advertisement Dual Output Supply Powers FPGAs from 3.3V and 5V Inputs Design Note 311 Haresh Patel Introduction FPGAs often require multiple power supplies: one for the core voltage (usually 1.8V, but sometimes as low as 1.2V) and at least one more for the I/O circuitry (often 2.5V). The available input supply is either 3.3V or 5V. One way to provide the multiple step-down conversions is via multiple switcher-based power supplies; however, this may be more complicated and cumbersome than is warranted, especially if the I/O does not draw much current. In such instances, the dual output LTC ®1704 is a simple and space-saving option. It can supply two voltages with its versatile high frequency switcher and its space-saving LDO controller. Circuit Description With a 5V input, the switcher channel can generate core voltages from 0.8V to 3.3V at currents up to 15A. This 550kHz switcher reduces required LC filter size while VIN 5V + C6 1μF 1 R2 18.7k C7 1μF R4 11k C8 C9 4700pF 68pF R8 1k R9 10k R10 8.06k The LDO uses an external pass transistor to regulate the I/O voltage. The LTC1704 provides base currents up to 30mA to control the output voltage under varying load conditions. The circuit shown in Figure 1 provides 2.5V at 2A from a 3.3V supply. The uncommitted collector and emitter of the external pass transistor gives the LDO versatility. It can convert 1.8V to 1.5V or 1.2V. At lower power levels, the input voltage can be as high as 5V. Output current would then be limited by thermal considerations. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and No RSENSE is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. D1 MBRO520LT1 C4 10μF 6.3V C1-C3 150μF 10V providing fast response to dynamic loads. Efficiency of the switcher is very high and features No RSENSE ™ technology, where the output current is sensed via the MOSFET’s on resistance, to improve efficiency compared with regulators using a sense resistor. 2 TG R12 6.8Ω BOOST 16 15 PVCC 3 14 LTC1704 IMAX BG 4 13 RUN/SS PGND 5 12 COMP PGOOD 6 11 VCC FB 7 10 REGDR REGLIM 8 9 GND REGFB SW L1 0.80μH CDEP105-0R8MC-88 Q1 FDS6670A R3 10Ω + Q2 FDS6670A C14-C16 180μF 4V D2 MBRS320T3 C17 1μF R5 4.99k R7 680k C11 1000pF R6 1.74k C12 2.2μF 10V + R11 806Ω VOUT2 2.5V 2A 3.3V INPUT Q3 D44H11 C10 100μF 4V C13 180pF C5 10μF 6.3V PGOOD DN311 F01 Figure 1. Dual Power Supply Using the LTC1704 05/03/311_conv VOUT1 1.8V 10A Figure 1 shows a dual output 1.8V/10A and 2.5V/2A circuit using the LTC1704. This is a typical FPGA application where 1.8V is the core voltage and 2.5V is for I/O. In this case, the switcher supplies 1.8V and the LDO supplies 2.5V, taking power from either 3.3V or 5V for the external pass transistor. The switcher channel uses all N-channel MOSFETs for improved efficiency and lower cost. R9 and R10 program the output voltage. Type III compensation—C9, R4, C8, R8 and C13—allows maximum flexibility in the choice of LC filter components. The current limit circuit uses the RDS(ON) of the bottom MOSFET to sense inductor current. A 10μA current from the IMAX pin flowing into R2 produces the reference voltage for current limit. The current limit circuit discharges the soft start capacitor C7 to control output current. The linear regulator uses an external high gain low VCESAT NPN series pass transistor Q3. The output voltage is 0.8V • (1 + R6/R11). The maximum output voltage is limited to (VCC – VDRV – VBE) and by (VC(Q5) – VCESAT ). 100 Figure 2 shows the efficiency of the 1.8V output over a 1A to 10A current range, over which the efficiency remains close to 90%. Figure 3 shows the load step response of the switcher to a 4A to 10A load step. At each edge of the load step, less than 50mV transient deviation occurs when using three 180μF 4V solid polymer capacitors. Conclusion The LTC1704 is suitable for applications requiring a high power switcher and a moderate power linear regulator where the cost and complexity of a second switcher would be unjustifiable. For applications that require more power from the 2nd output than is practical with a linear regulator, the LTC1702A is a good choice with its two switchers that can deliver up to 15A each. VIN = 5V 90 VOUT 50mV/DIV AC COUPLED 80 EFFICIENCY (%) The maximum driver voltage drop (VDRV ) is 1.1V at 30mA. Limiting the base drive current provides short circuit protection. R7 programs max base current drive. Pulling REGLIM down to below 0.8V turns off the LDO. 70 1.8VOUT 60 50 40 10A 30 4A 20 10 0 0A 20μs/DIV 0 5 OUTPUT CURRENT (A) DC311 F03 10 Figure 3. Step Load Response of 1.8V Output DN311 F02 Figure 2. 1.8V Efficiency Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn311f_conv LT/TP 0503 351.5K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2003