Sub-Milliohm DCR Current Sensing with Accurate Multiphase Current Sharing for High Current Power Supplies Muthu Subramanian, Tuan Nguyen and Theo Phillips The increasing functional complexity of electronic devices, combined with the desire for higher microprocessor computational speed and the quest for eco-friendly electronics, places stringent requirements on power supplies. High current supplies are expected to operate at top efficiency. In order to minimize conduction losses, power supplies are placed closer to the load, and multiple power stages are used on the same board. Individual power stages have had to shrink in size to fit the available board area. To achieve the best performance per board area, controllers must work with external power stages such as power blocks, DrMOS or external gate drivers with MOSFETs. SS VIN 7V TO 14V 20k 4.22k 40.2k VOUT LTC3861 VSNSOUT1 COMP2 FB2 SS CLKIN 500kHz EXTERNAL SYNC INPUT 1Ω 2.2µF 16V 53.6k RUN1 ILIM1 SGND ISNS1P ISNS1N ISNS2N ISNS2P SGND ILIM2 RUN2 VCC VIN RUN VIN VIN SS VCC RUN1 ILIM1 SGND ISNS1P ISNS1N ISNS2N ISNS2P SGND ILIM2 RUN2 34k 34 | January 2013 : LT Journal of Analog Innovation VIN RUN SS COUT2 : SANYO 2R5TPE330M9 COUT1 : MURATA GRM32ER60J107ME20 L1, L2, L3, L4 : COILCRAFT XAL1010-221ME VCC BOOT PHASE V FDMF6707B IN DISB VSWH PWM VDRV PGND VCIN SMOD CGND 0.22µF 2.87k L2 0.22µH 10k 0.22µF CIN4 22µF × 2 10k 16V 1Ω 2.2µF 16V 53.6k SS2 FREQ CLKIN CLKOUT PHSMD PGOOD2 PWMEN2 PWM2 VCC FB1 COMP1 VSNSP1,2 VSNSN1,2 VSNSOUT1,2 LTC3861 COMP2 FB2 2.87k 0.22µF 2.2µF 16V VCC RUN VCC SS1 VINSNS CONFIG IAVG PGOOD1 PWMEN1 PWM1 Figure 1. 4-phase, VIN =12V, VOUT = 0.9V/120A, step-down converter with DrMOS, fSW = 500kHz L1 0.22µH 10k 0.22µF VCC 34k 100pF 1µF IN VSWH DISB PWM PGND VDRV VCIN SMOD CGND 2.2µF 16V CIN3 22µF × 2 10k 16V 1Ω 2.2µF 16V VCC 5V BOOT PHASE V FDMF6707B VOUT 0.9V/ 120A SS2 FREQ CLKIN CLKOUT PHSMD PGOOD2 PWMEN2 PWM2 VCC VCC RUN FB1 COMP1 VSNSP1 VSNSN1 470pF 0.22µF CIN2 22µF × 2 10k 16V 100k 1µF 3.3nF VIN VCC VCC SS1 VINSNS CONFIG IAVG PGOOD1 PWMEN1 PWM1 374Ω 100pF 0.1µF CIN1 180µF VCC 5V 4.7nF IAVG BOOT PHASE FDMF6707B VIN VSWH DISB PWM PGND VDRV VCIN SMOD CGND 0.22µF 0.22µF VCC 1Ω 2.2µF 16V 2.2µF 16V 2.87k 10k 2.2µF 16V CIN5 22µF × 2 10k 16V L3 0.22µH BOOT PHASE V FDMF6707B IN VSWH DISB PWM PGND VDRV VCIN SMOD CGND 10k 0.22µF 2.87k L4 0.22µH COUT1 100µF × 8 6.3V COUT2 330µF × 12 2.5V design ideas The LTC3861 uses a constant-frequency voltage mode architecture, combined with a very low offset, high bandwidth error amplifier and a remote output sense differential amplifier per channel for excellent transient response and output regulation. 35 CURRENT IN EACH PHASE (A) 30 25 20 15 10 CHANNEL 4 CHANNEL 3 CHANNEL 2 CHANNEL 1 5 0 0 20 80 100 40 60 TOTAL LOAD CURRENT (A) Figure 3. Thermal image at 0.9V/120A, 400 FPM, fSW = 500kHz 120 Figure 2. Current sharing between the four phases with varying load current independent of any offsets between power ground and the controller’s ground. The LTC3861 is a multiphase dual output synchronous step-down DC/DC controller that can operate with power blocks, DrMOS and external gate drivers. It is flexible enough to operate as a dual output, 3+1 output, or up to a 12-phase single output step-down converter. In a voltage mode control loop, the error amplifier output is compared to a sawtooth ramp, which directly controls the converter duty cycle. The output voltage of the error amplifier depends on the magnitude of the error signal between the differentially sensed output voltage and the amplifier reference voltage. The 600mV reference has an accuracy of ±0.75% over a 0°C to 85°C temperature 100 90 EFFICIENCY (%) The LTC3861 uses a constant-frequency voltage mode architecture, combined with a very low offset, high bandwidth error amplifier and a remote output sense differential amplifier per channel for excellent transient response and output regulation. The error and differential amplifiers have a gain bandwidth of 40MHz, high enough not to affect the main loop compensation and transient behavior, especially when all ceramic low ESR output capacitors are used to minimize output ripple. The differential amplifiers sense the resistively divided feedback voltage differentially over the full output range from 0.6V to VCC – 0.5V, ensuring that the LTC3861 sees the actual output voltage, 80 range. This, combined with the low offset of the amplifiers, guarantees a total output regulation accuracy of ±1.3% over a –40°C to 125°C temperature range. The LTC3861 achieves outstanding line transient response using a feedforward correction scheme, which instantaneously adjusts the duty cycle to compensate for changes in input voltage, significantly reducing output overshoot and undershoot. This scheme makes the DC loop gain independent of the input voltage. The converter has a minimum on-time of 20ns, which is suitable for high stepdown ratio converters operating at high frequencies. The operating frequency is resistor programmable from 250kHz to 2.25MHz, or can be synchronized to an external clock through an onboard PLL. MULTIPHASE CURRENT SHARING 70 60 VIN = 12V VOUT = 0.9V fSW = 500kHz 0 20 40 80 60 ILOAD (A) 100 120 Figure 4. 4-phase, 0.9V/120A converter efficiency The controller allows the use of sense resistors or lossless inductor DCR current sensing to maintain current balance between phases and to provide overcurrent protection. In multiphase operation, the LTC3861 incorporates an auxiliary current January 2013 : LT Journal of Analog Innovation | 35 In multiphase operation, the LTC3861 incorporates an auxiliary current share loop, which is activated by configuring the FB pin and by adding an external capacitor on the IAVG pin. The maximum current sense mismatch between phases is ±1.25mV over the –40°C to 125°C temperature range. The current sharing accuracy between the four phases at full 120A load current is ±2.15%. 5mV (±0.28%) VOUT 2mV/DIV VOUT 20mV/DIV 60mV (±3.3%) 120A IOUT 20A/DIV 500ns/DIV 10µs/DIV Figure 5. Steady state voltage ripple share loop, which is activated by configuring the FB pin and by adding an external capacitor on the IAVG pin. The voltage on the IAVG pin corresponds to the instantaneous average inductor current of the master phase. Each slave phase integrates the difference between its inductor current and the master’s. A resistor connected to the ILIM pin sets the threshold for the positive and negative overcurrent fault protection comparator. The maximum current sense mismatch between phases is ±1.25mV over –40°C to 125°C temperature range. CIRCUIT PERFORMANCE Figure 1 shows a high efficiency 12V to 0.9V/120A 4-phase step-down converter with low DCR sensing. An inductor with DCR = 0.45mΩ is used in the design. The current sharing accuracy between the four phases at full 120A load current 36 | January 2013 : LT Journal of Analog Innovation 90A Figure 6. 30A Load step transient response from 90A to 120A is ±2.15%. Figure 2 shows the current sharing between phases as a function of varying load current. Figure 3 shows the thermal image at 120A load, and the hottest spot occurs on the MOSFETs of channels 2 and 3. The efficiency at full 120A load is close to 86%, as illustrated in Figure 4. Figure 5 shows the steady state voltage ripple as approximately ±0.3% of output voltage. Load step transient analysis was performed by stepping the load from 75% to 100% of full load. This resulted in a 30A load step from 90A to 120A. The peak to peak voltage overshoot and undershoot during a load step was 60mV, which is about ±3.3% of output voltage. CONCLUSION The LTC3861 is a voltage mode controller with accurate current sharing of up to 12 phases in parallel. Since it has a 3-state PWM output instead of a builtin gate driver output, the controller can be placed further from high current paths. Because output voltage is differentially sensed, offsets between power ground and the LTC3861’s ground do not affect load regulation. The LTC3861 works with DrMOS, power blocks, and external MOSFETs with an LTC4449 gate driver. It is used in high current distributed power systems, DSP, FPGA, and ASIC supplies, datacom and telecom systems, and industrial power supplies. The LTC3861 is available in a 36-pin 5mm × 6mm QFN package. In addition, the LTC3861-1 is a pin-compatible dropin replacement for the LTC3860, available in a 32-pin 5mm × 5mm QFN package. n