasing the RUN pin allows a small internal pull up current to pull the RUN pin above 1.26V and enable the controller. The RUN pin can be pulled up to an external supply of up to 60V or it can be driven directly by logic levels. The peak inductor current is measured through a sense resistor RSENSE placed across the VIN and SENSE pins. The resultant differential voltage from VIN to SENSE is proportional to the inductor current and is compared to the peak inductor current set point. During normal operation the P-channel power MOSFET is turned on when the clock leading edge sets the SR latch through the S input. The P-channel MOSFET is turned off through the SR latch R input when the differential voltage from VIN to SENSE is greater than the peak inductor current set point and the current comparator, ICMP, trips high. Power CAP and VIN Undervoltage Lockout (UVLO) Power for the P-channel MOSFET gate driver is derived from the CAP pin. The CAP pin is regulated to 8V below VIN in order to provide efficient P-channel operation. The power for the VCAP supply comes from an internal LDO, which regulates the VIN-CAP differential voltage. A minimum capacitance of 0.47µF (low ESR ceramic) is required between VIN and CAP to assure stability. For VIN ≤ 8V, the LDO will be in dropout and the CAP voltage will be at ground, i.e. the VIN-CAP differential voltage will equal VIN. If VIN-CAP is less than 3.25V (typical), the LTC3864 enters a UVLO state where the GATE is prevented from switching and most internal circuitry is shut down. In order to exit UVLO, the VIN-CAP voltage would have to exceed 3.5V (typical). The start-up of the output voltage VOUT is controlled by the voltage on the SS pin. When the voltage on the SS pin is less than the 0.8V internal reference, the VFB pin is regulated to the voltage on the SS pin. This allows the SS pin to be used to program a soft-start by connecting an external capacitor from the SS pin to signal ground. An internal 10µA pull-up current charges this capacitor, creating a voltage ramp on the SS pin. As the SS voltage rises from 0V to 0.8V, the output voltage VOUT rises smoothly from zero to its final value. Alternatively, the SS pin can be used to cause the startup of VOUT to track that of another supply. Typically, this requires connecting the SS pin to an external resistor divider from the other supply to ground. (See Applications Information section.) Under shutdown or UVLO, the SS pin is pulled to ground and prevented from ramping up. If the slew rate of the SS pin is greater than 1.2V/ms, the output will track an internal soft-start ramp instead of the SS pin. The internal soft-start will guarantee a smooth start-up of the output under all conditions, including in the case of a short-circuit recovery where the output voltage will recover from near ground. Light Load Current Operation (Burst Mode Operation or Pulse-Skipping Mode) The LTC3864 can be enabled to enter high efficiency Burst Mode operation or pulse-skipping mode at light loads. To select pulse-skipping operation, tie the PLLIN/MODE pin to signal ground. To select Burst Mode operation, float the PLLIN/MODE pin. In Burst Mode operation, if the VFB is higher than the reference voltage, the error amplifier will decrease the voltage on the ITH pin. When the ITH voltage drops below 0.425V, 3864fa 10 For more information www.linear.com/LTC3864 LTC3864 Operation the internal sleep signal goes high, enabling sleep mode. The ITH pin is then disconnected from the output of the error amplifier and held at 0.55V. In sleep mode, much of the internal circuitry is turned off, reducing the quiescent current to 40µA while the load current is supplied by the output capacitor. As the output voltage and hence the feedback voltage decreases, the error amplifier’s output will rise. When the output voltage drops enough, the ITH pin is reconnected to the output of the error amplifier, the sleep signal goes low, and the controller resumes normal operation by turning on the external P-MOSFET on the next cycle of the internal oscillator. In Burst Mode operation, the peak inductor current has to reach at least 25% of current limit for the current comparator, ICMP, to trip and turn the P-MOSFET back off, even though the ITH voltage may indicate a lower current setpoint value. When the PLLIN/MODE pin is connected for pulse-skipping mode, the LTC3864 will skip pulses during light loads. In this mode, ICMP may remain tripped for several cycles and force the external MOSFET to stay off, thereby skipping pulses. This mode offers the benefits of smaller output ripple, lower audible noise, and reduced RF interference, at the expense of lower efficiency when compared to Burst Mode operation. Frequency Selection and Clock Synchronization The switching frequency of the LTC3864 can be selected using the FREQ pin. If the PLLIN/MODE pin is not being driven by an external clock source, the FREQ pin can be tied to signal ground, floated, or programmed through an external resistor. Tying FREQ pin to signal ground selects 350kHz, while floating selects 535kHz. Placing a resistor between FREQ pin and signal ground allows the frequency to be programmed between 50kHz and 850kHz. The phase-locked loop (PLL) on the LTC3864 will synchronize the internal oscillator to an external clock source when connected to the PLLIN/MODE pin. The PLL forces the turn-on edge of the external P-channel MOSFET to be aligned with the rising edge of the synchronizing signal. The oscillator’s default frequency is based on the operating frequency set by the FREQ pin. If the oscillator’s default frequency is near the external clock frequency, only slight adjustments are needed for the PLL to synchronize the external P-channel MOSFET’s turn-on edge to the rising edge of the external clock. This allows the PLL to lock rapidly without deviating far from the desired frequency. The PLL is guaranteed from 75kHz to 750kHz. The clock input levels should be greater than 2V for HI and less than 0.5V for LO. Power Good and Fault Protection The PGOOD pin is an open-drain output. An internal N-channel MOSFET pulls the PGOOD pin low when the VFB pin voltage is outside a ±10% window from the 0.8V internal voltage reference. The PGOOD pin is also pulled low when the RUN pin is low (shut down). When the VFB pin voltage is within the ±10% window, the MOSFET is turned off and the pin is allowed to be pulled up by an external resistor to a source no greater than 6V. The PGOOD open-drain output has a 100µs delay before it can transition states. When the VFB voltage is above +10% of the regulated voltage of 0.8V, this is considered as an overvoltage condition and the external P-MOSFET is immediately turned off and prevented from ever turning on until VFB returns below +7.5%. In the event of an output short circuit or overcurrent condition that causes the output voltage to drop significantly while in current limit, the LTC3864 operating frequency will fold back. Anytime the output feedback VFB voltage is less than 50% of the 0.8V internal reference (i.e., 0.4V), frequency foldback is active. The frequency will continue to drop as VFB drops until reaching a minimum foldback frequency of about 18% of the setpoint frequency. Frequency foldback is designed, in combination with peak current limit, to limit current in start-up and short-circuit conditions. Setting the foldback frequency as a percentage of operating frequency assures that start-up characteristics scale appropriately with operating frequency. 3864fa For more information www.linear.com/LTC3864 11 LTC3864 Applications Information The LTC3864 is a current mode, constant frequency PWM controller for an asynchronous step-down DC/DC regulator with a P-channel power MOSFET acting as the main switch and a Schottky power diode acting as the commutating (catch) diode. The input range extends from 3.5V to 60V. The output range can be programmed from 0.8V to all the way up to VIN. The LTC3864 can transition from regulation to 100% duty cycle when the input voltage drops below the programmed output voltage. Additionally, the LTC3864 offers Burst Mode operation with 40µA quiescent current, which delivers outstanding efficiency in light load operation. The LTC3864 is a low pin count, robust and easy to use solution in applications which require high efficiency and operate with widely varying high voltage inputs. The typical application on the front page is a basic LTC3864 application circuit. The LTC3864 can sense the inductor current through a high side series sense resistor, RSENSE, placed between VIN and the source of the external PMOSFET. Once the required output voltage and operating frequency have been determined, external component selection is driven by load requirements, and begins with the selection of inductor and RSENSE. Next, the power MOSFET and catch diode are selected. Finally, input and output capacitors are selected. Output Voltage Programming The output voltage is programmed by connecting a feedback resistor divider from the output to the VFB pin as shown in Figure 1. The output voltage in steady state operation is set by the feedback resistors according to the equation: R VOUT = 0.8V • 1+ FB2 RFB1 VOUT LTC3864 RFB2 CFF VFB RFB1 3864 F01 Figure 1. Setting the Output Voltage Switching Frequency and Clock Synchronization The choice of operating frequency is a trade-off between efficiency and component size. Lowering the operating frequency improves efficiency by reducing MOSFET switching losses but requires larger inductance and/or capacitance to maintain low output ripple voltage. Conversely, raising the operating frequency degrades efficiency but reduces component size. The LTC3864 can free run at a user programmed switching frequency, or it can synchronize with an external clock to run at the clock frequency. When the LTC3864 is synchronized, the GATE pin will phase synchronize with the rising edge of the applied clock in order to turn the external P-MOSFET on. The switching frequency of the LTC3864 is programmed with the FREQ pin, and the external clock is applied at the PLLIN/MODE pin. Table 1 highlights the different states in which the FREQ pin can be used in conjunction with the PLLIN/MODE pin. Table 1 FREQ PIN PLLIN/MODE PIN FREQUENCY 0V DC Voltage 350kHz Floating DC Voltage 535kHz Resistor to GND DC Voltage 50kHz to 850kHz Any of the Above External Clock Phase Locked to External Clock To improve the transient response, a feedforward capacitor CFF may be used. Great care should be taken to route the VFB line away from noise sources, such as the inductor or the GATE signal that drives the external P-MOSFET. 3864fa 12 For more information www.linear.com/LTC3864 LTC3864 Applications Information The free-running switching frequency can be programmed from 50kHz to 850kHz by connecting a resistor from FREQ pin to signal ground. The resulting switching frequency as a function of resistance on FREQ pin is shown in Figure 2. 1000 900 FREQUENCY (kHz) 800 700 A reasonable starting point for ripple current is 40% of IOUT(MAX) at nominal VIN. The largest ripple current occurs at the highest VIN. To guarantee that the ripple current does not exceed a specified maximum, the inductance should be chosen according to: V VOUT L= 1– OUT f • ∆IL(MAX) VIN(MAX) 600 500 400 300 200 100 0 this requires a large inductor. There is a trade-off between component size, efficiency, and operating frequency. 15 25 35 45 55 65 75 85 95 105 115 125 FREQ PIN RESISTOR (kΩ) 3864 F02 Figure 2. Switching Frequency vs Resistor on FREQ Pin Set the free-running frequency to the desired synchronization frequency using the FREQ pin so that the internal oscillator is prebiased to approximately the synchronization frequency. While it is not required that the free-running frequency be near the external clock frequency, doing so will minimize synchronization time. Inductor Selection The operating frequency and inductor selection are interrelated in that higher operating frequencies allow the use of smaller inductor and capacitor values. A higher frequency generally results in lower efficiency because of MOSFET gate charge and transition losses. In addition to this basic trade-off, the effect of inductor value on ripple current and low current operation must also be considered. Given the desired input and output voltages, the inductor value and operation frequency determine the ripple current: V V ∆IL = OUT 1– OUT f •L VIN Lower ripple current reduces core losses in the inductor, ESR losses in the output capacitors and results in lower output ripple. Highest efficiency operation is obtained at low frequency with small ripple current. However, achieving Once the inductance value has been determined, the type of inductor must be selected. Core loss is independent of core size for a given inductor value, but it is very dependent on the inductance selected. As inductance increases, core losses decrease. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will increase. High efficiency converters generally cannot tolerate the core loss of low cost powdered iron cores, forcing the use of more expensive ferrite materials. Ferrite designs have very low core loss and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates hard, which means that inductance collapses abruptly when the peak design current is exceeded. This will result in an abrupt increase in inductor ripple current and output voltage ripple. Do not allow the core to saturate! A variety of inductors are available from manufacturers such as Sumida, Panasonic, Coiltronics, Coilcraft, Toko, Vishay, Pulse, and Würth. Current Sensing and Current Limit Programming The LTC3864 senses the inductor current through a current sense resistor, RSENSE, placed across the VIN and SENSE pins. The voltage across the resistor, VSENSE, is proportional to inductor current and in normal operation is compared to the peak inductor current setpoint. A current limit condition is detected when VSENSE exceeds 95mV. When the current limit threshold is exceeded, the P-channel MOSFET is immediately turned off by pulling the GATE voltage to VIN regardless of the controller input. 3864fa For more information www.linear.com/LTC3864 13 LTC3864 Applications Information The peak inductor current limit is equal to: 95mV IL(PEAK) ≅ RSENSE This inductor current limit would translate to an output current limit based on the inductor ripple: I LIMIT ≅ 95mV ∆IL – RSENSE 2 The SENSE pin is a high impedance input with a maximum leakage of ±2µA. Since the LTC3864 is a peak current mode controller, noise on the SENSE pin can create pulse width jitter. Careful attention must be paid to the layout of RSENSE. To ensure the integrity of the current sense signal, VSENSE, the traces from VIN and SENSE pins should be short and run together as a differential pair and Kelvin (4-wire) connected across RSENSE (Figure 3). drain current ID(MAX), and the MOSFET’s thermal resistance θJC(MOSFET) and θJA(MOSFET). The gate driver bias voltage VIN-VCAP is set by an internal LDO regulator. In normal operation, the CAP pin will be regulated to 8V below VIN. A minimum 0.1µF capacitor is required across the VIN and CAP pins to ensure LDO stability. If required, additional capacitance can be added to accommodate higher gate currents without voltage droop. In shutdown and Burst Mode operation, the CAP LDO is turned off. In the event of CAP leakage to ground, the CAP voltage is limited to 9V by a weak internal clamp from VIN to CAP. As a result, a minimum 10V VGS rated MOSFET is required. The power dissipated by the P-channel MOSFET when the LTC3864 is in continuous conduction mode is given by: PMOSFET ≅ D • IOUT2 • ρ t • RDS(ON) + I VIN2 • OUT • (CMILLER ) • 2 VIN RUP RDN + •f V – V – V V ( ) IN CAP MILLER MILLER VIN LTC3864 SENSE OPTIONAL FILTERING CF RSENSE RF MP 3864 F03 Figure 3. Inductor Current Sensing The LTC3864 has internal filtering of the current sense voltage which should be adequate in most applications. However, adding a provision for an external filter offers added flexibility and noise immunity, should it be necessary. The filter can be created by placing a resistor from the RSENSE resistor to the SENSE pin and a capacitor across the VIN and SENSE pins. where D is duty factor, RDS(ON) is on-resistance of P-MOSFET, ρt is temperature coefficient of on-resistance, RDN is the pull-down driver resistance specified at 0.9Ω typical and RUP is the pull-up driver resistance specified at 2Ω typical. VMILLER is the Miller effective VGS voltage and is taken graphically from the power MOSFET data sheet. The power MOSFET input capacitance C MILLER is the most important selection criteria for determining the transition loss term in the P-channel MOSFET but is not directly specified on MOSFET data sheets. CMILLER is a combination of several components, but it can be derived from the typical gate charge curve included on most data sheets (Figure 4). The curve is Power MOSFET Selection The LTC3864 drives a P-channel power MOSFET that serves as the main switch for the asynchronous stepdown converter. Important P-channel power MOSFET parameters include drain-to-source breakdown voltage VBR(DSS), threshold voltage VGS(TH), on-resistance RDS(ON), gate-to-drain reverse transfer capacitance CRSS, maximum VSG MILLER EFFECT G S D a b IGATE RLOAD + V – SD(TEST) QIN CMILLER = (QB – QA)/VSD(TEST) 3864 F04 (b) (a) Figure 4. (a) Typical P-MOSFET Gate Charge Characteristics and (b) Test Set-Up to Generate Gate Charge Curve 3864fa 14 For more information www.linear.com/LTC3864 LTC3864 Applications Information generated by forcing a constant current out of the gate of a common-source connected P-MOSFET that is loaded with a resistor, and then plotting the gate voltage versus time. The initial slope is the effect of the gate-to-source and gate-to-drain capacitances. The flat portion of the curve is the result of the Miller multiplication effect of the drainto-gate capacitance as the drain voltage rises across the resistor load. The Miller charge (the increase in coulombs on the horizontal axis from a to b while the curve is flat) is specified for a given VSD test voltage, but can be adjusted for different VSD voltages by multiplying by the ratio of the adjusted VSD to the curve specified VSD value. A way to estimate the CMILLER term is to take the change in gate charge from points a and b (or the parameter QGD on a manufacturer’s data sheet) and dividing it by the specified VSD test voltage, VSD(TEST). CMILLER ≅ Q GD VSD(TEST) The term with CMILLER accounts for transition loss, which is highest at high input voltages. For VIN < 20V, the highcurrent efficiency generally improves with larger MOSFETs, while for VIN > 20V, the transition losses rapidly increase to the point that the use of a higher RDS(ON) device with lower CMILLER actually provides higher efficiency. Schottky Diode Selection When the P-MOSFET is turned off, a power Schottky diode is required to function as a commutating diode to carry the inductor current. The average diode current is therefore dependent on the P-MOSFET’s duty factor. The worst case condition for diode conduction is a short-circuit condition where the Schottky must handle the maximum current as its duty factor approaches 100% (and the P-channel MOSFET’s duty factor approaches 0%). The diode therefore must be chosen carefully to meet worst case voltage and current requirements. The equation below describes the continuous or average forward diode current rating required, where D is the regulator duty factor. IF(AVG) ≅ I OUT(MAX) • (1–D) Once the average forward diode current is calculated, the power dissipation can be determined. Refer to the Schottky diode data sheet for the power dissipation PDIODE as a function of average forward current IF(AVG). PDIODE can also be iteratively determined by the two equations below, where VF(IOUT, TJ) is a function of both IF(AVG) and junction temperature TJ. Note that the thermal resistance θJA(DIODE) given in the data sheet is typical and can be highly layout dependent. It is therefore important to make sure that the Schottky diode has adequate heat sinking. TJ ≅ PDIODE • θJA(DIODE) PDIODE ≅ I F(AVG)• VF(IOUT,TJ) The Schottky diode forward voltage is a function of both IF(AVG) and TJ, so several iterations may be required to satisfy both equations. The Schottky forward voltage VF should be taken from the Schottky diode data sheet curve showing Instantaneous Forward Voltage. The forward voltage will increase as a function of both TJ and IF(AVG). The nominal forward voltage will also tend to increase as the reverse breakdown voltage increases. It is therefore advantageous to select a Schottky diode appropriate to the input voltage requirements. CIN and COUT Selection The input capacitance CIN is required to filter the square wave current through the P-channel MOSFET. Use a low ESR capacitor sized to handle the maximum RMS current. V VIN ICIN(RMS) ≅ IOUT(MAX) • OUT • –1 VIN VOUT The formula has a maximum at VIN = 2VOUT, where ICIN(RMS) = IOUT(MAX)/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life, which makes it advisable to derate the capacitor. The selection of COUT is primarily determined by the ESR required to minimize voltage ripple and load step transients. The ∆VOUT is approximately bounded by: 1 ∆VOUT ≤ ∆IL ESR+ 8 • f •C OUT 3864fa For more information www.linear.com/LTC3864 15 LTC3864 Applications Information Since ∆IL increases with input voltage, the output ripple is highest at maximum input voltage. Typically, once the ESR requirement is satisfied, the capacitance is adequate for filtering and has the necessary RMS current rating. Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirements. Dry tantalum, specialty polymer, aluminum electrolytic and ceramic capacitors are all available in surface mount packages. Specialty polymer capacitors offer very low ESR but have lower specific capacitance than other types. Tantalum capacitors have the highest specific capacitance, but it is important to only use types that have been surge tested for use in switching power supplies. Aluminum electrolytic capacitors have significantly higher ESR, but can be used in cost-sensitive applications provided that consideration is given to ripple current ratings and longterm reliability. Ceramic capacitors have excellent low ESR characteristics but can have a high voltage coefficient and audible piezoelectric effects. The high Q of ceramic capacitors with trace inductance can also lead to significant ringing. When used as input capacitors, care must be taken to ensure that ringing from inrush currents and switching does not pose an overvoltage hazard to the power switch and controller. To dampen input voltage transients, add a small 5μF to 40μF aluminum electrolytic capacitor with an ESR in the range of 0.5Ω to 2Ω. High performance through-hole capacitors may also be used, but an additional ceramic capacitor in parallel is recommended to reduce the effect of lead inductance. Discontinuous and Continuous Operation The LTC3864 operates in discontinuous conduction (DCM) until the load current is high enough for the inductor current to be positive at the end of the switching cycle. The output load current at the continuous/discontinuous boundary IOUT(CDB) is given by the following equation: I OUT(CDB) ≅ (VIN – VOUT)( VOUT+ VF ) 2 • L • f • (VIN + VF ) The continuous/discontinuous boundary is inversely proportional to the inductor value. Therefore, if required, IOUT(CDB) can be reduced by increasing the inductor value. External Soft-Start and Output Tracking Start-up characteristics are controlled by the voltage on the SS pin. When the voltage on the SS pin is less than the internal 0.8V reference, the LTC3864 regulates the VFB pin voltage to the voltage on the SS pin. When the SS pin is greater than the internal 0.8V reference, the VFB pin voltage regulates to the 0.8V internal reference. The SS pin can be used to program an external soft-start function or to allow VOUT to track another supply during start-up. Soft-start is enabled by connecting a capacitor from the SS pin to ground. An internal 10µA current source charges the capacitor, providing a linear ramping voltage at the SS pin that causes VOUT to rise smoothly from 0V to its final regulated value. The total soft-start time will be approximately: tSS = CSS • 0.8V 10µA When the LTC3864 is configured to track another supply, a voltage divider can be used from the tracking supply to the SS pin to scale the ramp rate appropriately. Two common implementations of tracking as shown in Figure 5a are coincident and ratiometric. For coincident tracking, make the divider ratio from the external supply the same as the divider ratio for the feedback voltage. Ratiometric tracking could be achieved by using a different ratio than the feedback (Figure 5b). Note that the soft-start capacitor charging current is always flowing, producing a small offset error. To minimize this error, select the tracking resistive divider values to be small enough to make this offset error negligible. Short-Circuit Faults: Current Limit and Foldback The maximum inductor current is inherently limited in a current mode controller by the maximum sense voltage. In the LTC3864, the maximum sense voltage is 95mV, measured across the inductor sense resistor RSENSE, placed across the VIN and SENSE pins. The output current limit is approximately: ILIMIT ≅ 95mV ∆I L – RSENSE 2 3864fa 16 For more information www.linear.com/LTC3864 LTC3864 Applications Information The current limit must be chosen to ensure that ILIMIT(MIN) > IOUT(MAX) under all operating conditions. The minimum current limit value should be greater than the inductor current required to produce maximum output power at worst case efficiency. Worst-case efficiency typically occurs at the highest VIN. Short-circuit fault protection is assured by the combination of current limit and frequency foldback. When the output feedback voltage VFB drops below 0.4V, the operating frequency f will fold back to a minimum value of 0.18 • f when VFB reaches 0V. Both current limit and frequency foldback are active in all modes of operation. In a shortcircuit fault condition, the output current is first limited by current limit and then further reduced by folding back the operating frequency as the short becomes more severe. Short-Circuit Recovery and Internal Soft-Start An internal soft-start feature guarantees a maximum positive output voltage slew rate in all operational cases. In a short-circuit recovery condition for example, the output recovery rate is limited by the internal soft-start so that output voltage overshoot and excessive inductor current buildup is prevented. The internal soft-start voltage and the external SS pin operate independently. The output will track the lower of the two voltages. The slew rate of the internal soft-start voltage is roughly 1.2V/ms, which translates to a total soft-start time of 650µs. If the slew rate of the SS pin is greater than 1.2V/ms the output will track the internal soft-start ramp. To assure robust fault recovery, the VOUT EXTERNAL SUPPLY VOLTAGE VOLTAGE EXTERNAL SUPPLY VOUT TIME TIME Coincident Tracking Ratiometric Tracking 3864 F05a Figure 5(a). Two Different Modes of Output Tracking EXT. V VOUT RFB2 TO VFB TO SS RFB1 RFB1 VOUT EXT. V RFB2 R1 TO SS R2 0.8V ≥ R1+ R2 EXT. V R2 RFB2 TO VFB RFB1 3864 F05b Coincident Tracking Setup Ratiometric Tracking Setup Figure 5(b): Setup for Ratiometric and Coincident Tracking 3864fa For more information www.linear.com/LTC3864 17 LTC3864 Applications Information internal soft-start feature is active in all operational cases. If a short-circuit condition occurs which causes the output to drop significantly, the internal soft-start will assure a soft recovery when the fault condition is removed. The internal soft-start assures a clean soft ramp-up from any fault condition that causes the output to droop, guaranteeing a maximum ramp rate in soft-start, short-circuit fault release, or output recovery from drop out. Figure 6 illustrates how internal soft-start controls the output ramp-up rate under varying scenarios. VIN INTERNAL SOFT-START INDUCED START-UP (NO EXTERNAL SOFT-START CAPACITOR) ~ 650µs TIME VOLTAGE (a) VOUT SHORT-CIRCUIT INTERNAL SOFT-START INDUCED RECOVERY TIME (b) VOLTAGE VIN VIN DROPOUT VOUT INTERNAL SOFT-START INDUCED RECOVERY TIME (c) The implications of both the UVLO rising and UVLO falling specifications must be carefully considered for low VIN operation. The UVLO threshold with VIN rising is typically 3.5V (with a maximum of 3.8V) and UVLO falling is typically 3.25V (with a maximum of 3.5V). The operating input voltage range of the LTC3864 is guaranteed to be 3.5V to 60V over temperature, but the initial VIN ramp must exceed 3.8V to guarantee start-up. For example, Figure 7 illustrates LTC3864 operation when an automotive battery droops during a cold crank condition. The typical automotive battery is 12V to 14V, which is more than enough headroom above 3.8V for the LTC3864 to start up. Onboard electronics which are powered by a DC/DC regulator require a minimum supply voltage for seamless operation during the cold crank condition, and the battery may droop close to these minimum supply requirements during a cold crank. The DC/DC regulator should not exacerbate the situation by having excessive dropout between the already suppressed battery voltage input and the output of the regulator which power these electronics. As seen in Figure 7, the LTC3864’s 100% duty cycle capability allows virtually no dropout (only the IOUT • (RSENSE + RDS(ON)) drop across the sense resistor and P-MOSFET if there is a significant IOUT) from the battery to the output. The 3.5V guaranteed UVLO point assures sufficient margin for continuous, uninterrupted operation in extreme cold crank battery drooping conditions. However, additional input capacitance or slower soft start-up time may be required at low VIN (e.g. 3.5V to 4.5V) in order to limit VIN droop caused by inrush currents, especially if the battery or input source has a sufficiently large input impedance. 3864 F06 Figure 6. Internal Soft-Start (a) Allows Soft Start-Up without an External Soft-Start Capacitor and Allows Soft Recovery from (b) a Short-Circuit or (c) a VIN Dropout VIN Undervoltage Lockout (UVLO) The LTC3864 is designed to accommodate applications requiring widely varying power input voltages from 3.5V to 60V. To accommodate the cases where VIN drops significantly once in regulation, the LTC3864 is VBATTERY 12V VOLTAGE VOLTAGE VOUT guaranteed to operate down to a VIN of 3.5V over the full temperature range. VOUT 5V LTC3864’s 100% DUTY CYCLE CAPABILITY ALLOWS VOUT TO RIDE VIN WITHOUT SIGNIFICANT DROP-OUT TIME 3864 F07 Figure 7. Typical Automotive Cold Crank 3864fa 18 For more information www.linear.com/LTC3864 LTC3864 Applications Information Minimum On-Time Considerations The minimum on-time, tON(MIN), is the smallest time duration that the LTC3864 is capable of turning on the power MOSFET, and is typically 220ns. It is determined by internal timing delays and the gate charge required to turn on the MOSFET. Low-duty-cycle applications may approach this minimum on-time limit, so care should be taken to ensure that: t ON(MIN) < I PPMOSTRL = VIN2 • OUT • (CMILLER) • 2 RUP RDN + (V – V ) – V •f IN CAP MILLER VMILLER VOUT VIN(MAX) • f If the duty cycle falls below what can be accommodated by the minimum on-time, the controller will skip cycles. However, the output voltage will continue to regulate. Efficiency Considerations The percent efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine the dominant contributors and therefore where efficiency improvements can be made. Percent efficiency can be expressed as: % Efficiency = 100% - (L1+L2+L3+…) where L1, L2, L3, etc., are the individual losses as a percentage of input power. Although all dissipative elements in the circuit produce losses, four main sources account for most of the losses in LTC3864 application circuits. 1. 2. Transition Loss: Transition loss of the P-channel MOSFET becomes significant only when operating at high input voltages (typically 20V or greater.) The P-channel transition losses (PPMOSTRL) can be determined from the following equation: I2R Loss: I2R losses result from the P-channel MOSFET resistance, inductor resistance, the current sense resistor, and input and output capacitor ESR. In continuous mode operation the average output current flows through L but is chopped between the P-channel MOSFET and the bottom side Schottky diode. The following equation may be used to determine the total I2R loss: PI2R ≈(I2OUT +∆I2L/12)•[ RDCR+D•(RDS(ON)+RSENSE + RESR(CIN))] + ∆I2L/ 12 • RESR(COUT) 3. Gate Charging Loss: Charging and discharging the gate of the MOSFET will result in an effective gate charging current. Each time the P-channel MOSFET gate is switched from low to high and low again, a packet of charge dQ moves from the capacitor across VIN – VCAP and is then replenished from ground by the internal VCAP regulator. The resulting dQ/dt current is a current out of VIN flowing to ground. The total power loss in the controller including gate charging loss is determined by the following equation: PCNTRL = VIN • (IQ + f •QG(PMOSFET)) 4. Schottky Loss: The Schottky diode loss is most significant at low duty factors (high step down ratios). The critical component is the Schottky forward voltage as a function of junction temperature and current. The Schottky power loss is given by the equation below. PDIODE ≅ (1–D)•IOUT • VF(IOUT,TJ) When making adjustments to improve efficiency, the input current is the best indicator of changes in efficiency. If changes cause the input current to decrease, then the efficiency has increased. If there is no change in input current, there is no change in efficiency. OPTI-LOOP® Compensation OPTI-LOOP compensation, through the availability of the ITH pin, allows the transient response to be optimized for a wide range of loads and output capacitors. The ITH pin not only allows optimization of the control loop behavior 3864fa For more information www.linear.com/LTC3864 19 LTC3864 Applications Information but also provides a test point for the step-down regulator ’s DC-coupled and AC-filtered closed-loop response. The DC step, rise time and settling at this test point truly reflects the closed-loop response. Assuming a predominantly second order system, phase margin and/or damping factor can be estimated using the percentage of overshoot seen at this pin. The bandwidth can also be estimated by examining the rise time at this pin. The ITH series RITH-CITH1 filter sets the dominant pole-zero loop compensation. Additionally, a small capacitor placed from the ITH pin to signal ground, CITH2, may be required to attenuate high frequency noise. The values can be modified to optimize transient response once the final PCB layout is done and the particular output capacitor type and value have been determined. The output capacitors need to be selected because their various types and values determine the loop feedback factor gain and phase. An output current pulse of 20% to 100% of full load current having a rise time of 1μs to 10μs will produce output voltage and ITH pin waveforms that will give a sense of the overall loop stability without breaking the feedback loop. The general goal of OPTI-LOOP compensation is to realize a fast but stable ITH response with minimal output droop due to the load step. For a detailed explanation of OPTI-LOOP compensation, refer to Application Note 76. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to ∆ILOAD • ESR, where ESR is the effective series resistance of COUT . ∆ILOAD also begins to charge or discharge COUT , generating a feedback error signal used by the regulator to return VOUT to its steady-state value. During this recovery time, VOUT can be monitored for overshoot or ringing that would indicate a stability problem. Connecting a resistive load in series with a power MOSFET, then placing the two directly across the output capacitor and driving the gate with an appropriate signal generator is a practical way to produce a realistic load-step condition. The initial output voltage step resulting from the step change in output current may not be within the bandwidth of the feedback loop, so this signal cannot be used to determine phase margin. This is why it is better to look at the ITH pin signal which is in the feedback loop and is the filtered and compensated feedback loop response. The gain of the loop increases with RITH and the bandwidth of the loop increases with decreasing CITH1. If RITH is increased by the same factor that CITH1 is decreased, the zero frequency will be kept the same, thereby keeping the phase the same in the most critical frequency range of the feedback loop. In addition, a feedforward capacitor, CFF, can be added to improve the high frequency response, as shown in Figure 1. Capacitor CFF provides phase lead by creating a high frequency zero with RFB2 which improves the phase margin. The output voltage settling behavior is related to the stability of the closed-loop system and will demonstrate overall performance of the step-down regulator. In some applications, a more severe transient can be caused by switching in loads with large (>10μF) input capacitors. If the switch connecting the load has low resistance and is driven quickly, then the discharged input capacitors are effectively put in parallel with COUT , causing a rapid drop in VOUT . No regulator can deliver enough current to prevent this problem. The solution is to limit the turn-on speed of the load switch driver. A Hot Swap™ controller is designed specifically for this purpose and usually incorporates current limiting, short-circuit protection and soft starting. Design Example Consider a step-down converter with the following specifications: VIN = 5V to 55V, VOUT = 5V, IOUT(MAX) = 2A, and f = 350kHz (Figure 8). The output voltage is programmed according to: R VOUT = 0.8V • 1+ FB2 RFB1 If RFB1 is chosen to be 80.6k, then RFB2 would have to be 422k. 3864fa 20 For more information www.linear.com/LTC3864 LTC3864 Applications Information Next, set the RSENSE resistor value to ensure that the converter can deliver a maximum output current of 2.0A with sufficient margin to account for component variations and worst-case operating conditions. Using a 30% margin factor: The FREQ pin is tied to signal ground in order to program the switching frequency to 350kHz. The on-time required at 55V to generate a 5V output can be calculated as: V 5V tON = OUT = ≈ 260ns VIN • f 55V • 350kHz RSENSE ≅ This on-time is larger than LTC3864’s minimum on-time with sufficient margin to prevent cycle skipping. Next, set the inductor value to give 60% worst-case ripple at maximum VIN = 55V. Use a more readily available 25mΩ sense resistor. The current limit is: 5V 5V L= 1– ≈ 10.8µH 350kHz •(0.6 • 2A) 55V ILIMIT ≅ Select 10µH, which is a standard value. 5V 5V 1– ≈ 1.3A ∆IL = 350kHz •10µH 55V CAP VIN MODE/PLLN RITH 9.53k CVIN 0.1µF LTC3864 CIN1 12µF 63V 90 RSENSE 25mΩ MP GATE ITH SW CITH2 100pF D1 FREQ PGOOD SGND PGND CIN1: NICHICON UPJ1J120MDD D1: DIODES INC SBR3U100LP L1: TOKO 1217AS-H-100M MP: FAIRCHILD FDMC5614P + 100 SENSE SS CITH1 3.3nF CIN2 4.7µF Efficiency VIN* 5.2V TO 55V EFFICIENCY (%) CCAP 0.47µF RUN CSS 0.1µF 95mV 1.3A – ≈ 3.15A 25mΩ 2 Next choose a P-channel MOSFET with the appropriate BVDSS and ID rating. In this example, a good choice is the Fairchild FDMC5614P (BVDSS = 60V, ID = 5.7A, RDS(ON) = 105mΩ, ρ100°C = 1.5, CMILLER = 100pF, θJA = 60°C/W). The expected power dissipation and the The resulting maximum ripple current is: RRUN 100k 95mV ≈ 27.5mΩ 1.3A 1.3 • 2A + 2 L1 10µH RPGD 100k CFF 47pF VOUT* 5V 47µF 2A ×2 RFB2 422k Burst Mode OPERATION 80 PULSE-SKIPPING 70 60 50 0.01 VIN = 12V VOUT = 5V 0.1 LOAD CURRENT (A) 1 3864 F08b VFB RFB1 80.6k 3864 F08a *VOUT FOLLOWS VIN WHEN 3.5V ≤ VIN ≤ 5.2V SEE DROPOUT BEHAVIOR IN TYPICAL PERFORMANCE CHARACTERISTICS Figure 8. Design Example (5V, 2A 350kHz Step-Down Converter) 3864fa For more information www.linear.com/LTC3864 21 LTC3864 Applications Information resulting junction temperature for the MOSFET can be calculated at TA = 70°C, VIN(MAX) = 55V and IOUT(MAX) = 2A: 5V (2A)2 •1.5 •105mΩ + 55V 0.9Ω 2Ω + (55V)2 •(2A / 2)•100pF • • 350kHz 8V – 3V 3V ≈ 57mW + 90mW = 147mW PPMOS = TJ = 70°C+147mW • 60°C/W ≈ 80°C The calculations can be repeated for VIN(MIN) = 5V: 5V (2A)2 •1.5 •105mΩ + 5V 2Ω 0.9Ω • 350kHz (5.2V)2 •100pF • + 5.2V – 3V 3V ≈ 630mW +1mW ≈ 631mW TJ = 70°C+ 631mW • 60°C / W ≈ 108°C PPMOS = Next choose an appropriate Schottky diode that will handle the power requirements. The Diodes Inc. SBR3U100LP Schottky diode is selected (VF(2A,125°C) = 0.5V, θJA = 61°C/W) for this application. The power dissipation and junction temperature at TA = 70°C can be calculated as: 5V • 0.5V ≈ 909mW PDIODE = 2A • 1– 55V TJ = 70°C+909mW • 61°C/W = 125°C These power dissipation calculations show that careful attention to heat sinking will be necessary. For the input capacitance, a combination of ceramic and electrolytic capacitors are chosen to handle the maximum RMS current of 1A. COUT will be selected based on the ESR that is required to satisfy the output voltage ripple requirement. For this design, two 47µF ceramic capacitors are chosen to offer low ripple in both normal operation and in Burst Mode operation. A soft-start time of 8ms can be programmed through a 0.1µF capacitor on the SS pin: CSS = 8ms •10µA = 0.1µF 0.8V Loop compensation components on the ITH pin are chosen based on load step transient behavior (as described under OPTI-LOOP Compensation) and is optimized for stability. A pull-up resistor is used on the RUN pin for FMEA compliance (see Failure Modes and Effects Analysis). Gate Driver Component Placement, Layout and Routing It is important to follow recommended power supply PC board layout practices such as placing external power elements to minimize loop area and inductance in switching paths. Be careful to pay particular attention to gate driver component placement, layout and routing. The effective CCAP capacitance should be greater than 0.1µF minimum in all operating conditions. Operating voltage and temperature both decrease the rated capacitance to varying degrees depending on dielectric type. The LTC3864 is a PMOS controller with an internal gate driver and bootstrapped LDO that regulates the differential CAP voltage (VIN – VCAP) to 8V nominal. The CCAP capacitance needs to be large enough to assure stability and provide cycleto-cycle current to the PMOS switch with minimum series inductance. We recommend a ceramic 0.47µF 16V capacitor with a high quality dielectric such as X5R or X7R. Some high current applications with large Qg PMOS switches may benefit from an even larger CCAP capacitance. Figure 9 shows the LTC3864 Generic Application Schematic which includes an optional current sense filter and series gate resistor. Figure 10 illustrates the recommended gate driver component placement, layout and routing of the GATE, VIN, SENSE and CAP pins and key gate driver components. It is recommended that the gate driver layout follow the example shown in Figure 10 to assure proper operation and long term reliability. 3864fa 22 For more information www.linear.com/LTC3864 LTC3864 Applications Information The LTC3864 gate driver should connect to the external power elements in the following manner. First route the VIN pin using a single low impedance isolated trace to the positive RSENSE resistor PAD without connection to the VIN plane. The reason for this precaution is that the VIN pin is internally Kelvin connected to the current sense comparator, internal VIN power and the PMOS gate driver. Connecting the VIN pin to the VIN power plane adds noise and can result in jitter or instability. Figure 10 shows a single VIN trace from the positive RSENSE pad connected to CSF, CCAP, VIN pad and CINB. The total trace length to RSENSE should be minimized and the capacitors CSF, CCAP and CINB should be placed near the VIN pin of the LTC3864. CINB VIN CIN CCAP CAP RUN CSS VIN CPITH SENSE SS GATE CITH CSF PLLIN/MODE RITH RFREQ RGATE RSENSE – Q1 D1 VOUT COUT LTC3864 L1 ITH RPGD FREQ SGND GROUND PLANE TO PGND RSF + PGOOD RFB2 PGND VFB RFB1 3864 F09 Figure 9. LTC3864 Generic Application Schematic with Optional Current Sense Filter and Series Gate Resistor CINB TO Q1 GATE RGATE GATE CSF TO RSENSE+ VIN SENSE CAP CCAP should be placed near the VIN and CAP pins. Figure 10 shows CCAP placed adjacent to the VIN and CAP pins with SENSE routed between the pads. This is the recommended layout and results in the minimum parasitic inductance. The gate driver is capable of providing high peak current. Parasitic inductance in the gate drive and the series inductance between VIN to CAP can cause a voltage spike between VIN and CAP on each switching cycle. The voltage spike can result in electrical over-stress to the gate driver and can result in gate driver failures in extreme cases. It is recommended to follow the example shown in Figure 10 for the placement of CCAP as close as is practical. RGATE resistor pads can be added with a 0Ω resistor to allow the damping resistor to be added later. The total length of the gate drive trace to the PMOS gate should be minimized and ideally be less than 1cm. In most cases with a good layout the RGATE resistor is not needed. The RGATE resistor should be located near the gate pin to reduce peak current through GATE and minimize reflected noise on the gate pin. The RSF and CSF pads can be added with a zero ohm resistor for RSF and CSF not populated. In most applications, external filtering is not needed. The current sense filter RSF and CSF can be added later if noise if demonstrated to be a problem. The bypass capacitor CINB is used to locally filter the VIN supply. CINB should be tied to the VIN pin trace and to the PGND exposed pad. The CINB positive pad should connect to RSENSE positive though the VIN pin trace. The CINB ground trace should connect to the PGND exposed pad connection. PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3864. 1. Multilayer boards with dedicated ground layers are preferable for reduced noise and for heat sinking pur- CCAP RSF TO RSENSE– 3864 F10 Figure 10. LTC3864 Recommended Gate Driver PC Board Placement, Layout and Routing 3864fa For more information www.linear.com/LTC3864 23 LTC3864 Applications Information poses. Use wide rails and/or entire planes for VIN, VOUT and GND for good filtering and minimal copper loss. If a ground layer is used, then it should be immediately below (and/or above) the routing layer for the power train components which consist of CIN, sense resistor, P-MOSFET, Schottky diode, inductor, and COUT. Flood unused areas of all layers with copper for better heat sinking. 2. Keep signal and power grounds separate except at the point where they are shorted together. Short signal and power ground together only at a single point with a narrow PCB trace (or single via in a multilayer board). All power train components should be referenced to power ground and all small signal components (e.g., CITH1, RFREQ, CSS etc.) should be referenced to signal ground. 3. Place CIN, sense resistor, P-MOSFET, inductor, and primary COUT capacitors close together in one compact area. The junction connecting the drain of P-MOSFET, cathode of Schottky, and (+) terminal of inductor (this junction is commonly referred to as switch or phase node) should be compact but be large enough to handle the inductor currents without large copper losses. Place the sense resistor and source of P-channel MOSFET as close as possible to the (+) plate of CIN capacitor(s) that provides the bulk of the AC current (these are normally the ceramic capacitors), and connect the anode of the Schottky diode as close as possible to the (–) terminal of the same CIN capacitor(s). The high dI/dt loop formed by CIN, the MOSFET, and the Schottky diode should have short leads and PCB trace lengths to minimize high frequency EMI and voltage stress from inductive ringing. The (–) terminal of the primary COUT capacitor(s) which filter the bulk of the inductor ripple current (these are normally the ceramic capacitors) should also be connected close to the (–) terminal of CIN. 4. Place pins 7 to 12 facing the power train components. Keep high dV/dt signals on GATE and switch away from sensitive small signal traces and components. 5. Place the sense resistor close to the (+) terminal of CIN and source of P-MOSFET. Use a Kelvin (4-wire) connection across the sense resistor and route the traces together as a differential pair into the VIN and SENSE pins. An optional RC filter could be placed near the VIN and SENSE pins to filter the current sense signal. 6. Place the resistive feedback divider RFB1/2 as close as possible to the VFB pin. The (+) terminal of the feedback divider should connect to the output regulation point and the (–) terminal of feedback divider should connect to signal ground. 7. Place the ceramic CCAP capacitor as close as possible to VIN and CAP pins. This capacitor provides the gate discharging current for the power P-MOSFET. 8. Place small signal components as close to their respective pins as possible. This minimizes the possibility of PCB noise coupling into these pins. Give priority to VFB, ITH, and FREQ pins. Use sufficient isolation when routing a clock signal into PLLIN /MODE pin so that the clock does not couple into sensitive small signal pins. Failure Mode and Effects Analysis (FMEA) A FMEA study on the LTC3864 has been conducted through adjacent pin opens and shorts. The device was tested in a step-down application (Figure 8) from VIN = 12V to VOUT = 5V with a current load of 1A on the output. One group of tests involved the application being monitored while each pin was disconnected from the PC board and left open while all other pins remained intact. The other group of tests involved each pin being shorted to its adjacent pins while all other pins were connected as it would be normally in the application. The results are shown in Table 2. For FMEA compliance, the following design implementations are recommended: • If the RUN pin is being pull-up to a voltage greater than 6V, then it is done so through a pull-up resistor (100k to 1M) so that the PGOOD pin is not damaged in case of a RUN to PGOOD short. • The gate of the external P-MOSFET be pulled through a resistor (20k to 100k) to the input supply, VIN so that the P-MOSFET is guaranteed to turn off in case of a GATE open. 3864fa 24 For more information www.linear.com/LTC3864 LTC3864 Applications Information Table 2 FAILURE MODE None RECOVERY WHEN FAULT IS REMOVED? BEHAVIOR VOUT IOUT IVIN f 5V 1A 453mA 350kHz N/A Normal Operation. 5V 1A 453mA 350kHz OK Pin already left open in normal application, so no difference. Pin Open Open Pin 1 (PLLIN/MODE) Open Pin 2 (FREQ) 5V 1A 453mA 535kHz OK Frequency jumps to default open value. Open Pin 3 (GND) 5V 1A 453mA 350kHz OK Exposed pad still provides GND connection to device. Open Pin 4 (SS) 5V 1A 453mA 350kHz OK External soft-start removed, but internal soft-start still available. Open Pin 5 (VFB) 0V 0A 0.7mA 0kHz OK Controller stops switching. VFB internally self biases HI to prevent switching. Open Pin 6 (ITH) 5V 1A 507mA 40kHz OK Output still regulating, but the switching is erratic. Loop not stable. Open Pin 7 (PGOOD) 5V 1A 453mA 350kHz OK No PGOOD output, but controller regulates normally. Open Pin 8 (RUN) 5V 1A 453mA 350kHz OK Controller does not start-up. Open Pin 9 (CAP) 5V 1A 453mA 350kHz OK More jitter during switching, but regulates normally. Open Pin 10 (SENSE) 0V 0A 0.7mA 0kHz OK SENSE internally prebiases to 0.6V below VIN. This prevents controller from switching. 5.4V 1A 597mA 20kHz OK VIN able to bias internally through SENSE. Regulates with high VOUT ripple. Open Pin 12 (GATE) 0V 0A Open Pin 13 (PGND) 5V Open Pin 11 (VIN) 0.7mA 0kHz OK Gate does not drive external power FET, preventing output regulation. 453mA 350kHz OK Pin 3 (GND) still provides GND connection to device. Pins Shorted Short Pins 1, 2 (PLLIN/MODE and FREQ) 5V 1A 453mA 350kHz OK Burst Mode operation disabled, but runs normally as in pulse-skipping mode. Short Pins 2, 3 (FREQ and GND) 5V 1A 453mA 0kHz OK FREQ already shorted to GND, so regulates normally. Short Pins 3, 4 (GND and SS) 0V 0A 0.7mA 0kHz OK SS short to GND prevents device from starting up. Short Pins 4, 5 (SS and VFB) 1V(DC) 3VP-P 50mA 9mA Erratic OK VOUT oscillates from 0V to 3V. Short Pins 5, 6 (VFB and ITH) 3.15V 625mA 181mA 350kHz OK Controller loop does not regulate to proper output voltage. Short Pins 7, 8 (PGOOD and RUN) 5V 1A 453mA 350kHz OK Controller does not start-up. Short Pins 8, 9 (RUN and CAP) 5V 1A 453mA 350kHz OK Able to start-up and regulate normally. Short Pins 9, 10 (CAP and SENSE) 0V 0A 181mA 0kHz OK CAP ~ VIN, which prevents turning on external P-MOSFET. Short Pins 10, 11 (SENSE and VIN) 5V 1A 453mA 50kHz OK Regulates with high VOUT ripple. Short Pins 11, 12 (VIN and GATE) 0V 0A 29mA 0kHz OK Power MOSFET is always kept OFF, preventing regulation. 3864fa For more information www.linear.com/LTC3864 25 LTC3864 Typical Applications 24V to 60V Input, 24V/1A Output at 750kHz Efficiency CCAP 0.47µF CIN2 2.2µF + RUN VIN MODE/PLLN CVIN 0.1µF VIN 24V TO 60V 90 80 EFFICIENCY (%) CAP CIN1 33µF 63V 100 RSENSE 50mΩ SENSE CITH1 6.8nF RITH 30.1k SS ITH MP GATE LTC3864 CITH2 100pF RFREQ 97.6k PGOOD SGND VFB PGND CIN1: NICHICON UPJ1J100MPD D1: DIODES INC SBR3U100LP L1: TOKO 1217AS-H-470M MP: VISHAY/SILICONIX SI7113DN 70 60 PULSE-SKIPPING 50 L1 47µH RPGD2 768k D1 FREQ Burst Mode OPERATION 10µF VOUT* 24V 1A 40 30 0.01 RFB2 887k RPGD1 200k VIN = 48V VOUT = 24V 0.1 LOAD CURRENT (A) 1 3864 TA02b RFB1 30.1k 3864 TA02a *VOUT FOLLOWS VIN WHEN 3.5V ≤ VIN ≤ 24V 3.5V to 48V Input, 1.8V/4A Output at 100kHz Efficiency CIN2 10µF ×2 CAP RUN CSS 0.1µF CITH1 10nF VIN MODE/PLLN ITH LTC3864 GATE CITH2 100pF RFREQ 24.3k 70 RSENSE 15mΩ MP D1 PGOOD FREQ SGND CIN1 33µF 63V SENSE SS RITH 14k CVIN 0.1µF + EFFICIENCY (%) CCAP 0.47µF 80 VIN 3.5V TO 48V VFB PGND L1 10µH 100µF ×2 RPGD 100k RFB2 102k + VOUT 1.8V 4A 330µF 6.3V Burst Mode OPERATION 60 PULSE-SKIPPING 50 40 30 0.01 VIN = 12V VOUT = 1.8V 0.1 1 LOAD CURRENT (A) 3864 TA03b RFB1 80.6k 3864 TA03a CIN1: SANYO 63ME33AX D1: VISHAY V10P10 L1: WÜRTH 7447709100 MP: VISHAY/SILICONIX SI7461DP 3864fa 26 For more information www.linear.com/LTC3864 LTC3864 Typical Applications 12V to 58V Input, 12V/2A Output at 535kHz CCAP 0.47µF CIN2 4.7µF + CAP VIN MODE/PLLN CVIN 0.1µF VIN 12V TO 58V Efficiency 90 RSENSE 30mΩ SENSE CITH1 3300pF SS RITH 11.3k LTC3864 MP GATE L1 22µH ITH CITH2 100pF D1 FREQ RPGD2 549k PGND CIN1: SANYO 63ME33AX D1: DIODES INC SBR3U100LP L1: TOKO 1217AS-H-220M MP: VISHAY/SILICONIX SI7465DP 10µF ×2 VOUT* 12V 2A VFB RPGD1 402k 70 PULSE-SKIPPING 60 RFB2 845k PGOOD SGND Burst Mode OPERATION 80 EFFICIENCY (%) RUN CIN1 33µF 63V VIN = 48V VOUT = 12V 50 0.01 0.1 LOAD CURRENT (A) RFB1 60.4k 1 3864 TA04b 3864 TA04a *VOUT FOLLOWS VIN WHEN 3.5V ≤ VIN ≤ 12V Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. DE/UE Package 12-Lead Plastic DFN (4mm × 3mm) (Reference LTC DWG # 05-08-1695 Rev D) 4.00 ±0.10 (2 SIDES) 7 0.70 ±0.05 3.60 ±0.05 2.20 ±0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 0.40 ± 0.10 12 R = 0.05 TYP 3.30 ±0.05 1.70 ± 0.05 R = 0.115 TYP PIN 1 TOP MARK (NOTE 6) 0.200 REF 3.30 ±0.10 3.00 ±0.10 (2 SIDES) 1.70 ± 0.10 0.75 ±0.05 6 0.25 ± 0.05 1 PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER (UE12/DE12) DFN 0806 REV D 0.50 BSC 2.50 REF 2.50 REF 0.00 – 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING PROPOSED TO BE A VARIATION OF VERSION (WGED) IN JEDEC PACKAGE OUTLINE M0-229 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 3864fa For more information www.linear.com/LTC3864 27 LTC3864 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. MSE Package 12-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1666 Rev F) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 ±0.102 (.112 ±.004) 5.23 (.206) MIN 2.845 ±0.102 (.112 ±.004) 0.889 ±0.127 (.035 ±.005) 6 1 1.651 ±0.102 (.065 ±.004) 1.651 ±0.102 3.20 – 3.45 (.065 ±.004) (.126 – .136) 12 0.65 0.42 ±0.038 (.0256) (.0165 ±.0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 0.35 REF 4.039 ±0.102 (.159 ±.004) (NOTE 3) 0.12 REF DETAIL “B” CORNER TAIL IS PART OF DETAIL “B” THE LEADFRAME FEATURE. FOR REFERENCE ONLY 7 NO MEASUREMENT PURPOSE 0.406 ±0.076 (.016 ±.003) REF 12 11 10 9 8 7 DETAIL “A” 0° – 6° TYP 3.00 ±0.102 (.118 ±.004) (NOTE 4) 4.90 ±0.152 (.193 ±.006) GAUGE PLANE 0.53 ±0.152 (.021 ±.006) DETAIL “A” 1.10 (.043) MAX 0.18 (.007) SEATING PLANE 0.22 – 0.38 (.009 – .015) TYP 1 2 3 4 5 6 0.650 (.0256) BSC NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX 6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL NOT EXCEED 0.254mm (.010") PER SIDE. 0.86 (.034) REF 0.1016 ±0.0508 (.004 ±.002) MSOP (MSE12) 0911 REV F 3864fa 28 For more information www.linear.com/LTC3864 LTC3864 Revision History REV DATE DESCRIPTION A 6/14 Modified VIN to CAP capacitance PAGE NUMBER Updated Notes 2 and 3 1, 8, 10, 21, 25, 26, 28 2 3864fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representaFor more information www.linear.com/LTC3864 tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. 29 LTC3864 Typical Application 3.5V to 38V Input, 3.3V/3A Output at 300kHz CIN2 10µF ×2 CAP RUN CSS 0.1µF CITH1 10nF VIN CIN1 33µF 63V RSENSE 20mΩ LTC3864 ITH MP GATE VOUT 3.3V 47µF 3A ×2 L1 6.8µH RFREQ 42.2k SGND PGND RPGD 100k 60 PULSE-SKIPPING 40 0.01 RFB2 634k PGOOD FREQ 70 50 CITH2 100pF D1 Burst Mode OPERATION 80 SENSE SS RITH 20k CVIN 0.1µF MODE/PLLN + 90 EFFICIENCY (%) CCAP 0.47µF Efficiency VIN 3.5V TO 60V VIN = 12V VOUT = 3.3V 0.1 LOAD CURRENT (A) 1 3864 TA05b VFB RFB1 200k 3864 TA05a CIN1: SANYO 63ME33AX D1: VISHAY V15P45S L1: WÜRTH 7447709100 MP: VISHAY/SILICONIX Si7611DN Related Parts PART NUMBER DESCRIPTION COMMENTS LTC3891 60V, Low IQ, Synchronous Step-Down DC/DC Controller Phase-Lockable Fixed Frequency 50kHz to 900kHz 4V ≤ VIN ≤ 60V, 0.8V ≤ VOUT ≤ 24V, IQ = 50µA LTC3890 60V, Low IQ, Dual 2-Phase Synchronous Step-Down DC/DC Controller Phase-Lockable Fixed Frequency 50kHz to 900kHz 4V ≤ VIN ≤ 60V, 0.8V ≤ VOUT ≤ 24V, IQ = 50µA LTC3824 60V, Low IQ, Step-Down DC/DC Controller, 100% Duty Cycle Selectable Fixed Frequency 200kHz to 600kHz 4V≤ VIN ≤ 60V, 0.8V ≤ VOUT ≤ VIN, IQ = 40µA, MSOP-10E LT3845A 60V, Low IQ, Single Output Synchronous Step-Down DC/DC Controller Synchronizable Fixed Frequency 100kHz to 600kHz 4V ≤ VIN ≤ 60V, 1.23V ≤ VOUT ≤ 36V, IQ = 120µA, TSSOP-16 LTC3863 60V Low IQ Inverting DC/DC Controller PLL Fixed Frequency 75kHz to 750kHz, 3.5V ≤ VIN ≤ 60V –150V ≤ VOUT ≤ –0.4V, IQ = 70µA, 3mm × 4mm DFN-12, MSOP-12 LTC3834/LTC3834-1 LTC3835/LTC3835-1 Low IQ, Single Output Synchronous Step-Down DC/DC Controller with 99% Duty Cycle Phase-Lockable Fixed Frequency 140kHz to 650kHz, 4V ≤ VIN ≤ 36V, 0.8V ≤ VOUT ≤ 10V, IQ = 30µA/80µA LTC3857/LTC3857-1 LTC3858/LTC3858-1 Low IQ, Dual Output 2-Phase Synchronous Step-Down DC/DC Controllers with 99% Duty Cycle Phase-Lockable Fixed Frequency 50kHz to 900kHz, 4V ≤ VIN ≤ 38V, 0.8V ≤ VOUT ≤ 24V, IQ = 50µA/170µA LTC3859AL Low IQ, Triple Output Buck/Buck/Boost Synchronous DC/DC Controller All Outputs Remain in Regulation Through Cold Crank 2.5V ≤ VIN ≤ 38V, VOUT(BUCKS) Up to 24V, VOUT(BOOST) Up to 60V, IQ = 28µA 3864fa 30 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 For more information www.linear.com/LTC3864 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LTC3864 LT 0614 REV A • PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 2012