High Efficiency, High Density 3-Phase Supply Delivers 60A with Power Saving Stage Shedding, Active Voltage Positioning and Nonlinear Control for Superior Load Step Response Design Note 489 Jian Li and Kerry Holliday Introduction The LTC ®3829 is a feature-rich single output 3-phase 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 LTC3829’s features include: • 4.5V to 38V input range and 0.6V to 5V output range • 3-phase operation for low input current ripple and output voltage ripple with Stage Shedding™ mode to yield high light load efficiency • On-chip drivers in a 38-pin 5mm × 7mm QFN (or 38-pin FE) package to satisfy demanding space requirements L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks and Stage Shedding is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. VIN VIN 10μF 16V X5R 2.2Ω VIN 100k 0.1μF DIFFOUT 30.1k 13.5k 13 14 40.2k 15 VOSENSE – 1 VOSENSE + 2 0Ω 66.5Ω DIFFOUT EXTVCC PGOOD ITEMP 38 4 23 17 37 39 9 RUN IFAST MODE ILIM VIN INTVCC VFB BOOST1 ITH ISET TG1 DIFFN SW1 DIFFP BG1 DIFFOUT BOOST2 EXTVCC PGOOD TG2 ITEMP SW2 GND BG2 BOOST3 TK/SS 5 C21 1000pF 6 7 C22 1000pF 4.7μF 16V Q1 L1 0.33μH D1 CMDSH-3 Q3 Q4 8 11 C23 1000pF TG3 SW3 BG3 S1P 180μF 16V S1N 25 RSENSE1 0.001Ω 24 33 32 0.1μF SW1 30 BG1 0.1μF 10μF 16V X5R Q1,Q5,Q9: RJK0305DPB Q3,Q4,Q7,Q8,Q11,Q12: RJK0330DPB TG2 28 SW2 29 BG2 VOSENSE+ Q5 D3 CMDSH-3 L2 0.33μH S2P S2N VOUT 19 20 330μF 2.5V SANYO w2 + VIN 26 27 100μF 6.3V X5R TG1 31 D2 CMDSH-3 LTC3829 AVP SENSE1+ CSS 0.1μF CLKOUT VIN 7V TO 14V + GND 36 SENSE3+ 1nF 34 SENSE2– CLKOUT PLLIN 47pF SENSE1– SENSE2+ 20.0k FREQ 18 16 10 35 3 180μF 16V INTVCC PLLIN SENSE3– 100pF MODE RUN 40.2k + 0.1μF Q7 RSENSE2 0.001Ω Q8 10Ω 330μF 2.5V SANYO w2 100μF 6.3V X5R 21 + 22 VIN 12 10Ω 10μF 16V X5R R26 100Ω S3N VOSENSE – R25 100Ω S3P TG3 R24 100Ω S2N R23 100Ω Q9 L3 0.33μH Q11 Q12 SW3 S2P S3P R22 100Ω S1N R21 100Ω BG3 S1P S3N RSENSE3 0.001Ω 100μF 6.3V X5R + 330μF 2.5V SANYO w2 DN489 F01 Figure 1. A 1.5V/60A 3-Phase Converter Featuring the LTC3829 04/11/489 GND VOUT 1.5V 60A • Remote output voltage sensing and inductor DCR temperature compensation for accurate regulation • Active voltage positioning (AVP) and nonlinear control ensure impressive load transient performance 1.5V/60A, 3-Phase Power Supply Figure 1 shows a 7V to 14V input, 1.5V/60A output application. The LTC3829’s three channels run 120° out-of-phase, which reduces input RMS current ripple and output voltage ripple compared to single-channel solutions. Each phase uses one top MOSFET and two bottom MOSFETs to provide up to 20A of output current. The LTC3829 includes unique features that maximize efficiency, including strong gate drivers, short dead times and a programmable Stage Shedding mode, where two of the three phases shut down at light load. Onset of Stage Shedding mode can be programmed from no load to 30% load. Figure 2 shows the efficiency of this regulator at over 86.5% with a 12V input and a 1.5V/60A output with Stage Shedding mode, dramatically increasing light load efficiency. The current mode control architecture of the LTC3829 ensures that DC load current is evenly distributed among the three channels, as shown in Figure 3. Dynamic, cycleby-cycle current sharing performance is similarly tight in the face of load transients. 90 1 PHASE 3 PHASE A fast and controlled transient response is another important requirement for modern power supplies. The LTC3829 includes two features that reduce the peakto-peak output voltage excursion during a load step: programmable nonlinear control or programmable active voltage positioning (AVP). Figure 4 shows the transient response without these features enabled. Figure 5 shows that nonlinear control improves peak-to-peak response by 17%. Figure 6 shows that AVP can achieve a 50% reduction in the amplitude of voltage spikes. Conclusion The LTC3829’s tiny 5mm × 7mm 38-pin QFN package belies its expansive feature set. It produces high efficiency with a combination of strong integrated drivers and Stage Shedding/Burst Mode® operation. It supports temperature compensated DCR sensing for high reliability. AVP and nonlinear control improve transient response with minimum output capacitance. Voltage tracking, multichip operation and external sync capability fill out its menu of features. The LTC3829 is ideal for high current applications such as telecom and datacom systems, industrial and computer systems. IO 20A/DIV 85 EFFICIENCY (%) 200mV VO 100mV/DIV 60A 100μs/DIV VIN = 12V VO = 1.5V fSW = 400kHz L = 330nH RSENSE = 1mΩ QT = RJK0305DPB QB = 2w RJK0330DPB 80 DN489 F04 Figure 4. Transient Performance without AVP and Nonlinear Control 166mV VO 100mV/DIV STAGE SHEDDING CCM 75 1 10 LOAD CURRENT (A) 100 DN489 F02 Figure 2. Efficiency Comparison of Stage Shedding vs CCM VOLTAGE ACROSS SENSE RESISTOR (mV) 22 18 60A 100μs/DIV VIN = 12V VO = 1.5V 20 IO 20A/DIV DN489 F05 Figure 5. Transient Performance with Nonlinear Control 16 14 12 VO 100mV/DIV 10 8 125mV 6 4 FIRST PHASE SECOND PHASE THIRD PHASE 2 0 –2 0 10 20 30 40 LOAD CURRENT (A) 50 60 DN489 F03 IO 20A/DIV 60A 100μs/DIV DN489 F06 Figure 6. Transient Performance with AVP Figure 3. Current Sharing Performance Between Phases Data Sheet Download www.linear.com For applications help, call (408) 432-1900, Ext. 3967 Linear Technology Corporation dn489f LT/AP 0411 226K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2011 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●