4-Phase Power Supply Delivers 120A in Tiny Footprint, Features Ultralow DCR Sensing for High Efficiency Yingyi Yan, Haoran Wu and Jian Li The LTC3875 is a feature-rich dual-output synchronous buck controller that meets the power density demands of modern high speed, high capacity data processing systems, telecom systems, industrial equipment and DC power distribution systems. The LTC3875 delivers high efficiency with reliable current mode control, ultralow DCR sensing and strong integrated drivers in a 6mm × 6mm 40-pin QFN. Multiple LTC3875s can be paralleled to provide higher current, or it can be combined with the LTC3874 to deliver the same performance with a smaller footprint. The LTC3874 is a small footprint (4mm × 5mm QFN), dual PolyPhase® current mode synchronous step-down slave controller (phase extender). It is suitable for high current, multiphase applications when paired with a companion master controller, such as LTC3875. The LTC3874 can use sub-milliohm DC resistance power inductors to optimize efficiency. Immediate response to system faults guarantees reliability of the total solution. 1V V OUT, 120A CONVERTER WITH PARALLEL LTC3875s The LTC3875 can be easily configured as dual-phase, single-output operation for high current outputs. This design can be expanded with more converters and phases in parallel for even higher current. Figure 1 shows a 4.5V~14V input, singleoutput application schematic using two LTC3875s. The LTC3875s’ four channels run with 90° phase shift, reducing input RMS current ripple and required capacitor size. Figure 1. A single-output, 4-phase (1.0V/120A) converter Each phase supports 30A of current with one top MOSFET and one bottom MOSFET. The LTC3875 employs a unique current sensing architecture to enhance its signalto-noise ratio, enabling current mode control even with a small sense signal from a very low inductor DCR—1mΩ or less. As a result, efficiency is high and jitter is low. Current mode control yields fast cycle-by-cycle current limit, current sharing and easy feedback compensation. The LTC3875 can sense a DCR value as low as 0.2mΩ with careful PCB layout. The LTC3875 uses two positive sense pins SNSD+ and SNSA+ to acquire signals. The filter time constant of the SNSD+ should match the L/DCR of the output inductor, while the filter at SNSA+ should have a bandwidth five times larger than that of SNSD+. Moreover, an 35 EFFICIENCY (%) 90 85 80 75 VIN = 12V VOUT = 1V fSW = 400kHz 70 65 0 20 40 60 80 100 VIN = 12V VOUT = 1.0V IOUT = 120A 200 LFM airflow 10 | July 2015 : LT Journal of Analog Innovation Figure 3. Thermal scan of 4-channel regulator VIN = 12V VOUT = 1V 30 25 20 15 10 CHANNEL 1 CHANNEL 2 CHANNEL 3 CHANNEL 4 5 0 120 ILOAD (A) Figure 2. Efficiency of circuit in Figure 1 INDIVIDUAL CHANNEL CURRENT (A) 95 0 20 40 80 60 ILOAD (A) 100 120 Figure 4. DC current sharing is balanced among the four channels, even at very high current loads design features 2.2Ω 10µF ×2 4.7µF D1 VIN TG2 MT2 0.25µH DCR = 0.32mΩ VOUT INTVCC BOOST2 0.1µF L2 PHASMD SNSD2+ 220nF 715Ω 220nF SNS1– SNSA2+ SNSA1+ VOSNS2+ ITH1 VOSNS1– ITH2 VOSNS2– IFAST TK/SS1 EXTVCC TK/SS2 PGOOD FREQ 220pF 100k TAVG 13.3k 220nF 220nF 715Ω 100k 3.01k ILIM TRSET2 4.02k 330µF 2.5V ×12 VOSNS1+ RUN2 CLKOUT 0.1µF + 100µF 6.3V ×14 3.57k SNSD1+ RUN1 2.2nF ENTEMPB SNS2– L1 MB1 BG1 LTC3875 TCOMP1 TCOMP2 VOUT 1V 120A 0.25µH DCR = 0.32mΩ 0.1µF SW1 BG2 3.57k 1µF MT1 TG1 BOOST1 SW2 MB2 10µF ×2 D2 VIN 4.5V TO 14V 20k TRSET1 1k MODE/PLLIN SGND/PGND GND 10µF ×2 4.7µF D3 0.25µH DCR = 0.32mΩ VOUT L4 BG1 LTC3875 TCOMP1 TCOMP2 PHASMD 3.57k SNSD2+ 220nF 220nF SNSA2+ SNSA1+ RUN1 VOSNS1+ RUN2 VOSNS2+ ITH1 VOSNS1– ITH2 VOSNS2– 0.1µF IFAST TK/SS1 EXTVCC TK/SS2 PGOOD FREQ TRSET2 100k TAVG L3 MB3 3.57k SNSD1+ SNS1– 0.25µH DCR = 0.32mΩ 0.1µF ENTEMPB SNS2– CLKOUT 100pF SW1 BG2 1µF MT3 TG1 BOOST1 SW2 MB4 715Ω INTVCC BOOST2 0.1µF 10µF ×2 D4 VIN TG2 MT4 2.2Ω 220nF 220nF 715Ω 3.01k ILIM TRSET1 MODE/PLLIN SGND/PGND 1k D1–D4: CMDSH-3 L1–L4: WÜRTH 744301025 MTx: BSC050NE2LS MBx: BSC010NE2LSI GND July 2015 : LT Journal of Analog Innovation | 11 Figure 5. A single-output, 4-phase (1.0V/120A) converter featuring LTC3875 and LTC3874 2.2Ω 10µF ×2 4.7µF D2 VIN TG2 MT2 0.25µH DCR = 0.32mΩ L2 INTVCC BOOST2 0.1µF SW1 SNSD2+ 220nF 715Ω 220nF ENTEMPB SNS1– SNSA2+ SNSA1+ RUN2 VOSNS2+ ITH1 VOSNS1– ITH2 VOSNS2– 0.1µF TK/SS1 EXTVCC TK/SS2 PGOOD 220pF 100k 220nF 715Ω 100k 3.01k ILIM FREQ TRSET2 4.02k 13.3k 220nF IFAST CLKOUT 2.2nF TAVG 20k TRSET1 1k MODE/PLLIN SGND/PGND GND 2.2Ω 10µF ×2 D4 4.7µF 0.25µH DCR = 0.32mΩ TG0 BOOST0 0.1µF L3 MB4 715Ω 220nF SW1 BG1 ISENSE0 ISENSE1+ ISENSE0– ISENSE1– LOWDCR FAULT1 MODE0 ILIM MODE1 ITH0 PHASMD SYNC 12 | July 2015 : LT Journal of Analog Innovation 0.25µH DCR = 0.32mΩ L4 MB3 715Ω 220nF EXTVCC FAULT0 ITH1 120k 0.1µF LTC3874 + 1µF MT3 BOOST1 BG0 RUN1 100pF INTVCC TG1 SW0 RUN0 2N7002 10µF ×2 D3 VIN MT4 + 100µF 6.3V ×14 VOSNS1+ RUN1 10nF VOUT 1V 120A 3.57k SNSD1+ SNS2– L1 MB1 BG1 LTC3875 TCOMP1 TCOMP2 PHASMD 0.25µH DCR = 0.32mΩ 0.1µF BG2 3.57k 1µF MT1 TG1 BOOST1 SW2 MB2 10µF ×2 D1 VIN 4.5V TO 14V FREQ GND 75k GND D1–D4: CMDSH-3 L1–L4: WÜRTH 744301025 MTx: BSC050NE2LS MBx: BSC010NE2LSI 330µF 2.5V ×12 design features The LTC3875 delivers an outsized set of features for its small 6mm × 6mm 40-pin QFN. It offers high efficiency with reliable current mode control, ultralow DCR sensing and strong integrated drivers. Tracking, multichip operation, and external sync capability fill out its menu of features. additional temperature compensation circuit can be used to guarantee the accurate current limit over a wide temperature range, and DCR variation. Efficiency can be optimized with an ultralow DCR inductor. As shown in Figure 2, the total solution efficiency in forced continuous mode (CCM) is 87.1% at 12V input and 1.0V, 120A output. The hot spot (bottom MOSFET) temperature rise is 58.1°C with 200 LFM airflow as shown in Figure 3, where the ambient temperature is about 25°C. The DC current sharing among the four channels is shown in Figure 4. The difference at full load is about 2.0A (±3.5%) with a 0.32mΩ DCR inductor. THE LTC3874 SLAVE CONTROLLER REDUCES SOLUTION SIZE AND COMPONENT COUNT IN ALTERNATE 1V, 120A CONVERTER Figure 5 shows an alternative to the 4.5V~14V input, single-output application shown in Figure 1—in this case using an LTC3875 and an LTC3874. The LTC3874 phase extender acts as a slave controller, but it supports all the programmable features as well as fault protection. • ITH pins of the LTC3875 and LTC3874 are connected for current sharing. •The CLKOUT pin of the LTC3875 is connected to the SYNC pin of the LTC3874 to synchronize switching frequency. •The MODE pin of the LTC3874 is connected to PGOOD, which allows DCM operation during start-up period for pre-bias load condition. •The FAULT pin of the LTC3874 is pulled up to the INTVCC pin and is connected to the PGOOD pin of LTC3875 via a TK/SS pin voltage-controlled MOSFET. When the PGOOD pin is pulled low due to a fault, the LTC3874 can shut down both channels for protection purposes. CONCLUSION The LTC3875 delivers an outsized set of features for its small 6mm × 6mm 40-pin QFN. It offers high efficiency with reliable current mode control, ultralow DCR sensing and strong integrated drivers. Tracking, multichip operation, and external sync capability fill out its menu of features. Furthermore, the slave controller LTC3874 offers a smaller footprint solution when paired with the LTC3875. The LTC3875 and LTC3874 are ideal for high current applications, such as telecom and datacom systems, industrial and computer systems applications. n Like the LTC3875, the LTC3874’s current mode control is accurate even with sense signals from an inductor DCR below 1mΩ. Compared to the master LTC3875, the LTC3874 simplifies pinout and uses only one set of RC components for DCR current sensing. The filter time constant of the RC filter should have a bandwidth five times larger than that of the L/DCR of the output inductor. The total solution efficiency and thermal performance is similar to that of the two-LTC3875 solution. The DC current sharing among four channels is accurate. The difference at full load is about 1.6A with a 0.32m Ω DCR inductor. July 2015 : LT Journal of Analog Innovation | 13