Design Solutions 41 February 2004 Dual Output DC/DC Converter Solutions for Xilinx FPGA Based Systems Charlie Zhao INTRODUCTION Xilinx FPGAs require at least two power supplies: VCCINT for core circuitry and VCCO for I/O interface. For the latest Xilinx FPGAs, including Virtex-II Pro, Virtex-II and Spartan-3, a third auxiliary supply, VCCAUX may be needed. In most cases, VCCAUX can share a power supply with VCCO. The core voltages, VCCINT, for most Xilinx FPGAs, range from 1.2V to 2.5V. Some mature products have 3V, 3.3V or 5V core voltages. Table 1 shows the core voltage requirement for most of the FPGA device families. Typical I/O voltages (VCCO) vary from 1.2V to 3.3V. The auxiliary voltage VCCAUX is 2.5V for Virtex-II Pro and Spartan-3, and is 3.3V for Virtex-II. Each FPGA family has a specific quiescent supply current, ranging from under 100mA to about 2A. For applications with multiple FPGAs, the core supply current can be higher than 10A. With multiple voltage rails in today’s systems (FPGA, DDR memory, data converter ICs, etc.), supply sequencing and tracking are quite important for proper start-up and shutdown. Ramp time requirement should also be satisfied. For example, the recommended ramp time (tCCPO) for the core voltage VCCINT is less than 50ms during power-on. Some Xilinx FPGA families also have minimum VCCINT ramp time requirements. New dual output DC/DC regulators from Linear Technology, the LTC®3407, LTC3736 and LTC3708, greatly simplify the design of an optimal power supply solution for systems using Xilinx FPGAs. LTC3407: Dual Synchronous, 600mA, DC/DC Regulator The LTC3407 is a dual synchronous step-down DC/DC converter with integrated power switches. It provides a compact and high efficiency power solution for FPGAs with supply currents up to 600mA. The switching regulator operates from a 2.5V to 5.5V input voltage range and has an adjustable output range from 0.6V to 5V. Its internal 1A switches provide up to 96% efficiency, eliminating the need for external MOSFETs and Schottky diodes. Figure 1 is an application example for 2.5V/600mA and 1.8V/ 600mA supplies. Figure 2 shows the efficiency curves of the circuit vs load current. VIN = 2.5V TO 5.5V C1 10µF 6.3V RUN2 VIN RUN1 MODE/SYNC POR LTC3407 L2 2.2µH VOUT2 = 2.5V AT 600mA SW2 R4 887k R3 280k RESET L1 2.2µH SW1 C5 22pF C3 10µF 6.3V R5 100k C4 22pF VOUT1 = 1.8V AT 600mA VFB1 VFB2 R2 GND C1, C2, C3: TAIYO YUDEN JMK316BJ106ML R1 887k 443.5k C2 10µF 6.3V L1, L2: MURATA LQH32CN2R2M33 AN95 F01 Figure 1. High Efficiency 2.5V/600mA and 1.8V/600mA Regulators , LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. Table 1. Core Voltage Requirement for Xilinx FPGA Families VCCINT Virtex-II Pro Virtex-II Virtex-E Extended Memory 1.5V 1.5V 1.8V Virtex-E Virtex Spartan-3 Spartan-IIE Spartan-II Spartan-XL Spartan 1.8V 2.5V 1.2V 1.8V 2.5V 3.3V 5V 1 Design Solutions 41 100 load current requirements for most FPGA applications. The LTC3736 receives input from 2.7V to 9.8V and produces output voltages ranging from 0.6V to 9.5V. Figure␣ 3 shows that up to 95% efficiency is achieved. An application example is shown in Figure 4. 95 2.5V EFFICIENCY (%) 90 1.8V 85 80 75 In contrast to single-phase operation, the two channels of a 2-phase switching converter are operated 180 degrees out of phase. This technique interleaves the current pulses coming from the topside MOSFET switches, greatly reducing the total RMS input ripple current. This in turn allows the use of smaller and lower cost input capacitors, reduces the EMI attenuation requirement and improves operating efficiency. 70 VIN = 3.3V Burst Mode OPERATION NO LOAD ON OTHER CHANNEL 65 60 10 100 LOAD CURRENT (mA) 1 1000 AN95 F02 Figure 2. LTC3407 Efficiency Curve The LTC3407 has a constant 1.5MHz switching frequency, allowing the use of tiny inductors and capacitors. Selectable Burst Mode® operation provides high efficiency at light loads. The IC has short-circuit protection and a power-on reset (power good) output. It is available in small thermally enhanced 10-lead MSOP and 3 × 3 DFN packages. 100 VO = 2.5V EFFICIENCY (%) 90 LTC3736: 2-Phase, Dual Synchronous, DC/DC Controller for 5A Loads 80 70 60 50 40 The LTC3736 is a 2-phase dual synchronous step down DC/DC controller. Power supplies using the LTC3736 can provide 5A at both outputs with a 5V input, meeting the C3 1µF 10V Q1B Si7540DP LTC3736 20 Q1A Si7540DP VOUT1 2.5V (5A AT 5VIN) + C6 150µF 4V 21 L1 1.5µH IHLP2525CZ-01 17 22 19 18 R3 118k 1% 14 6 23 24 R4 187k 1% R6 59k 1% 1 C11 220pF R7 59k 1% R8 15k VO = 1.8V VIN = 3.3V 10 100 1000 LOAD CURRENT (mA) 1 AN95 F03 Figure 3. Efficiency vs Load Current for the LTC3736 Converter R1 10Ω 5 VIN PGND1 SENSE1+ TG1 SW1 BG1 SYNC/FCB RUN/SS TRACK IPRG1 VFB1 ITH1 4 VIN 2.7V TO 9.8V C1 22µF 10V R2 1M Q2B Si7540DP 25 PGND 12 PGND2 11 SENSE2+ 15 TG2 10 SW2 13 BG2 16 PGND 9 PGOOD 3 PLLLPF 2 IPRG2 7 VFB2 8 ITH2 L2 1.5µH IHLP2525CZ-01 Q2A Si7540DP C9 150µF 4V + R5 118k 1% C14 220pF R10 59k 1% R9 15k AN95 F04 Figure 4. High Efficiency 2.5V/5A and 1.8V/5A Dual Output Converter with Output Tracking 2 10000 VOUT2 1.8V (5A AT 5VIN) Design Solutions 41 Figure 5 compares the input waveforms for a representative single-phase dual switching converter to the 2-phase dual switching converter. Figure 6 shows how the RMS input current varies for single phase and 2-phase dual controllers with 2.5V and 1.8V outputs over a wide input voltage range. For most applications, 2-phase operation will reduce the input capacitor RMS current requirement to that of just one channel operating at maximum current and 50% duty cycle. The LTC3736 has a default switching frequency at 550kHz, but can be externally synchronized from 300kHz to 750kHz. The LTC3736 provides output tracking for controlled ramp-up of two supply rails, programmable current limit, output overvoltage protection, power good output and selectable Burst Mode operation for high efficiency light load operations. The IC is available in the tiny (4mm × 4mm) thermally enhanced QFN package or the 24-lead SSOP package. LTC3708: 2-phase, Dual Synchronous, DC/DC Controller for 15A Loads The core supply voltages of the latest Xilinx FPGAs have decreased towards 1V. The Virtex-II pro family requires 1.5V VCCINT and the Spartan-3 family needs only 1.2V VCCINT. In the meantime, these FPGAs demand more current from the power supplies. Some systems use more than ten FPGAs per board so the resulting total supply current can easily exceed 10A. The LTC3708-based dual output supply is an ideal choice for such applications. Single Phase Dual Controller 2-Phase Dual Controller SW1 (V) SW2 (V) The LTC3708 is a 2-phase dual synchronous step-down DC/DC controller with a wide input voltage range: from 3.3V up to 36V. Its output voltage can be programmed down to 0.6V. Figure 7 shows the schematic of a dual output 2.5V/15A and 1.8V/15A converter. As shown in Figure 8, up to 95% efficiency can be achieved. The LTC3708 has output voltage up/down tracking capability. The IC allows both coincident or ratiometric tracking, as shown in Figures 9 and 10. The ramp rate can be selected by a soft-start capacitor from RUN/SS pin to ground. Multiple LTC3708s can easily be daisy-chained in applications requiring more than two voltages to be tracked. The 2-phase operation of the LTC3708 reduces power loss and noise, and lowers the input-filtering requirement. The constant on time, valley current mode control in the LTC3708 allows fast transient response and thus minimizes the number of output capacitors. An internal phaselocked loop allows the IC to be synchronized to an external clock for applications with more than two output rails. The LTC3708 also features programmable current limit, output overvoltage protection and power good output. The IC is available in the 5mm × 5mm QFN package. Conclusion An optimal power solution for multirail supply systems incorporating the latest Xilinx FPGAs should provide multiple outputs with supply tracking/sequencing. As board real estate becomes more expensive, the power supply must be more efficient and smaller while supplying higher current in high-end applications. Linear Technology’s latest dual output power management ICs: LTC3407, LTC3736 and LTC3708, successfully address these challenges. For data sheets and additional information on other power solutions for Xilinx FPGAs, visit Linear Technology’s web site at www.linear.com. 2.0 INPUT CAPACITOR RMS CURRENT IL1 IL2 IIN 1.8 SINGLE PHASE DUAL CONTROLLER 1.6 1.4 2- PHASE DUAL CONTROLLER 1.2 1.0 0.8 0.6 0.4 VOUT1 = 2.5V/2A VOUT2 = 1.8V/2A 0.2 AN95 F05 0 2 Figure 5. Example Waveforms for a Single-Phase Dual Converter vs the 2-Phase LTC3736 3 4 8 6 5 7 INPUT VOLTAGE (V) 9 10 AN95 F06 Figure 6. RMS Input Current Comparision Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 3 Design Solutions 41 VIN 3.3V TO 28V CIN 10µF 35V ×4 + 4.7µF CMDSH-3 17 VCC M1 27 28 L1 1.4µH + COUT1 330µF 4V ×2 12.1k 0.22µF 26 25 M3 D1 B340A 22 19.1k 24 23 3 6 1.5M VIN 29 2 10 33k 6.04k CMDSH-3 10Ω 1µF VOUT1 2.5V 15A M1, M2: RENESAS HAT2168 M3, M4: RENESAS HAT2165 COUT1: SANYO POSCAP 4TPD330M COUT2: SANYO POSCAP 2R5TPD470M L1, L2: PANASONIC ETQP3H 5V 1 10k 0.01µF 6.04k 180pF 21 30 DRVCC PWRGD TG2 TG1 BOOST2 BOOST1 SW1 SENSE1+ LTC3708 SENSE2+ BG2 BG1 SENSE1– SW2 SENSE2– PGND2 PGND1 VFB1 VFB2 TRACK2 FCB ION1 ION2 ITH1 ITH2 INTLPF EXTLPF RUN/SS TRACK1 SGND 5 VRNG1 VRNG2 32 11 0.1µF M2 14 13 L2 1.2µH 0.22µF 15 16 VOUT2 1.8V 15A D2 B340A M4 20 + 12.1k 18 COUT2 470µF 2.5V ×2 19 7 31 12 1.07M fIN VIN 8 9 4 10k 6.04k 0.01µF 100k 25k 33k 5V 180pF AN95 F07 Figure 7. High Efficiency 2.5V/15A and 1.8V/15A Dual Output Converter with Output Tracking 100 EFFICIENCY (%) 95 90 85 80 75 70 0.01 20VIN TO 2.5VOUT 5VIN TO 2.5VOUT 20VIN TO 1.8VOUT 5VIN TO 1.8VOUT 0.1 1 LOAD CURRENT (A) 10 15 AN95 F08 Figure 8. Efficiency vs Load Current for the LTC3708 Converter 500mV/DIV 500mV/DIV 2ms/DIV AN95 F09 Figure 9. Up/Down Coincident Tracking 4 Linear Technology Corporation 2ms/DIV AN95 F10 Figure 10. Up/Down Ratiometric Tracking dsol41 LT/TP 0204 1K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2004