Dual Phase Buck Controller Drives High Density 1.2V/60A Supply with Sub-Milliohm DCR Sensing Mike Shriver Designers of low output voltage rails for communication, networking, server and industrial systems are challenged to achieve greater load currents and higher efficiency in diminishing board space. The LTC3774 dual output buck controller eases this burden by interfacing easily with DrMOS devices, providing high efficiency and small size by integrating MOSFET and gate driver in the same package. The LTC3774 can sense current across the inductor’s DCR, with values as low as 0.2mΩ, improving efficiency by eliminating the need for a discrete sense resistor. The LTC3774’s peak current mode architecture provides cycle-by-cycle current limit, inherent cycle-by-cycle current sharing and easy to design type II compensation. HIGH EFFICIENCY CONVERTER WITH A SMALL FOOTPRINT Figure 1 shows a dual phase 1.2V/60A LTC3774 converter operating at a switching frequency of 400kHz. The power stage for each phase is the FDMF6820A DrMOS, which comes in a 6mm × 6mm QFN package, and a 0.3µ H single winding ferrite inductor with a typical DCR of 0.325mΩ. The resulting full load efficiency is 89.8%, as shown in Figure 2. The core of the converter has a current density of 50A /in2. DrMOS INTERFACE The PWM outputs of the LTC3774 are designed to drive DrMOS devices with a 3-state PWM input. When the PWM signal is high, the top FET is on, and when the PWM signal is low, the bottom FET is on. When the PWM signal is floating, both the top FET and bottom FET are off. This state is used to block inductor reverse current when the LTC3774 is set up for either pulse-skipping mode or Burst Mode operation, providing a smooth turn-on into a prebiased output. The PWM outputs of the LTC3774 can 32 | July 2014 : LT Journal of Analog Innovation interface with power block devices and gate drivers with external MOSFETs. produces less than a 1°C temperature difference between the two phases. DCR SENSING The LTC3774 provides accurate output voltage regulation. The output of each phase is sensed with a differential amplifier placed after the feedback divider to compensate for any PCB IR drops. The total regulated feedback voltage accuracy is ±0.75% over temperature. The output voltage range of the LTC3774 is 0.6V to 3.5V. The ultralow DCR sensing capability is a result of an innovative current sensing technique that improves the signal-tonoise ratio of the current sense signal. The external filter tied to the SNSA+ pin amplifies the AC portion of the DCRsensed current; the DC current is sensed via the SNSD+ pin, internally amplified and summed with the AC portion. This reconstructed current sense signal seen by the LTC3774’s current comparator is effectively amplified by a factor of five, allowing the converter to remain stable and retain current limit accuracy for inductor DCR values as low as 0.2mΩ. The LTC3774 offers five current limit settings between 10mV and 30mV with a worst-case error over temperature of ±1.25mV. With current mode contol, current sharing between phases is tightly balanced, as shown by the thermal image shown in Figure 3. The 1.2V/60A converter operating at full load PolyPhase OPERATION AND IMPROVING ROBUSTNESS The LTC3774 features CLKIN and CLKOUT pins for PolyPhase operation up to 12 phases. PolyPhase operation reduces ripple current for the input capacitors and in cases where the phases are tied together, reduces output voltage ripple and provides faster load step response. Further improvements in the reliability of single output, redundant (N + 1), PolyPhase converters can be achieved by placing Hot Swap™ circuits on the input and ideal diode circuits on the output of each phase. If a MOSFET failure occurs, the fault is isolated and the output is protected and continues to regulate. Reliability design ideas 10k 22pF VCIN 10k INTVCC 330pF 931Ω + ITEMP1 ITH1 VOSNS1– VOSNS1+ TK/SS1 HIZB1 PWMEN1 PWM1 RUN1 GND 0.22µF COUT1 100µF 4× 0.22µF 0.22µF 10k 1µF 5V BIAS 1Ω 10k 2.2µF VIN 22µF CGND 22µF Figure 1. Dual phase, 1.2V/60A LTC3774 converter operating at fSW = 400kHz, 7V ≤ VIN ≤ 14V PGND VIN The LTC3774 is a high performance dual output buck controller intended for low output voltage, high output current supplies with DrMOS and ultralow DCR inductors. It yields high efficiency, an accurate current limit, a precise 0.6V ±0.75% feedback voltage and fault isolation. n EFFICIENCY (%) 20 90 15 POWER STAGE: DrMOS = FDMF6820A RBOOST = 2.2Ω L = WÜRTH 744301033 (0.33µH, 0.32mΩ) 80 12 8 75 70 POWER LOSS (W) CONCLUSION 95 85 L2 0.33µH L1, L2: WÜRTH 744301033 COUT1: MURATA GRM31CR60J107ME39L COUT2: SANYO 2R5TPE330M9 10k Other features include soft recovery from an output overload, optional NTC compensated DCR sensing, a phase-lockable switching frequency range of 200kHz to 1.2MHz and an input voltage range of 4.5V to 38V. COUT2 330µF 6× 931Ω 0.22µF PHASE VSWH 10k is further improved with the LTC3774’s HIZB pin, which floats the PWM output when a fault is detected, allowing for a more predictable shutdown of the DrMOS. 1.2V/60A VOUT 2.2Ω PWM BOOT FDMF6820A VIN 4.64k 2.2µF VCIN INTVCC VDRV 2.2Ω 180µF LTC3774 DISB VIN 0.22µF SNSD1+ SNS1– SNSA1+ PGOOD1 PGOOD2 SNSA2+ SNS2– SNSD2+ ITEMP2 ITH2 VOSNS2– VOSNS2+ TK/SS2 HIZB2 PWMEN2 PWM2 RUN2 GND 4.7µF 10k SMOD INTVCC ILIM1 PHSMD FREQ MODE/PLLIN CLKOUT INTVCC VIN ILIM2 L1 0.33µH 4.64k 10k 10k 37.4k 0.22µF PHASE VSWH DISB 22µF CGND 22µF PGND VIN 3.01k 3.3nF 2.2Ω PWM BOOT FDMF6820A VIN 7V TO 14V RUN1 2.2µF VDRV 2.2µF 0.01µF VIN 5V BIAS 1Ω SMOD 10k 4 0 10 20 30 40 50 LOAD CURRENT (A) 60 70 0 Figure 2. Efficiency and power loss curves for circuit shown in Figure 1. VIN = 12V, VOUT = 1.2V Figure 3. Thermal image of the circuit shown in Figure 1. fSW = 400kHz, VIN = 12V, VOUT = 1.2V, IOUT = 60A, no airflow and 21°C ambient July 2014 : LT Journal of Analog Innovation | 33