advertisement Triple Output 3-Phase Controller Saves Space and Improves Performance in High Density Power Converters Design Note 409 Mike Shriver Today’s telecommunications, server and network applications require power from a multitude of voltage rails. Having more than ten rails ranging from 5V to 1V or less is common. These boards are typically crowded with heat-producing FPGAs or microprocessors, thus demanding power converters that are both compact and highly efficient. Furthermore, the converters may need to meet other requirements such as a fast load step response and rail tracking. 220kHz, 400kHz or 560kHz, or it can be synchronized to an external clock between 160kHz and 700kHz. The controller can step down from input voltages as high as 36V and the output voltage can be programmed from 0.6V to 5V. Figure 1 shows a high density triple output DC/DC converter with each output delivering up to 5A using the LTC3773 controller. Figure 2 shows the efficiency of each output versus load current; where up to 93% efficiency is achieved. Reductions in space are realized by the use of dual channel FETs and a switching frequency of 400kHz which permits the use of 7mm × 7mm ferrite inductors. The LTC® 3773 switching regulator meets and even goes beyond the above requirements. This device is a 3-phase, triple output synchronous buck controller with built-in gate drivers packaged in either a 5mm × 7mm QFN or a 36-pin SSOP. Its switching frequency can be set to 68.1k POWER DOWN VOUT1 POWER DOWN VOUT2 10Ω POWER DOWN VOUT3 1000pF , LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 10Ω VCC 4.5V TO 7V PGOOD 150pF 6 10k 7 10k 8 0.01µF 9 0.01µF 10 11 12 20k SW2 ITH2 TG2 LTC3773 ITH3 BOOST2 VFB2 BOOST3 VFB3 TG3 TRACK2 SW3 TRACK3 BG1 SENSE2– SENSE2+ 14 15 16 17 18 31.6k 10Ω 1000pF L2, L3: TDK RLF7030T-1R5M5R4 10Ω PGOOD SENSE1+ PHASEMD SENSE1– SDB1 SGND 13 L1: TDK RLF7030T-2R2M5R4 BOOST1 10Ω 19 VDR 31 30 1 0.1µF 29 5 2.2µF 10Ω 6, 7 L1 2.2µH 28 27 26 CMDSH-3 CMDSH-3 24 0.1µF 22µF X5R 8 Si4816BDY 1 23 5 22 4 6, 7 VIN 4.7µF 25V L2 1.5µH 7mΩ 21 VOUT2 2.5V/5A + COUT2 220µF 4V 22µF X5R 2, 3 20 4.7µF 8 Si4816BDY 1 6, 7 0.1µF VIN 4.7µF 25V L3 1.5µH 7mΩ VOUT3 1.8V/5A COUT3 220µF 4V 4 2, 3 COUT1 COUT2 COUT3,: SANYO POSCAP 4TPE220MF Figure 1. High Density 5A Converter. Total Circuit Size = 1.5in2, with Components on Both Sides 02/15/409 VOUT1 3.3V/5A + 2, 3 0.1µF 25 7mΩ COUT1 220µF 4V 4 5 1000pF 10Ω BG2 BG3 5.9k 5 8 Si4816BDY CMDSH-3 SW1 CLKOUT 1nF 4 150pF 4.7µF 25V 32 ITH1 PLLIN/FC 150pF 8.2k 1nF 33 TG1 PLLFLTR 15k 3 34 VFB1 VCC 20k 1nF 35 TRACK1 SENSE3+ 2 36 SDB2 PGND 1 SENSE3– 0.01µF 37 SDB3 38 39 VIN 4.5V TO 20V 47µF 25V + 10k + 22µF X5R DN409 F01 Switching the three rails out of phase results in improved performance and reduced cost. The use of triple phase operation instead of single phase can result in a reduction of the input capacitor ripple current by over 50% as shown in Figure 3, allowing the use of less input capacitance. The outputs of two or more phases can be tied together Compensation of each rail is achieved with an RC network on the ITH pin (error amplifier output). The external ITH compensation and the current mode topology allow the designer to easily stabilize a converter with the minimal amount of output capacitance using a variety of capacitor types including conductive polymer, tantalum and ceramic while still achieving a fast load step response (see Figure 4). 94 93 3.3V EFFICIENCY (%) 92 91 90 2.5V 89 which results in output ripple current reduction as well and a faster load step response. Up to six phases can be synchronized using the CLKOUT pin (on the QFN part only). Fast and accurate current sharing among the parallel phases is a result of the LTC3773’s peak current mode architecture. 1.8V 88 87 86 85 84 1 0 2 3 LOAD CURRENT (A) 4 5 DN409 F02 INPUT CAPACITOR RIPPLE CURRENT (AMPS RMS) Figure 2. Efficiency of the LTC3773 Converter at VIN = 12V, fSW = 400kHz. One Rail Enabled at a Time 7 Other features of the LTC3773 include rail tracking and sequencing, a PGOOD signal, and three selectable light load operating modes (continuous conduction mode, Burst Mode® operation and pulse skip mode). Conclusion Now designers have a clear and practical solution when they need a compact and cost effective triple supply rail requirement in their telecom, server or network systems. 6 5 SINGLE PHASE 1.8VOUT(AC) 50mV/DIV 4 3 TRIPLE PHASE 2 5A LOAD STEP 2A/DIV 1 2.5A 0 4 6 8 10 12 14 16 INPUT VOLTAGE (V) 18 20 DN409 F03 Figure 3. Input Capacitor Ripple Current Comparison for Single Phase and Triple Phase Operation VOUT1 = 3.3V/5A, VOUT2 = 2.5V/5A, VOUT3 = 1.8V/5A Single Phase: φ1,2,3 = 0º Triple Phase: φ1,2,3 = 0º,120º, 240º 50µs/DIV DN409 F04 VIN = 12V Figure 4. 1.8V Load Step Response Data Sheet Download For applications help, call (408) 432-1900, Ext. 2134 www.linear.com Linear Technology Corporation dn409f LT/TP 0207 409K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2006 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●