Dual Output Step-Down Controller Produces 10% Accurate, Efficient and Reliable High Current Rails – Design Note 478 Mike Shriver and Theo Phillips Introduction The LTC ®3855 makes it possible to generate high current rails with the accuracy and efficiency to satisfy the most demanding requirements of today’s leading edge network, telecommunications and server applications. This 2-phase, dual output synchronous buck controller includes strong gate drivers that support operation with per-phase currents above 20A. The accurate 0.6V ±0.75% reference and its integrated differential amplifier (diff amp) allow remote sensing of the output of critical rails. This controller has an output voltage range from 0.6V to 12.5V when used without the diff amp and from 0.6V to 3.3V with the diff amp. The LTC3855 uses the reliable peak current mode architecture to achieve a fast and accurate current limit and real time current sharing. Its current sense comparators are designed to sense the inductor current with either a sense resistor or with inductor DCR sensing. DCR sensing offers 17.4k RNTC2 100k NEXT TO L2 the advantage of reduced conducted power losses, since the current is measured using the voltage drop across the already-present inductor DC resistance—eliminating the losses incurred by adding a sense resistor. The trade-off is that DCR sensing is less accurate than a dedicated sense resistor because the DCR varies from part to part and over temperature. The LTC3855 uses an innovative scheme to improve the accuracy of DCR sensing by compensating for the DCR’s variation with temperature. 1.5V/20A and 1.2V/20A Buck Converter with Remote Sensing and NTC Compensated DCR Sensing Figure 1 shows a 1.5V/20A and 1.2V/20A dual phase converter with DCR sensing, operating at 325kHz. High efficiency is achieved with the strong gate drivers, L, LT, LTC, LTM, Burst Mode, PolyPhase, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 17.4k 49.9k RNTC1 100k NEXT TO L1 0.1μF 49.9k 82.5k 325kHz 20k ITH1 BOOST1 VFB1 PGND1 SGND TK/SS2 EXTVCC SENSE2– PGND2 34.0k + 4.42k L1 0.47μH DCR = 1.0mΩ TYP, 1.2mΩ MAX CMDSH-3 2.2Ω 4.7μF 0.1μF COUT1 100μF 6.3V s2 M2 RJK0330DPB s2 + COUT2 330μF 2.5V s2 VOUT1 1.5V 20A L1, L2: VISHAY IHLP5050FD-01, 0.47μH COUT1, COUT3: MURATA GRM31CR60J107ME39l COUT2, COUT4: SANYO 2R5TPE330M9 RNTC1, RNTC2: MURATA NCP18WF104J03RB 0.1μF CMDSH-3 DCR = 1.0mΩ TYP, 1.2mΩ MAX M3 RJK0305DPB 100k PGOOD1 PGOOD2 100k M4 RJK0330DPB s2 L2 0.47μH 4.42k 34.0k COUT3 100μF 6.3V s2 DN478 F01 Figure 1. Dual 1.5V/20A and 1.2V/20A Converter Operating at fSW = 325kHz. The Entire Circuit Fits within 1.7in2 with Both Sides of the Board Populated 05/10/478 VIN 4.5V TO 14V 180μF 16V BOOST2 SW2 20k NC DIFFP 0.1μF PGOOD2 BG2 PGOOD1 SENSE2+ ILIM2 0.1μF INTVCC ILIM1 1.2nF 0.1μF VIN LTC3855 ITH2 DIFFN 5.23k VFB2 RUN2 20k M1 RJK0305DPB BG1 DIFFOUT 150pF SW1 CLKOUT TG1 TG2 1nF 150pF PHSASMD FREQ TK/SS1 6.04k MODE/PLLIN ITEMP2 RUN1 0.1μF ITEMP1 SENSE1– 30.1k SENSE1+ 22μF s3 + COUT4 330μF 2.5V s2 VOUT2 1.2V 20A 95 30 5A 25 IOUT(MAX) 20 DCR SENSING IMPLEMENTED WITH TEMPERATURE COMPENSATION WITHOUT TEMPERATURE COMPENSATION 15 6 EFFICIENCY 10 90 5 1.5V 1.2V 85 4 1.5V 1.2V 80 3 POWER LOSS 75 2 VIN = 12V fSW = 325kHz MODE = CCM 70 POWER LOSS (W) EFFICIENCY (%) 35 CURRENT LIMIT (A) optimized dead-time and DCR sensing. The typical full load efficiency for the 1.5V and 1.2V rails is 89.5% and 87.8%, respectively (see Figure 2). The 1.2V output is remotely sensed with the diff amp. As a result, the 1.2V rail’s output accuracy is unaffected by the voltage drops across the VOUT and GND planes. The load step response for the 1.2V rail is shown in Figure 3. 1 65 0 0 5 10 15 LOAD CURRENT (A) 20 25 DN478 F02 Figure 2. Efficiency and Power Loss of the 1.5V/20A and 1.2V/20A Converter 1.2VO(AC) 100mV/DIV 20A LOAD STEP 10A/DIV 10A 50μs/DIV DN478 F03 Figure 3. 50% to 100% Load Step Response for the 1.2V Rail at VIN = 12V The LTC3855 features precise current limit thresholds of 30mV, 50mV and 75mV, selected via the ILIM pins. The current limit threshold can be raised by biasing the ITEMP pins below 500mV. Since the ITEMP pins source 10μA of current, the peak current sense voltage can be increased by inserting a resistance of less than 50k from the ITEMP pin to ground. By placing an inexpensive NTC thermistor next to the inductor and connecting this thermistor to a linearization network from the ITEMP pin to ground, the current limit temperature coefficient can be greatly reduced. As Figure 4 illustrates, the compensated current limit is 20% higher than the uncompensated current limit at 110°C. Another use for the ITEMP pins is to increase the current limit for conventional DCR sense and RSENSE applications. PolyPhase® Operation The LTC3855 provides inherently fast cycle-by-cycle current sharing due to its peak current mode architecture, Data Sheet Download www.linear.com 0 100 40 60 80 20 INDUCTOR TEMPERATURE (°C) 120 DN478 F04 Figure 4. Measured Current Limit of the 1.2V Rail Over Temperature with and without Temperature Compensation plus very tight DC current sharing for single output PolyPhase applications. Up to 12-phase operation can be achieved by daisy chaining the CLKOUT and MODE/ PLLIN pins and by programming the phase separation with the PHASMD pins. A major advantage of PolyPhase operation is the reduction of the required input and output capacitance due to ripple current cancellation. Also, single output PolyPhase applications have a faster load step response due to a smaller clock delay. Other Important Features The switching frequency of the LTC3855 can be programmed between 250kHz and 770kHz with a resistor placed from the FREQ pin to ground or synchronized to an external clock in this frequency range using its internally compensated phase lock loop. High efficiency at light load is achieved by selecting either Burst Mode® operation or discontinuous mode operation, as opposed to continuous conduction mode. The LTC3855 can be used for inputs up to 38V, and its 100ns typical minimum ontime allows for high step-down ratios. The LTC3855 has a TK/SS pin for programmable soft-start or rail tracking, and dedicated RUN and PGOOD pins for each channel. The LTC3855 comes in either a 6mm × 6mm QFN or a thermally enhanced 38-lead TSSOP package. Conclusion The LTC3855 is a high performance dual output buck converter intended for low output voltage, high output current supplies. It provides the user with the benefits of a precise 0.6V 0.75% reference, an accurate current limit and high efficiency. For applications help, call (408) 432-1900, Ext. 3720 Linear Technology Corporation dn478f LT/TP 0510 116K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2010 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●