LTC3831 High Power Synchronous Switching Regulator Controller for DDR Memory Termination DESCRIPTION FEATURES n n n n n n n n n n n n n High Power Switching Regulator Controller for DDR Memory Termination VOUT Tracks 1/2 of VIN or External VREF No Current Sense Resistor Required Low Input Supply Voltage Range: 3V to 8V Maximum Duty Cycle > 91% Over Temperature Drives All N-Channel External MOSFETs High Efficiency: Over 95% Possible Programmable Fixed Frequency Operation: 100kHz to 500kHz External Clock Synchronization Operation Programmable Soft-Start Low Shutdown Current: <10μA Overtemperature Protection Available in 16-Pin Narrow SSOP Package The LTC®3831 is a high power, high efficiency switching regulator controller designed for DDR memory termination. The LTC3831 generates an output voltage equal to 1/2 of an external supply or reference voltage. The LTC3831 uses a synchronous switching architecture with N-channel MOSFETs. Additionally, the chip senses output current through the drain-source resistance of the upper N-channel FET, providing an adjustable current limit without a current sense resistor. The LTC3831 operates with input supply voltage as low as 3V and with a maximum duty cycle of > 91%. It includes a fixed frequency PWM oscillator for low output ripple operation. The 200kHz free-running clock frequency can be externally adjusted or synchronized with an external signal from 100kHz to above 500kHz. In shutdown mode, the LTC3831 supply current drops to <10μA. APPLICATIONS n n n L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. DDR SDRAM Termination SSTL_2 Interface SSTL_3 Interface TYPICAL APPLICATION VDDQ 2.5V 5V Efficiency vs Load Current + MBR0530T1 PVCC1 VCC IMAX MBRS340T3 1k LO 1.2μH 0.1μF VTT 1.25V ±6A LTC3831 IFB FREQSET RC 15k CC 1500pF Q1 10k TG SS 0.01μF 130k SHDN C1 33pF 90 80 BG SHDN PGND COMP GND R+ R– Q2 MBRS340T3 + CIN: SANYO POSCAP 6TPB330M COUT: SANYO POSCAP 4TPB470M Q1, Q2: SILICONIX Si4410DY FB EFFICIENCY (%) PVCC2 0.1μF 4.7μF CIN 330μF ×2 0.1μF 1μF + 100 70 60 50 40 30 COUT 470μF ×3 TA = 25°C VIN = 2.5V VOUT = 1.25V 20 10 0 3831 F01 0 1 3 4 2 LOAD CURRENT (A) 5 6 2831 G01 Figure 1. Typical DDR Memory Termination Application 3831fb 1 LTC3831 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) TOP VIEW Supply Voltage VCC ..............................................................................9V PVCC1,2 ......................................................................14V Input Voltage IFB, IMAX ..................................................... –0.3V to 14V R+, R–, FB, SHDN, FREQSET .......... –0.3V to VCC to 0.3V Junction Temperature (Note 9) ............................. 125°C Operating Temperature Range Note 4) .....–40°C to 85°C Storage Temperature Range...................–65°C to 150°C Lead Temperature (Soldering, 10 sec) .................. 300°C TG 1 16 BG PVCC1 2 15 PVCC2 PGND 3 14 VCC GND 4 13 IFB R– 5 12 IMAX FB 6 11 FREQSET R+ 7 10 COMP SHDN 8 9 SS GN PACKAGE 16-LEAD PLASTIC SSOP TJMAX = 125°C, θJA = 130°C/W ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3831EGN#PBF LTC3831EGN#TRPBF 3831 16-Lead Plastic SSOP –40°C to 85°C LTC3831IGN#PBF LTC3831IGN#TRPBF 3831 16-Lead Plastic SSOP –40°C to 85°C LEAD BASED FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3831EGN LTC3831EGN#TR 3831 16-Lead Plastic SSOP –40°C to 85°C LTC3831IGN LTC3831IGN#TR 3831 16-Lead Plastic SSOP –40°C to 85°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/ ELECTRICAL CHARACTERISTICS The ● denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC, PVCC1, PVCC2 = 5V, VR+ = 2.5V, VR– = GND, unless otherwise noted. (Note 2) SYMBOL PARAMETER VCC Supply Voltage CONDITIONS PVCC PVCC1, PVCC2, Voltage VUVLO Undervoltage Lockout Voltage VFB Feedback Voltage VR+ = 2.5V, VR– = 0V, VCOMP = 1.25V ΔVOUT Output Load Regulation Output Line Regulation IOUT = 0A to 10A (Note 6) VCC = 4.75V to 5.25V IVCC Supply Current Figure 2, VSHDN = VCC VSHDN = 0V l l 0.7 1 1.6 10 mA μA IPVCC PVCC Supply Current Figure 2, VSHDN = VCC (Note 3) VSHDN = 0V l l 14 0.1 20 10 mA μA ΔfOSC Internal Oscillator Frequency FREQSET Floating l 200 240 kHz VSAWL VCOMP at Minimum Duty Cycle 1.2 V VSAWH VCOMP at Maximum Duty Cycle 2.2 V (Note 7) MIN TYP MAX l 3 5 8 V l 3 13.2 V 2.4 2.9 V 1.25 1.269 V l 1.231 2 0.1 160 UNITS mV mV 3831fb 2 LTC3831 ELECTRICAL CHARACTERISTICS The ● denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC, PVCC1, PVCC2 = 5V, VR+ = 2.5V, VR– = GND, unless otherwise noted. (Note 2) SYMBOL PARAMETER CONDITIONS VCOMPMAX Maximum VCOMP VFB = 0V, PVCC1 = 8V ΔfOSC/ΔIFREQSET Frequency Adjustment AV Error Amplifier Open-Loop DC Gain ● gm Error Amplifier Transconductance ● ICOMP Error Amplifier Output Sink/Source Current IMAX IMAX Sink Current VIMAX = VCC IMAX Sink Current Tempco VIMAX = VCC (Note 6) VIH SHDN Input High Voltage MIN TYP MAX 2.85 V 10 kHz/μA 46 55 dB 520 650 780 100 ● 9 4 12 12 VIL SHDN Input Low Voltage IIN SHDN Input Current VSHDN = VCC ● ISS Soft-Start Current VSS = 0V, VIMAX = 0V, VIFB = VCC ● ISSIL Maximum Soft-Start Sink Current Undercurrent Limit VIMAX = VCC, VIFB = 0V, VSS = VCC (Note 8), PVCC1 = 8V R+ R+ Input Resistance tr, tf Driver Rise/Fall Time Figure 2, PVCC1 = PVCC2 = 5V (Note 5) ● tNOV Driver Nonoverlap Time Figure 2, PVCC1 = PVCC2 = 5V (Note 5) ● DCMAX Maximum TG Duty Cycle Figure 2, VFB = 0V (Note 5), PVCC1 = 8V ● μA μA ppm/°C V ● 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: All currents into device pins are positive; all currents out of device pins are negative. All voltages are referenced to ground unless otherwise specified. Note 3: Supply current in normal operation is dominated by the current needed to charge and discharge the external FET gates. This will vary with the LTC3831 operating frequency, operating voltage and the external FETs used. Note 4: The LTC3831EGN is guaranteed to meet performance specifications from 0°C to 85°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LTC 3831IGN is guaranteed to meet performance specifications over the full –40°C to 85°C temperature range. 15 20 2.4 –8 μmho μA 3300 ● UNITS 0.8 V 0.1 1 μA –12 –16 μA 1.6 mA 49.5 kΩ 80 250 ns 25 120 250 ns 91 95 % Note 5: Rise and fall times are measured using 10% and 90% levels. Duty cycle and nonoverlap times are measured using 50% levels. Note 6: Guaranteed by design, not subject to test. Note 7: PVCC1 must be higher than VCC by at least 2.5V for TG to operate at 95% maximum duty cycle and for the current limit protection circuit to be active. Note 8: The current limiting amplifier can sink but cannot source current. Under normal (not current limited) operation, the output current will be zero. Note 9: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125°C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability. 3831fb 3 LTC3831 TYPICAL PERFORMANCE CHARACTERISTICS 1.270 1.260 TA = 25°C REFER TO FIGURE 1 NEGATIVE OUTPUT CURRENT INDICATES CURRENT SINKING 1.265 1.260 Error Amplifier Transconductance vs Temperature Line Regulation 1.258 6 1.254 4 1.252 2 1.250 0 1.248 –2 1.246 –4 1.244 –6 1.242 –8 VFB (V) VOUT (V) 8 1.256 1.250 1.245 1.240 1.235 1.230 –4 –6 –2 2 0 OUTPUT CURRENT (A) 4 1.240 6 4 3 6 7 5 SUPPLY VOLTAGE (V) 3831 G02 1.260 10 1.255 5 1.250 0 1.245 –5 1.240 –10 1.235 –15 –25 0 50 25 TEMPERATURE (°C) –20 100 75 200 160 140 120 100 80 60 40 –50 –25 0 50 75 25 TEMPERATURE (°C) 100 550 500 –50 –25 600 200 190 180 –25 75 0 25 50 TEMPERATURE (°C) 100 125 3831 G07 Oscillator (VSAWH – VSAWL) vs External Sync Frequency 1.5 TA = 25°C 1.4 500 TA = 25°C 1.3 VSAWH – VSAWL (V) OSCILLATOR FREQUENCY (kHz) 210 125 45 3831 G06 FREQSET FLOATING 220 100 50 Oscillator Frequency vs FREQSET Input Current 230 50 25 75 0 TEMPERATURE (°C) 55 40 –50 125 240 OSCILLATOR FREQUENCY (kHz) 600 60 180 Oscillator Frequency vs Temperature 400 300 200 1.2 1.1 1.0 0.9 0.8 0.7 100 170 160 –50 650 Error Amplifier Open-Loop Gain vs Temperature 3831 G04 250 700 3831 G05 ERROR AMPLIFIER OPEN-LOOP GAIN (dB) 15 ERROR AMPLIFIER SINK/SOURCE CURRENT (μA) 20 REFER TO FIGURE 1 OUTPUT = NO LOAD 1.230 –50 750 Error Amplifier Sink/Source Current vs Temperature ΔVOUT (mV) VOUT (V) 1.265 –10 800 3831 G03 Output Temperature Drift 1.270 8 ΔVFB (mV) 1.255 10 TA = 25°C ERROR AMPLIFIER TRANSCONDUCTANCE (μmho) Load Regulation 0.6 –25 0 25 50 75 TEMPERATURE (°C) 100 125 3831 G08 0 –40 10 –20 –10 0 –30 FREQSET INPUT CURRENT (μA) 20 3831 G09 0.5 100 300 200 400 EXTERNAL SYNC FREQUENCY (kHz) 500 3831 G10 3831fb 4 LTC3831 TYPICAL PERFORMANCE CHARACTERISTICS Maximum TG Duty Cycle vs Temperature VFB = 0V REFER TO FIGURE 3 98 IMAX SINK CURRENT (μA) MAXIMUM G1 DUTY CYCLE (%) 99 97 96 95 94 93 Output Overcurrent Protection 20 1.4 18 1.2 16 OUTPUT VOLTAGE (V) 100 IMAX Sink Current vs Temperature 14 12 10 8 91 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 4 –50 –25 125 100 SOFT-START SOURCE CURRENT (μA) OUTPUT CURRENT LIMIT (A) 7 6 5 4 3 2 1 50 25 0 75 TEMPERATURE (°C) 100 2.00 –9 1.75 –10 –11 –12 –13 –14 –15 75 50 25 TEMPERATURE (°C) 0 100 1.6 2.6 2.5 2.4 2.3 2.2 2.1 –25 50 25 0 75 TEMPERATURE (°C) 100 1.25 1.00 0.75 0.50 0.25 0 –150 125 125 3831 G17 –125 –100 –50 –75 VIFB – VIMAX (mV) 1.5 PVCC Supply Current vs Oscillator Frequency 90 FREQSET FLOATING TA = 25°C 80 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 TG AND BG LOADED WITH 6800pF, PVCC1,2 = 12V 70 60 50 TG AND BG LOADED WITH 1000pF, PVCC1,2 = 5V 40 30 TG AND BG LOADED WITH 6800pF, PVCC1,2 = 5V 20 10 0.5 0.4 –50 0 –25 3831 G16 PVCC SUPPLY CURRENT (mA) 3.0 2.0 –50 1.50 VCC Operating Supply Current vs Temperature VCC OPERATING SUPPLY CURRENT (mA) UNDERVOLTAGE LOCKOUT THRESHOLD VOLTAGE (V) Undervoltage Lockout Threshold Voltage vs Temperature 2.7 10 TA = 25°C 3831 G15 3831 G14 2.8 8 6 4 OUTPUT CURRENT (A) Soft-Start Sink Current vs (VIFB – VIMAX) –8 –16 –50 –25 125 2.9 2 3831 G13 SOFT-START SINK CURRENT (mA) REFER TO FIGURE 1 8 TA = 25°C REFER TO FIGURE 1 0 125 Soft-Start Source Current vs Temperature 9 –25 0.4 3831 G12 Output Current Limit Threshold vs Temperature 0 –50 0.6 0 75 50 25 TEMPERATURE (°C) 0 3831 G11 10 0.8 0.2 6 92 1.0 –25 50 25 0 75 TEMPERATURE (°C) 100 125 3831 G18 0 0 400 100 300 200 OSCILLATOR FREQUENCY (kHz) 500 3831 G19 3831fb 5 LTC3831 TYPICAL PERFORMANCE CHARACTERISTICS PVCC Supply Current vs Gate Capacitance 50 TG Rise/Fall Time vs Gate Capacitance 200 TA = 25°C TA = 25°C 160 TG RISE/FALL TIME (ns) PVCC SUPPLY CURRENT (mA) 180 40 PVCC1,2 = 12V 30 20 PVCC1,2 = 5V 10 VOUT 50mV/ DIV 140 120 tf AT PVCC1,2 = 5V 100 tr AT PVCC1,2 = 5V 80 40 0 1 2 3 4 5 6 7 8 9 10 GATE CAPACITANCE AT TG AND BG (nF) ILOAD 2A/DIV 60 tf AT PVCC1,2 = 12V 20 0 Transient Response 0 50μs/DIV 3831 G22 tr AT PVCC1,2 = 12V 0 1 2 3 4 5 6 7 8 9 10 GATE CAPACITANCE AT TG AND BG (nF) 3831 G20 3831 G21 PIN FUNCTIONS TG ( Pin 1): Top Driver Output. Connect this pin to the gate of the upper N-channel MOSFET, Q1. This output swings from PGND to PVCC1. It remains low if BG is high or during shutdown mode. resistor divider. The FB pin is servoed to the ratiometric reference under closed-loop conditions. The LTC3831 can operate with a minimum VFB of 1.1V and maximum VFB of (VCC – 1.75V). PVCC1 (Pin 2): Power Supply Input for TG. Connect this pin to a potential of at least VIN + VGS(ON)(Q1). This potential can be generated using an external supply or a simple charge pump connected to the switching node between the upper MOSFET and the lower MOSFET. SHDN (Pin 8): Shutdown. A TTL compatible low level at SHDN for longer than 100μs puts the LTC3831 into shutdown mode. In shutdown, TG and BG go low, all internal circuits are disabled and the quiescent current drops to 10μA max. A TTL compatible high level at SHDN allows the part to operate normally. This pin also double as an external clock input to synchronize the internal oscillator with an external clock. PGND (Pin 3): Power Ground. Both drivers return to this pin. Connect this pin to a low impedance ground in close proximity to the source of Q2. Refer to the Layout Consideration section for more details on PCB layout techniques. GND (Pin 4): Signal Ground. All low power internal circuitry returns to this pin. To minimize regulation errors due to ground currents, connect GND to PGND right at the LTC3831. R–, R+ (Pins 5, 7): These two pins connect to the internal resistor divider that generate the internal ratiometric reference for the error amplifier. The reference voltage is set at 0.5 • (VR+ – VR–). SS (Pin 9): Soft-Start. Connect this pin to an external capacitor, CSS, to implement a soft-start function. If the LTC3831 goes into current limit, CSS is discharged to reduce the duty cycle. CSS must be selected such that during power-up, the current through Q1 will not exceed the current limit level. COMP (Pin 10): External Compensation. This pin internally connects to the output of the error amplifier and input of the PWM comparator. Use a RC + C network at this pin to compensate the feedback loop to provide optimum transient response. FB (Pin 6): Feedback Voltage. FB senses the regulated output voltage either directly or through an external 3831fb 6 LTC3831 PIN FUNCTIONS FREQSET (Pin 11): Frequency Set. Use this pin to adjust the free-running frequency of the internal oscillator. With the pin floating, the oscillator runs at about 200kHz. A resistor from FREQSET to ground speeds up the oscillator; a resistor to VCC slows it down. through a 1k resistor. The 1k resistor is required to prevent voltage transients from damaging IFB.This pin is used for sensing the voltage drop across the upper N-channel MOSFET, Q1. VCC (Pin 14): Power Supply Input. All low power internal circuits draw their supply from this pin. This pin requires a 4.7μF bypass capacitor to GND. IMAX (Pin 12): Current Limit Threshold Set. IMAX sets the threshold for the internal current limit comparator. If IFB drops below IMAX with TG on, the LTC3831 goes into current limit. IMAX has an internal 12μA pull-down to GND. Connect this pin to the main VIN supply at the drain of Q1, through an external resistor to set the current limit threshold. Connect a 0.1μF decoupling capacitor across this resistor to filter switching noise. PVCC2 (Pin 15): Power Supply Input for BG. Connect this pin to the main high power supply. BG (Pin 16): Bottom Driver Output . Connect this pin to the gate of the lower N-channel MOSFET, Q2. This output swings from PGND to PVCC2. It remains low when TG is high or during shutdown mode. To prevent output undershoot during a soft-start cycle, BG is held low until TG first goes high (FFBG in the Block Diagram). IFB (Pin 13): Current Limit Sense. Connect this pin to the switching node at the source of Q1 and the drain of Q2 BLOCK DIAGRAM SHDN DISABLE GATE DRIVE LOGIC AND THERMAL SHUTDOWN 100μs DELAY VCC POWER DOWN INTERNAL OSCILLATOR PVCC1 – FREQSET + COMP S Q TG R Q PVCC2 PWM BG FFBG 12μA S QSS SS POR Q PGND ENABLE BG R FB R+ ERR + MIN – – MAX + + – 24k VREF + 3% 750Ω VREF – 3% VREF VREF VREF + 3% 750Ω CC 2.2V QC DISABLE ILIM – IFB + IMAX VREF – 3% 24k R– 12μA + 1.2V PVCC1 GND 3830 BD V – VCC1 + 2.5V 3831fb 7 LTC3831 TEST CIRCUITS PVCC + VSHDN VCC 10μF SHDN NC NC VFB VCOMP 2.5V R– PVCC2 PVCC1 VCC SS FREQSET FB COMP R+ 0.1μF IFB TG TG RISE/FALL 6800pF LTC3831 BG IMAX GND PGND BG RISE/FALL 6800pF 3831 F02 Figure 2 APPLICATIONS INFORMATION OVERVIEW The LTC3831 is a voltage mode feedback, synchronous switching regulator controller (see Block Diagram) designed for use in high to medium power, DDR memory termination. It includes an onboard PWM generator, a ratiometric reference, two high power MOSFET gate drivers and all necessary feedback and control circuitry to form a complete switching regulator circuit. The PWM loop nominally runs at 200kHz. The LTC3831 is designed to generate an output voltage that tracks at 1/2 of the external voltage connected between the R+ and R– pins. The LTC3831 can be used to generate the termination voltage, VTT, for interface like the SSTL_2 where VTT is a ratio of the interface supply voltage, VDDQ. It is a requirement in the SSTL_2 interface standard for VTT to track the interface supply voltage to improve noise immunity. Using the LTC3831 to supply the interface termination voltage allows large current sourcing and sinking through the termination resistors during bus transitions. The LTC3831 includes a current limit sensing circuit that uses the topside external N-channel power MOSFET as a current sensing element, eliminating the need for an external sense resistor. Also included is an internal softstart feature that requires only a single external capacitor to operate. In addition, the part features an adjustable oscillator which can free run or synchronize to an external signal with frequencies from 100kHz to 500kHz, allowing added flexibility in external component selection. THEORY OF OPERATION Primary Feedback Loop The LTC3831 senses the output voltage of the circuit through the FB pin and feeds this voltage back to the internal transconductance error amplifier, ERR. The error amplifier compares the output voltage to the internal ratiometric reference, VREF, and outputs an error signal to the PWM comparator. VREF is set to 0.5 multiplied by the voltage difference between the R+ and R– pins, using an internal resistor divider. This error signal is compared with a fixed frequency ramp waveform, from the internal oscillator, to generate a pulse width modulated signal. This PWM signal drives the external MOSFETs through the TG and BG pins. The resulting chopped waveform is filtered by LO and COUT which closes the loop. Loop compensation is achieved with an external compensation network at the COMP pin, the output node of the error amplifier. MIN, MAX Feedback Loops Two additional comparators in the feedback loop provide high speed output voltage correction in situations where the error amplifier may not respond quickly enough. MIN compares the feedback signal to a voltage 3% below VREF. If the signal is below the comparator threshold, the MIN comparator overrides the error amplifier and forces the loop to maximum duty cycle, >91%. Similarly, the MAX comparator forces the output to 0% duty cycle if the feed3831fb 8 LTC3831 APPLICATIONS INFORMATION back signal is greater than 3% above VREF. To prevent these two comparators from triggering due to noise, the MIN and MAX comparators’ response times are deliberately delayed by two to three microseconds. These two comparators help prevent extreme output perturbations with fast output load current transients, while allowing the main feedback loop to be optimally compensated for stability. Thermal Shutdown The LTC3831 has a thermal protection circuit that disables both gate drivers if activated. If the chip junction temperature reaches 150°C, both TG and BG are pulled low. TG and BG remain low until the junction temperature drops below 125°C, after which, the chip resumes normal operation. Soft-Start and Current Limit The LTC3831 includes a soft-start circuit that is used for start-up and current limit operation. The SS pin requires an external capacitor, CSS, to GND with the value determined by the required soft-start time. An internal 12μA current source is included to charge CSS. During power-up, the COMP pin is clamped to a diode drop (B-E junction of QSS in the Block Diagram) above the voltage at the SS pin. This prevents the error amplifier from forcing the loop to maximum duty cycle. The LTC3831 operates at low duty cycle as the SS pin rises above 0.6V (VCOMP ≈ 1.2V). As SS continues to rise, QSS turns off and the error amplifier takes over to regulate the output. The MIN comparator is disabled during soft-start to prevent it from overriding the soft-start function. The LTC3831 includes yet another feedback loop to control operation in current limit. Just before every falling edge of TG, the current comparator, CC, samples and holds the voltage drop measured across the external upper MOSFET, Q1, at the IFB pin. CC compares the voltage at IFB to the voltage at the IMAX pin. As the peak current rises, the measured voltage across Q1 increases due to the drop across the RDS(ON) of Q1. When the voltage at IFB drops below IMAX, indicating that Q1’s drain current has exceeded the maximum level, CC starts to pull current out of CSS, cutting the duty cycle and controlling the output current level. The CC comparator pulls current out of the SS pin in proportion to the voltage difference between IFB and IMAX. Under minor overload conditions, the SS pin falls gradually, creating a time delay before current limit takes effect. Very short, mild overloads may not affect the output voltage at all. More significant overload conditions allow the SS pin to reach a steady state, and the output remains at a reduced voltage until the overload is removed. Serious overloads generate a large overdrive at CC, allowing it to pull SS down quickly and preventing damage to the output components. By using the RDS(ON) of Q1 to measure the output current, the current limiting circuit eliminates an expensive discrete sense resistor that would otherwise be required. This helps minimize the number of components in the high current path. The current limit threshold can be set by connecting an external resistor RIMAX from the IMAX pin to the main VIN supply at the drain of Q1. The value of RIMAX is determined by: RIMAX = (ILMAX)(RDS(ON)Q1)/IIMAX where: ILMAX = ILOAD + (IRIPPLE/2) ILOAD = Maximum load current IRIPPLE = Inductor ripple current = ( VIN – VOUT )( VOUT ) ( fOSC )(LO )( VIN ) fOSC = LTC3831 oscillator frequency = 200kHz LO = Inductor value RDS(ON)Q1 = On-resistance of Q1 at ILMAX IIMAX = Internal 12μA sink current at IMAX The RDS(ON) of Q1 usually increases with temperature. To keep the current limit threshold constant, the internal 12μA sink current at IMAX is designed with a positive temperature coefficient to provide first order correction for the temperature coefficient of RDS(ON)Q1. In order for the current limit circuit to operate properly and to obtain a reasonably accurate current limit threshold, the IIMAX and IFB pins must be Kelvin sensed at Q1’s drain 3831fb 9 LTC3831 APPLICATIONS INFORMATION and source pins. In addition, connect a 0.1μF decoupling capacitor across RIMAX to filter switching noise. Otherwise, noise spikes or ringing at Q1’s source can cause the actual current limit to be greater than the desired current limit set point. Due to switching noise and variation of RDS(ON), the actual current limit trip point is not highly accurate. The current limiting circuitry is primarily meant to prevent damage to the power supply circuitry during fault conditions. The exact current level where the limiting circuit begins to take effect will vary from unit to unit as the RDS(ON) of Q1 varies. Typically, RDS(ON) varies as much as ±40% and with ±25% variation on the LTC3831’s IMAX current, this can give a ±65% variation on the current limit threshold. The RDS(ON) is high if the VGS applied to the MOSFET is low. This occurs during power up, when PVCC1 is ramping up. To prevent the high RDS(ON) from activating the current limit, the LTC3831 disables the current limit circuit if PVCC1 is less than 2.5V above VCC. To ensure proper operation of the current limit circuit, PVCC1 must be at least 2.5V above VCC when TG is high. PVCC1 can go low when TG is low, allowing the use of an external charge pump to power PVCC1. VIN LTC3831 RIMAX + 12μA CC CIN 12 IMAX IFB – + 0.1μF TG Q1 LO 1k 13 BG Q2 + VOUT COUT 3831 F03 Figure 3. Current Limit Setting Oscillator Frequency The LTC3831 includes an onboard current controlled oscillator that typically free-runs at 200kHz. The oscillator frequency can be adjusted by forcing current into or out of the FREQSET pin. With the pin floating, the oscillator runs at about 200kHz. Every additional 1μA of current into/out of the FREQSET pin decreases/increases the frequency by 10kHz. The pin is internally servoed to 1.265V, connect- ing a 50k resistor from FREQSET to ground forces 25μA out of the pin, causing the internal oscillator to run at approximately 450kHz. Forcing an external 10μA current into FREQSET cuts the internal frequency to 100kHz. An internal clamp prevents the oscillator from running slower than about 50kHz. Tying FREQSET to VCC forces the chip to run at this minimum speed. Shutdown The LTC3831 includes a low power shutdown mode, controlled by the logic at the SHDN pin. A high at SHDN allows the part to operate normally. A low level at SHDN for more than 100μs forces the LTC3831 into shutdown mode. In this mode, all internal switching stops, the COMP and SS pins pull to ground and Q1 and Q2 turn off. The LTC3831 supply current drops to <10μA, although offstate leakage in the external MOSFETs may cause the total VIN current to be somewhat higher, especially at elevated temperatures. If SHDN returns high, the LTC3831 reruns a soft-start cycle and resumes normal operation. External Clock Synchronization The LTC3831 SHDN pin doubles as an external clock input for applications that require a synchronized clock. An internal circuit forces the LTC3831 into external synchronization mode if a negative transition at the SHDN pin is detected. In this mode, every negative transition on the SHDN pin resets the internal oscillator and pulls the ramp signal low. This forces the LTC3831 internal oscillator to lock to the external clock frequency. The LTC3831 internal oscillator can be externally synchronized from 100kHz to 500kHz. Frequencies above 300kHz can cause a decrease in the maximum obtainable duty cycle as rise/fall time and propagation delay take up a larger percentage of the switch cycle. The low period of this clock signal must not be >100μs or else the LTC3831 enters into the shutdown mode. Figure 4 describes the operation of the external synchronization function. A negative transition at the SHDN pin forces the internal ramp signal low to restart a new PWM cycle. Notice that the ramp amplitude is lowered as the external clock frequency goes higher. The effect of this decrease in ramp amplitude increases the open-loop gain of the 3831fb 10 LTC3831 APPLICATIONS INFORMATION supply input) by at least one power MOSFET VGS(ON) for efficient operation. An internal level shifter allows PVCC1 to operate at voltages above VCC and VIN, up to 14V maximum. This higher voltage can be supplied with a separate supply, or it can be generated using a charge pump. SHDN TRADITIONAL SYNC METHOD WITH EARLY RAMP TERMINATION 200kHz FREE RUNNING RAMP SIGNAL RAMP SIGNAL WITH EXT SYNC Gate drive for the bottom MOSFET Q2 is provided through PVCC2. This supply only need to be above the power MOSFET VGS(ON) for efficient operation. PVCC2 can also be driven from the same supply/charge pump for the PVCC1, or it can be connected to a lower supply to improve efficiency. RAMP AMPLITUDE ADJUSTED LTC3831 KEEPS RAMP AMPLITUDE CONSTANT UNDER SYNC 3831 F04 Figure 6 shows a doubling charge pump circuit that can be used to provide 2VIN gate drive for Q1. The charge pump consists of a Schottky diode from VIN to PVCC1 and a 0.1μF capacitor from PVCC1 to the switching node at the drain of Q2. This circuit provides 2VIN – VF to PVCC1 while Q1 is ON and VIN – VF while Q1 is OFF where VF is the forward voltage of the Schottky diode. Ringing at the drain of Q2 can cause transients above 2VIN at PVCC1; if VIN is higher Figure 4. External Synchronization Operation VCC controller feedback loop. As a result, the loop crossover frequency increases and it may cause the feedback loop to be unstable if the phase margin is insufficient. To overcome this problem, the LTC3831 monitors the peak voltage of the ramp signal and adjust the oscillator charging current to maintain a constant ramp peak. PVCC2 PVCC1 VIN TG Q1 LO INTERNAL CIRCUITRY VOUT BG + COUT Q2 LTC3831 3831 F05 Input Supply Considerations/Charge Pump The LTC3831 requires four supply voltages to operate: VIN for the main power input, PVCC1 and PVCC2 for MOSFET gate drive and a clean, low ripple VCC for the LTC3831 internal circuitry (Figure 5). In many applications, VCC can be powered from VIN through an RC filter. This supply can be as low as 3V. The low quiescent current (typically 800μA) allows the use of relatively large filter resistors and correspondingly small filter capacitors. 100Ω and 4.7μF usually provide adequate filtering for VCC. For best performance, connect the 4.7μF bypass capacitor as close to the LTC3831 VCC pin as possible. Gate drive for the top N-channel MOSFET Q1 is supplied from PVCC1. This supply must be above VIN (the main power Figure 5. Supplies Input VIN OPTIONAL USE FOR VIN ≥ 7V DZ 12V 1N5242 MBR0530T1 PVCC2 PVCC1 TG 0.1μF Q1 LO VOUT BG + Q2 LTC3831 COUT 3831 F06a Figure 6. Doubling Charge Pump 3831fb 11 LTC3831 APPLICATIONS INFORMATION than 7V, a 12V zener diode should be included from PVCC1 to PGND to prevent transients from damaging the circuitry at PVCC1 or the gate of Q1. For applications with a lower VIN supply, a tripling charge pump circuit shown in Figure 7 can be used to provide 2VIN and 3VIN gate drive for the external top and bottom MOSFETs respectively. This circuit provides 3VIN – 3VF to PVCC1 while Q1 is ON and 2VIN – 2VF to PVCC2 where VF is the forward voltage of the Schottky diode. The circuit requires the use of Schottky diodes to minimize forward drop across the diodes at start-up. The tripling charge pump circuit can rectify any ringing at the drain of Q2 and provide more than 3VIN at PVCC1; a 12V zener diode should be included from PVCC1 to PGND to prevent transients from damaging the circuitry at PVCC1 or the gate of Q1. The charge pump capacitors for PVCC1 refresh when the BG pin goes high and the switch node is pulled low by Q2. The BG on time becomes narrow when the LTC3831 operates at maximum duty cycle (95% typical) which can occur if the input supply rises more slowly than the soft-start capacitor or the input voltage droops during load transients. If the BG on time gets so narrow that the switch node fails to pull completely to ground, the charge pump voltage may collapse or fail to start causing excessive dissipation in external MOSFET Q1. This is most likely with low VCC voltages and high switching frequencies, coupled with large external MOSFETs that slow the BG and switch node slew rates. The LTC3831 overcomes this problem by sensing the PVCC1 voltage when TG is high. If PVCC1 is less than 2.5V above VCC, the maximum TG duty cycle is reduced to 70% by clamping the COMP pin at 1.8V (QC in the Block Diagram). This increases the BG on time and allows the charge pump capacitors to be refreshed. DZ 12V 1N5242 10μF 1N5817 VIN 1N5817 1N5817 PVCC2 PVCC1 TG 0.1μF 0.1μF Q1 LO VOUT BG + Q2 COUT 3831 F07 LTC3831 Figure 7. Tripling Charge Pump Connecting the Ratiometric Reference Input The LTC3831 derives its ratiometric reference, VREF, using an internal resistor divider. The top and bottom of the resistor divider is connected to the R+ and R – pins respectively. This permits the output voltage to track at a ratio of the differential voltage at R+ and R–. The LTC3831 can operate with a minimum VFB of 1.1V and maximum VFB of (VCC – 1.75V). With R– connected to GND, this gives a VR+ input range of 2.2V to (2 • VCC – 3.5V). If VR+ is higher than the permitted input voltage, increase the VCC voltage to raise the input range. In a typical DDR memory termination application as shown in Figure 1, R+ is connected to VDDQ, the supply voltage of the interface, and R– to GND. The output voltage VTT is connected to the FB pin, so VTT = 0.5 • VDDQ. If a ratio greater than 0.5 is desired, it can be achieved using an external resistor divider connected to VTT and FB pin. Figure 8 shows an application that generates a VTT of 0.6 • VDDQ. For applications using an external supply to power PVCC1, this supply must also be higher than VCC by at least 2.5V to ensure normal operation. 3831fb 12 LTC3831 APPLICATIONS INFORMATION VDDQ 2.5V 5V + MBR0530T1 1μF 0.1μF + 4.7μF PVCC1 IMAX 0.1μF Q1 MBRS340T3 1k LO 1.2μH 0.1μF VTT 1.5V ±6A LTC3831 IFB FREQSET RC 15k CC 1500pF 10k TG SS 0.01μF 130k SHDN C1 33pF PVCC2 VCC CIN 330μF ×2 PGND COMP GND R+ R– MBRS340T3 Q2 BG SHDN + CIN: SANYO POSCAP 6TPB330M COUT: SANYO POSCAP 4TPB470M Q1, Q2: SILICONIX Si4410DY COUT 470μF ×3 2k 1% FB 10k 1% 3831 F08 Figure 8. Typical Application with VTT = 0.6 • VDDQ Power MOSFETs Two N-channel power MOSFETs are required for most LTC3831 circuits. These should be selected based primarily on threshold voltage and on-resistance considerations. Thermal dissipation is often a secondary concern in high efficiency designs. The required MOSFET threshold should be determined based on the available power supply voltages and/or the complexity of the gate drive charge pump scheme. In 3.3V input designs where an auxiliary 12V supply is available to power PVCC1 and PVCC2, standard MOSFETs with RDS(ON) specified at VGS = 5V or 6V can be used with good results. The current drawn from this supply varies with the MOSFETs used and the LTC3831’s operating frequency, but is generally less than 50mA. LTC3831 applications that use 5V or lower VIN voltage and doubling/tripling charge pumps to generate PVCC1 and PVCC2, do not provide enough gate drive voltage to fully enhance standard power MOSFETs. Under this condition, the effective MOSFET RDS(ON) may be quite high, raising the dissipation in the FETs and reducing efficiency. Logiclevel FETs are the recommended choice for 5V or lower voltage systems. Logic-level FETs can be fully enhanced with a doubler/tripling charge pump and will operate at maximum efficiency. After the MOSFET threshold voltage is selected, choose the RDS(ON) based on the input voltage, the output voltage, allowable power dissipation and maximum output current. In a typical LTC3831 circuit operating in continuous mode, the average inductor current is equal to the output load current. This current flows through either Q1 or Q2 with the power dissipation split up according to the duty cycle: V DC(Q1) = OUT VIN V V –V DC(Q2) = 1– OUT = IN OUT VIN VIN The RDS(ON) required for a given conduction loss can now be calculated by rearranging the relation P = I2R. RDS(ON)Q1 = RDS(ON)Q2 = PMAX(Q1) DC(Q1) • (ILOAD )2 PMAX(Q2) DC(Q2) • (ILOAD )2 = = VIN • PMAX(Q1) VOUT • (ILOAD )2 VIN • PMAX(Q2) (VIN – VOUT ) • (ILOAD )2 PMAX should be calculated based primarily on required efficiency or allowable thermal dissipation. A typical high efficiency circuit designed for 2.5V input and 1.25V at 5A 3831fb 13 LTC3831 APPLICATIONS INFORMATION output might allow no more than 3% efficiency loss at full load for each MOSFET. Assuming roughly 90% efficiency at this current level, this gives a PMAX value of: (1.25V)(5A/0.9)(0.03) = 0.21W per FET Table 1 highlights a variety of power MOSFETs that are for use in LTC3831 applications. and a required RDS(ON) of: RDS(ON)Q1 = RDS(ON)Q2 = (2.5V) • (0.21W) 2 Inductor Selection = 0.017Ω (1.25V)(5A) (2.5V) • (0.21W) (2.5V – 1.25V)(5A)2 higher PMAX value in the RDS(ON) calculations generally decreases the MOSFET cost and the circuit efficiency and increases the MOSFET heat sink requirements. = 0.017Ω Note that while the required RDS(ON) values suggest large MOSFETs, the power dissipation numbers are only 0.21W per device or less; large TO-220 packages and heat sinks are not necessarily required in high efficiency applications. Siliconix Si4410DY or International Rectifier IRF7413 (both in SO-8) or Siliconix SUD50N03-10 (TO-252) or ON Semiconductor MTD20N03HDL (DPAK) are small footprint surface mount devices with RDS(ON) values below 0.03Ω at 5V of VGS that work well in LTC3831 circuits. Using a The inductor is often the largest component in an LTC3831 design and must be chosen carefully. Choose the inductor value and type based on output slew rate requirements. The maximum rate of rise of inductor current is set by the inductor’s value, the input-to-output voltage differential and the LTC3831’s maximum duty cycle. In a typical 2.5V input 1.25V output application, the maximum rise time will be: DCMAX • (VIN – VOUT ) 1.138 A = LO LO μs where LO is the inductor value in μH. With proper frequency compensation, the combination of the inductor and output Table 1. Recommended MOSFETs for LTC3831 Applications PARTS RDS(ON) AT 25ºC (mΩ) RATED CURRENT (A) TYPICAL INPUT CAPACITANCE CISS (pF) θJC (°C/W) TJMAX (°C) 1.8 175 Siliconix SUD50N03-10 T0-252 19 15 at 25°C 10 at 100°C 3200 Siliconix Si4410DY SO-8 20 10 at 25°C 8 at 70°C 2700 ON Semiconductor MTD20N03DHL D PAK 35 20 at 25°C 16 at 100°C 880 1.67 150 Fairchild FDS6670A SO-8 8 13 at 25°C 3200 25 150 Fairchild FDS6680 SO-8 10 11.5 at 25°C 2070 25 150 ON Semiconductor MTB75N03HDL DS PAK 9 75 at 25°C 59 at 100°C 4025 1 150 IR IRL3103S DD PAK 19 64 at 25°C 45 at 100°C 1600 1.4 175 IR IRLZ44 TO-220 28 50 at 25°C 36 at 100°C 3300 1 175 Fuji 2SK1388 TO-220 37 35 at 25°C 1750 2.08 150 150 Note: Please refer to the manufacturer’s data sheet for testing conditions and detailed information. 3831fb 14 LTC3831 APPLICATIONS INFORMATION capacitor values determine the transient recovery time. In general, a smaller value inductor improves transient response at the expense of ripple and inductor core saturation rating. A 2μH inductor has a 0.57A/μs rise time in this application, resulting in a 8.8μs delay in responding to a 5A load current step. During this 8.8μs, the difference between the inductor current and the output current is made up by the output capacitor. This action causes a temporary voltage droop at the output. To minimize this effect, the inductor value should usually be in the 1μH to 5μH range for most LTC3831 circuits. To optimize performance, different combinations of input and output voltages and expected loads may require different inductor values. Once the required value is known, the inductor core type can be chosen based on peak current and efficiency requirements. Peak current in the inductor will be equal to the maximum output load current plus half of the peakto-peak inductor ripple current. Ripple current is set by the inductor value, the input and output voltage and the operating frequency. The ripple current is approximately equal to: (V − V ) • (VOUT ) IRIPPLE = IN OUT fOSC • LO • VIN fOSC = LTC3831 oscillator frequency = 200kHz LO = Inductor value Solving this equation with our typical 2.5V to 1.25V application with 2μH inductor, we get: (2.5V – 1.25V) • 1.25V = 1.56AP-P 200kHz • 2μH • 2.5V Peak inductor current at 5A load: 5A + (1.56A/2) = 5.78A The ripple current should generally be between 10% and 40% of the output current. The inductor must be able to withstand this peak current without saturating, and the copper resistance in the winding should be kept as low as possible to minimize resistive power loss. Note that in circuits not employing the current limit function, the current in the inductor may rise above this maximum under short circuit or fault conditions; the inductor should be sized accordingly to withstand this additional current. Inductors with gradual saturation characteristics are often the best choice. Input and Output Capacitors A typical LTC3831 design places significant demands on both the input and the output capacitors. During normal steady load operation, a buck converter like the LTC3831 draws square waves of current from the input supply at the switching frequency. The peak current value is equal to the output load current plus 1/2 the peak-to-peak ripple current. Most of this current is supplied by the input bypass capacitor. The resulting RMS current flow in the input capacitor heats it and causes premature capacitor failure in extreme cases. Maximum RMS current occurs with 50% PWM duty cycle, giving an RMS current value equal to IOUT/2. A low ESR input capacitor with an adequate ripple current rating must be used to ensure reliable operation. Note that capacitor manufacturers’ ripple current ratings are often based on only 2000 hours (3 months) lifetime at rated temperature. Further derating of the input capacitor ripple current beyond the manufacturer’s specification is recommended to extend the useful life of the circuit. Lower operating temperature has the largest effect on capacitor longevity. The output capacitor in a buck converter under steadystate conditions sees much less ripple current than the input capacitor. Peak-to-peak current is equal to inductor ripple current, usually 10% to 40% of the total load current. Output capacitor duty places a premium not on power dissipation but on ESR. During an output load transient, the output capacitor must supply all of the additional load current demanded by the load until the LTC3831 adjusts the inductor current to the new value. ESR in the output capacitor results in a step in the output voltage equal to the ESR value multiplied by the change in load current. A 5A load step with a 0.05Ω ESR output capacitor results in a 250mV output voltage shift; this is 20% of the output voltage for a 1.25V supply! Because of the strong relationship between output capacitor ESR and output load transient response, choose the output capacitor for ESR, 3831fb 15 LTC3831 APPLICATIONS INFORMATION Electrolytic capacitors, such as the Sanyo MV-WX series, rated for use in switching power supplies with specified ripple current ratings and ESR, can be used effectively in LTC3831 applications. OS-CON electrolytic capacitors from Sanyo and other manufacturers give excellent performance and have a very high performance/size ratio for electrolytic capacitors. Surface mount applications can use either electrolytic or dry tantalum capacitors. Tantalum capacitors must be surge tested and specified for use in switching power supplies. Low cost, generic tantalums are known to have very short lives followed by explosive deaths in switching power supply applications. Other capacitor series that can be used include Sanyo POSCAPs and the Panasonic SP line. The ESR of the output capacitor and the output capacitor value form a zero at the frequency: fESR = 1/ ⎡⎣2π(ESR)(COUT )⎤⎦ The compensation network used with the error amplifier must provide enough phase margin at the 0dB crossover frequency for the overall open-loop transfer function. The zero and pole from the compensation network are: fZ = 1/[2π(RC)(CC)] and fP = 1/[2π(RC)(C1)] respectively. Figure 9b shows the Bode plot of the overall transfer function. Although a mathematical approach to frequency compensation can be used, the added complication of input and/or output filters, unknown capacitor ESR, and gross operating A common way to lower ESR and raise ripple current capability is to parallel several capacitors. A typical LTC3831 application might exhibit 5A input ripple current. Sanyo OS-CON capacitors, part number 10SA220M (220μF/10V), feature 2.3A allowable ripple current at 85°C; three in parallel at the input (to withstand the input ripple current) meet the above requirements. Similarly, Sanyo POSCAP 4TPB470M (470μF/4V) capacitors have a maximum rated ESR of 0.04Ω, three in parallel lower the net output capacitor ESR to 0.013Ω. LTC3831 VFB – not for capacitance value. A capacitor with suitable ESR will usually have a larger capacitance value than is needed to control steady-state output ripple. COMP 10 VTT 6 ERR + RC VREF C1 CC 3831 F09a Figure 9a. Compensation Pin Hook-Up Feedback Loop Compensation Loop stability is affected by the values of the inductor, the output capacitor, the output capacitor ESR, the error amplifier transconductance and the error amplifier compensation network. The inductor and the output capacitor create a double pole at the frequency: fLC = 1/ ⎡⎣2π (LO )(COUT ) ⎤⎦ fSW = LTC3831 SWITCHING FREQUENCY fCO = CLOSED-LOOP CROSSOVER FREQUENCY fZ LOOP GAIN The LTC3831 voltage feedback loop is compensated at the COMP pin, which is the output node of the error amplifier. The feedback loop is generally compensated with an RC + C network from COMP to GND as shown in Figure 9a. 20dB/DECADE fP fLC fESR fCO FREQUENCY 3830 F10b Figure 9b. Bode Plot of the LTC3831 Overall Transfer Function 3831fb 16 LTC3831 APPLICATIONS INFORMATION point changes with input voltage, load current variations, all suggest a more practical empirical method. This can be done by injecting a transient current at the load and using an RC network box to iterate toward the final values, or by obtaining the optimum loop response using a network analyzer to find the actual loop poles and zeros. Table 2 shows the suggested compensation component value for 2.5V to 1.25V applications based on the 470μF Sanyo POSCAP 4TPB470M output capacitors. Table 2. Recommended Compensation Network for 2.5V to 1.25V Applications Using Multiple Paralleled 470μF Sanyo POSCAP 4TPB470M Output Capacitors Table 3. Recommended Compensation Network for 2.5V to 1.25V Applications Using Multiple Paralleled 1500μF Sanyo MV-WX Output Capacitors Table 3 shows the suggested compensation component values for 2.5V to 1.25V applications based on 1500μF Sanyo MV-WX output capacitors. L1 (μH) COUT(μF) RC(kΩ) CC(nF) C1(pF) L1 (μH) COUT(μF) RC(kΩ) CC(nF) C1(pF) 1.2 1410 6.8 3.3 33 1.2 4500 20 1.5 120 1.2 2820 15 3.3 33 1.2 6000 27 1 82 1.2 4700 22 1.5 33 1.2 9000 43 0.47 56 2.4 1410 15 10 33 2.4 4500 51 1 56 2.4 2820 36 3.3 10 2.4 6000 62 1 33 2.4 4700 47 4.7 10 2.4 9000 82 0.47 27 4.7 1410 33 10 10 4.7 4500 82 3.3 33 4.7 2820 68 22 10 4.7 6000 100 1 15 4.7 4700 120 10 10 4.7 9000 150 1 15 PVCC VIN 100Ω + 1μF 4.7μF VCC 1μF PVCC1 LTC3831 GND C1 PGND CIN 0.1μF Q1 TG IMAX NC 10k + PVCC2 FREQSET IFB SHDN R+ COMP BG SS FB OPTIONAL MBRS340T3 1k VOUT MBRS340T3 Q2 + R– RC CC GND CSS LO COUT PGND PGND GND 3830 F11 Figure 10. Typical Schematic Showing Layout Considerations 3831fb 17 LTC3831 APPLICATIONS INFORMATION LAYOUT CONSIDERATIONS When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3831. These items are also illustrated graphically in the layout diagram of Figure 10. The thicker lines show the high current paths. Note that at 5A current levels or above, current density in the PC board itself is a serious concern. Traces carrying high current should be as wide as possible. For example, a PCB fabricated with 2oz copper requires a minimum trace width of 0.15” to carry 5A. 1. In general, layout should begin with the location of the power devices. Be sure to orient the power circuitry so that a clean power flow path is achieved. Conductor widths should be maximized and lengths minimized. After you are satisfied with the power path, the control circuitry should be laid out. It is much easier to find routes for the relatively small traces in the control circuits than it is to find circuitous routes for high current paths. 2. The GND and PGND pins should be shorted directly at the LTC3831. This helps to minimize internal ground disturbances in the LTC3831 and prevents differences in ground potential from disrupting internal circuit operation. This connection should then tie into the ground plane at a single point, preferably at a fairly quiet point in the circuit such as close to the output capacitors. This is not always practical, however, due to physical constraints. Another reasonably good point to make this connection is between the output capaci- tors and the source connection of the bottom MOSFET Q2. Do not tie this single point ground in the trace run between the Q2 source and the input capacitor ground, as this area of the ground plane will be very noisy. 3. The small-signal resistors and capacitors for frequency compensation and soft-start should be located very close to their respective pins and the ground ends connected to the signal ground pin through a separate trace. Do not connect these parts to the ground plane! 4. The VCC, PVCC1 and PVCC2 decoupling capacitors should be as close to the LTC3831 as possible. The 4.7μF and 1μF bypass capacitors shown at VCC, PVCC1 and PVCC2 will help provide optimum regulation performance. 5. The (+) plate of CIN should be connected as close as possible to the drain of the upper MOSFET, Q1. An additional 1μF ceramic capacitor between VIN and power ground is recommended. 6. The VFB pin is very sensitive to pickup from the switching node. Care should be taken to isolate VFB from possible capacitive coupling to the inductor switching signal. 7. In a typical SSTL application, if the R+ pin is to be connected to VDDQ, which is also the main supply voltage for the switching regulator, do not connect R+ along the high current flow path; it should be connected to the SSTL interface supply output. R– should be connected to the interface supply GND. 8. Kelvin sense IMAX and IFB at Q1’s drain and source pins. 3831fb 18 LTC3831 PACKAGE DESCRIPTION GN Package 16-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 05-08-1641) .189 – .196* (4.801 – 4.978) .045 ±.005 16 15 14 13 12 11 10 9 .254 MIN .009 (0.229) REF .150 – .165 .229 – .244 (5.817 – 6.198) .0165 ± .0015 .150 – .157** (3.810 – 3.988) .0250 BSC RECOMMENDED SOLDER PAD LAYOUT 1 .015 ± .004 × 45° (0.38 ± 0.10) .007 – .0098 (0.178 – 0.249) 2 3 4 5 6 7 .0532 – .0688 (1.35 – 1.75) 8 .004 – .0098 (0.102 – 0.249) 0° – 8° TYP .016 – .050 (0.406 – 1.270) NOTE: 1. CONTROLLING DIMENSION: INCHES INCHES 2. DIMENSIONS ARE IN (MILLIMETERS) .008 – .012 (0.203 – 0.305) TYP .0250 (0.635) BSC GN16 (SSOP) 0204 3. DRAWING NOT TO SCALE *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE 3831fb 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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 19 LTC3831 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC1530 High Power Synchronous Switching Regulator Controller SO-8 with Current Limit. No RSENSE™ required LTC1628 Family Dual High Efficiency 2-Phase Synchronous Step-Down Controllers Constant Frequency, Standby 5V and 3.3V LDOs, 3.5 ≤ VIN ≤ 36V LTC1702 Dual High Efficiency 2-Phase Synchronous Step-Down Controller 550kHz, 25MHz GBW Voltage Mode, VIN ≤ 7V, No RSENSE LTC1703 Dual 550kHz Synchronous 2-Phase Switching Regulator Controller with Mobile VID LTC1702 with Mobile VID for Portable Systems LTC1705 Dual 550kHz Synchronous 2-Phase Switching Regulator Controller with 5-Bit VID Plus LDO Provides Core, I/O and CLK Supplies for Portable Systems LTC1709 Family 2-Phase, 5-Bit Desktop VID Synchronous Step-Down Controllers Current Mode, VIN to 36V, IOUT Up to 42A, Various VID Tables LTC1736 Synchronous Step-Down Controller with 5-Bit Mobile VID control Fault Protection, Power Good, 3.5V to 36V Input, Current Mode LTC1753 5-Bit Desktop VID Prorammable Synchronous Switching Regulator 1.3V to 3.5V Programmable Output Using Internal 5-Bit DAC LTC1778 Wide Operating Range/Step-Down Controller, No RSENSE VIN Up to 36V, Current Mode, Power Good LTC1873 Dual Synchronous Switching Regulator with 5-Bit Desktop VID 1.3V to 3.5V Programmable Core Output Plus I/O Output LTC1929 2-Phase, Synchronous High Efficiency Converter with Mobile VID Current Mode Ensures Accurate Current Sensing VIN Up to 36V, IOUT Up to 40A LTC3413 3A, Monolithic Synchronous Regulator for DDR/QDR Memory Termination Low RDS(ON) Internal Switch: 85mΩ, ±3A Output Current (Sink and Source), VOUT = VREF/2 LTC3713 Low Input Voltage, High Power, No RSENSE, Step-Down Synchronous Controller Minimum VIN: 1.5V, Uses Standard Logic-Level N-Channel MOSFETs LTC3778 Wide Operating Range, No RSENSE, Step-Down Controller VIN Up to 36V, Current Mode, Power Good, Stable with Ceramic COUT LTC3717 Wide VIN Step-Down Controller for DDR Memory Termination Current Mode Operation, VOUT = 1/2 VIN, VOUT (VTT) Tracks VIN (VDDQ), No RSENSE, Symmetrical Sink and Source Output Current Limit LTC3718 Bus Termination Supply for Low Voltage VIN 1.5V ≤ VIN, Generates 5V Gate Drive for Standard N-Ch MOSFETs, 2A ≤ IOUT ≤ 25A LTC3832 High Power Synchronous Switching Regulator Controller VOUT as low as 0.6V No RSENSE is a trademark of Linear Technology Corporation. 3831fb 20 Linear Technology Corporation LT 0908 REV B • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2001