High Efficiency PolyPhase Power Supply Delivers 30A per Phase with Ultralow Inductor DCR Sensing and Fast Transient Response Jian Li and Gina Le 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’s features include: 60k D1 CB1 0.1µF IFAST PHASMD CLKOUT PGOOD MODE/PLLIN BOOST1 SW1 EXTVCC BG1 TAVG M2 715Ω 220nF 220nF RB1 13.3k COUT3 470µF ×2 RUN1,2 COUT4 100µF ×2 RA1 20k 1.5nF 220pF 10k 22 | January 2014 : LT Journal of Analog Innovation M3 D1, D2: CMDSH-3 L1: WÜRTH 744301025 0.25µH DCR = 0.32mΩ L2: WÜRTH 744301033 0.33µH DCR = 0.32mΩ M1, M3: BSC050NE2LS M2, M4: BSC010NE2LSI L2 0.33µH (0.32mΩ DCR) CB2 0.1µF SW2 M4 BG2 SNS1– SNS2– SNSD2+ TCOMP2 VOSNS2+ VOSNS2– ITH2 FREQ TK/SS1 TK/SS2 SELECT PIN ABBREVIATIONS SNSA1+, SNSA2+: Positive AC Current Sense Comparator Inputs SNSD1+, SNSD2+: Positive DC Current Sense Comparator Inputs SNS1–, SNS2–: Negative AC and DC Current Sense Comparator Inputs 60k 270µF 50V BOOST2 TRSET1 SNSA1+ 0.1µF D2 TG2 PGND TRSET2 SNSA2+ SNSD1+ TCOMP1 VOSNS1+ VOSNS1– ITH1 10µF ×4 4.7µF INTVCC ENTMPB TG1 LTC3875 M1 L1 0.25µH (0.32mΩ DCR) VIN ILIM 20k + Similar to LTC3866, the LTC3875 employs a unique current sensing architecture to enhance its signal-to-noise ratio, enabling current mode control via a small sense signal of a very low inductor DCR, 1mΩ or less. As a result, the efficiency is greatly VIN 4.5V TO 14V Figure 1. A dual-output converter (1.0V at 30A and 1.5V at 30A) featuring the LTC3875 VOUT1 1V 30A Figure 1 shows a typical 4.5V~14V input, dual-output solution. The LTC3875’s two channels run relative to each other with a 180° phase shift, reducing the input RMS current ripple and capacitor size. Each phase has one top MOSFET and one bottom MOSFET to provide up to 30A of output current. •Fast transient response, facilitating high density design with less output capacitance. •Remote output voltage sensing and ±0.5% reference (0.6V) window for accurate regulation. •On-chip drivers in a 6mm × 6mm QFN package to satisfy demanding space requirements. •Easy parallel multiphase operation for high current applications. •4.5V to 38V input range and 0.6V to 3.5V output range •Proprietary current mode architecture enhances the signal-to-noise ratio of the current sensing signal, allowing the use of ultralow DCR power inductors to maximize efficiency and reduce switching jitter. 3.57k DUAL-OUTPUT CONVERTER (1.0V AT 30A AND 1.5V AT 30A) 0.1µF 931Ω 4.64k 220nF 220nF RB2 30.1k 1.5nF 100k 150pF 10k RA2 20k VOUT2 1.5V 30A COUT1 100µF ×2 + VOSNS1+, VOSNS2+: Positive Inputs of Remote Sensing Differential Amplifiers VOSNS1–, VOSNS2–: Negative Inputs of Remote Sensing Differential Amplifiers TK/SS1, TK/SS2: Voltage Tracking and Soft Start Inputs COUT2 470µF ×2 design features The LTC3875 can sense a DCR value as low as 0.2mΩ with careful PCB layout. Moreover, an additional temperature compensation circuit can be used to guarantee the accurate current limit over a wide temperature range. 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 additional temperature compensation circuit can be used to guarantee the accurate current limit over a wide temperature range. Efficiency can be optimized with ultralow DCR inductor. As shown in Figure 2, the total solution efficiency in forced continuous mode (CCM) is 87.3% at 1.0V/30A output, and 89.8% at 1.5V/30A. The hot spot (bottom MOSFET) temperature rise is 57°C without any airflow as shown in Figure 3, where the ambient temperature is about 23°C. The LTC3875 features fast transient response and minimizes undershoot through a proprietary solution. Peak current mode control is widely adopted in switching converters due to its cycleby-cycle peak current limit and easy compensation. However, the inherent switching cycle delay of peak current mode control results in large undershoot of the output voltage when there is a load step-up. The LTC3875 overcomes undershoot by using a dynamic switching frequency adjustment scheme. The internal 95 90 EFFICIENCY (%) improved and the jitter is reduced. The current mode control yields fast cycleby-cycle current limit, current sharing and simplified feedback compensation. 85 80 75 70 VOUT1 = 1V VOUT2 = 1.5V VIN = 12V fSW = 400kHz 0 5 10 20 15 ILOAD (A) 25 30 VIN = 12V, VOUT1 = 1.0V/30A, VOUT2 = 1.5V/30A, NO AIRFLOW Figure 2. Efficiency comparison of the two channels Figure 3. Thermal test results transient detector can detect a large voltage undershoot, leading the LTC3875 to run the power stage at twice the preset switching frequency for about 20 cycles. after the fast transient is enabled. In other words, with fast transient enabled, the LTC3875 can achieve the same transient performance as without, but with 20% less output capacitance, increasing power density and reducing total cost. Compared to other nonlinear control methods, the response scheme used by the LTC3875 is linear, simplifying overall design. Figure 4 shows that switching cycle delay is reduced from 2.18µs to 1.2µs and voltage undershoot is reduced from 95mV to 67.5mV (29% reduction) at 15A load up Figure 4. Transient comparison (a) Fast transient disabled VOUT 50mV/DIV (b) Fast transient enabled 95mV VOUT 50mV/DIV 67.5mV DOUBLING THE CLK VSW 10V/DIV VSW 10V/DIV IOUT 10A/DIV 0A–15A IOUT 10A/DIV 0A–15A 10µs/DIV VIN = 12V VOUT = 1.5V 15A LOAD STEP 10µs/DIV VIN = 12V VOUT = 1.5V 15A LOAD STEP January 2014 : LT Journal of Analog Innovation | 23 VIN 4.5V TO 14V Figure 5. Converter uses two channels of the LTC3875 for a single 1V output with a 60A rating 60k D1 ILIM 20k L1 0.25µH (0.32mΩ DCR) 3.57k BOOST1 SW1 EXTVCC BG1 TAVG M2 715Ω 220nF VOUT COUT3 470µF ×2 COUT4 100µF ×2 INTVCC PHASMD CLKOUT PGOOD IFAST MODE/PLLIN ENTMPB TG1 LTC3875 CB1 0.1µF 220nF + VIN RUN1,2 M1 220pF Figure 6. DC current sharing of 60A solution shown in Figure 5 VIN = 12V VOUT = 1V TRSET1 SNSA1+ PGND TRSET2 SNSA2+ SNS1– SNS2– SNSD1+ TCOMP1 VOSNS1+ VOSNS1– ITH1 SNSD2+ TCOMP2 VOSNS2+ VOSNS2– ITH2 FREQ TK/SS1 TK/SS2 10 20 40 30 ILOAD (A) 50 PHASE CURRENT (A) 24 | January 2014 : LT Journal of Analog Innovation 3.57k 220nF 220nF RB 13.3k 1.5nF 100k 10k RA 20k VOUT 1V 60A COUT1 100µF ×2 + COUT2 470µF ×2 VOSNS1+, VOSNS2+: Positive Inputs of Remote Sensing Differential Amplifiers VOSNS1–, VOSNS2–: Negative Inputs of Remote Sensing Differential Amplifiers TK/SS1, TK/SS2: Voltage Tracking and Soft Start Inputs CONCLUSION Figure 7. Dynamic current sharing of 60A solution shown in Figure 5 10 0 715Ω The DC current sharing between the two channels is shown in Figure 6. The difference at full load is around 1.6A with 0.32mΩ DCR inductor. Thanks to the peak current mode control architecture, 15 0 M4 the dynamic current sharing is also very good, as shown in Figure 7. IOUT 20A/DIV IL1, IL2 10A/DIV PHASE 1 PHASE 2 L2 0.25µH (0.32mΩ DCR) can be paralleled and phase-interleaved for even higher current if required. 25 5 M3 CB2 0.1µF BG2 VOUT 100mV/DIV 20 D1, D2: CMDSH-3 L1, L2: WÜRTH 744301025 0.25µH DCR = 0.32mΩ M1, M3: BSC050NE2LS M2, M4: BSC010NE2LSI SW2 35 30 270µF 50V BOOST2 SELECT PIN ABBREVIATIONS SNSA1+, SNSA2+: Positive AC Current Sense Comparator Inputs SNSD1+, SNSD2+: Positive DC Current Sense Comparator Inputs SNS1–, SNS2–: Negative AC and DC Current Sense Comparator Inputs The LTC3875 can be easily configured as a dual-phase single-output converter for higher current solutions. Figure 5 shows a buck converter that produces a 1V, 60A output from a 12V input. Multiple ICs D2 TG2 0.1µF SINGLE OUTPUT, DUAL PHASE, HIGH CURRENT CONVERTER (12V TO 1V AT 6A) 10µF ×4 4.7µF 20µs/DIV 60 VIN = 12V VOUT = 1V 15A LOAD STEP The LTC3875 delivers high efficiency with reliable current mode control, ultralow DCR sensing and strong integrated drivers in a 6mm × 6mm 40-pin QFN. It supports temperature-compensated DCR sensing for high reliability. Its fast transient response can help improve the transient response with minimum output capacitance. Tracking, multichip operation, and external sync capability fill out its menu of features. The LTC3875 is ideal for high current applications, such as telecom and datacom systems, industrial and computer systems applications. n