DESIGN IDEAS LTC1709 Low Cost, High Efficiency 42A Converters with VID Control Reduce Input and Output Capacitors by Wei Chen Introduction used, resulting in a faster load transient response. This, plus the 5-bit VID table, makes these devices particularly attractive for CPU power supply applications. Two VID tables are available to comply with the VRM 8.4 (LTC1709-8) and VRM9.0 (LTC1709-9) specifications. The LTC1709-8/LTC1709-9 are dual, current mode, PolyPhase™ controllers that drive two synchronous buck stages out of phase. This architecture reduces the number of input and output capacitors without increasing the switching frequency. The relatively low switching frequency and integrated high current MOSFET drivers help provide high powerconversion efficiency for low voltage, high current applications. Because of the output ripple current cancellation, lower value inductors can be C1 1000pF Design Example Figure 1 shows the schematic diagram of a 42A power supply for the AMD Athlon microprocessor. With only one IC, eight tiny SO-8 MOS- D2, C18 AND C19 ARE NEEDED ONLY IF VIN IS < 5V; OTHERWISE, THEY CAN BE OMITTED AND POINTS A AND B SHORTED INTVCC R2 2.7k 1 2 C5 0.01µF 3 4 R4 51k 5 C7 120pF C8 1.2nF R1 10Ω C2 0.1µF C4 0.1µF R6 15k D2 BAT54S A R3 10k 6 7 R7 15k 8 9 10 C12 470pF 11 12 13 14 15 1000pF 16 17 18 FETs and two 1µH low profile, surface mount inductors, an efficiency of 86% is achieved for a 5V input and a 1.6V/42A output. Greater than 85% efficiency can be maintained throughout the load range of 3A–42A, as shown in Figure 2. Because of the low input voltage, the reverse recovery losses in the body diodes of the bottom MOSFETs are not significant. No Schottky diodes are required in parallel with the bottom MOSFETs in this application. LTC1709EG-8 RUN/SS NC SENSE1 + TG1 SENSE1 – SW1 BOOST1 EAIN VIN PLLFLTR PLLIN BG1 NC EXTVCC ITH INTVCC SGND PGND VDIFFOUT BG2 VOS– BOOST2 VOS+ SW2 SENSE2 – TG2 SENSE2 + PGOOD ATTENOUT VBIAS ATTENIN VID4 VID0 VID3 VID1 VID2 B C18 0.1µF Q1 Q2 5VIN+ + C3 1µF CIN 5VIN– C19 0.1µF 36 35 34 33 L1 1µH C6 0.47µF 32 31 30 29 28 Q3 Q4 Q5 Q6 R5 0.002Ω C16 1µF D1 BAT54A C10 2.2µF + C11 10µF 6.3V 27 26 C14 0.47µF 25 24 C13 1µF L2 1µH 23 22 R9 10Ω 21 C17 0.1µF 20 Q7 R8 0.002Ω VOUT+ C20 1µF Q8 + 19 COUT VOUT– R10 51Ω 100k 5V VID0 VID1 VID2 VID3 VID4 PGOOD CIN: 4 RUBYCON ALUM ELECT CAPACITORS 1500µF AT 6.3V COUT: 6 RUBYCON ALUM ELECT CAPACITORS 1500µF AT 6.3V OR 4 SANYO OS-CON 2R5SP1200M L1, L2: SUMIDA CEPH149-1R0MC Q1 TO Q8: FAIRCHILD FDS7760A OR SILICONIX Si4874 FREQUENCY = 200kHz R11 51Ω VOSENSE+ VOSENSE– (714) 668-8998 (619) 661-6835 (847) 956-0667 (408) 822-2126 (800) 554-5565 Figure 1. Schematic diagram of a 42A power supply using the LTC1709 24 Linear Technology Magazine • May 2000 DESIGN IDEAS 100 Table 1. Comparison of input and output ripple current for single-phase and dual-phase configurations (L = 1µH, fS = 200kHz) 1 1 19.7 10.9 2 10.1 2.9 90 EFFICIENCY (%) Input Ripple Output Ripple Phases Current (ARMS) Current (AP-P) VIN = 5V VOUT = 1.6V fS = 200kHz VOUT 50mV/DIV 80 70 1 Assumes that the single-phase circuit uses two 1.0µH/21A inductors in parallel to provide 42A output. ILOAD 20A/DIV 60 50 Table 1 compares the input and output ripple currents for single-phase and 2-phase configurations. A 2-phase converter reduces the input ripple current by 50% and the output ripple current by 75% compared to a singlephase design. The reduction in the cost and size of the input and output capacitors is significant. Figure 3 shows the measured load transient waveform. The load current changes between 2A and 42A with a slew rate of about 30A/µs. Output capacitor type and size requirements are dominated by the total ESR of the output capacitor network. Six low cost aluminum electrolytic caps (Rubycon, 1500µF/6.3V) are needed on the SMBus Fan, continued from page 23 LTC1694, which also appears in Figure 1, is a dual SMBus accelerator/ pull-up device that may be used in conjunction with the LTC1695. Boost-Start Timer, Thermal Shutdown and Overcurrent Clamp Features A DC fan typically requires a starting voltage higher than its minimum stall voltage. For example, a Micronel 5V fan requires a 3.5V starting voltage, but once started, it will run until its terminal voltage drops below 2.1V (its stall voltage). Thus, the user needs to ensure that the fan starts up properly before programming the fan voltage to a value lower than the starting voltage. Monitoring the fan’s DC current for stall conditions does not help because some fans consume almost the same amount of current at the same terminal voltage in both stalled and operating conditions. Another approach is to detect the absence of Linear Technology Magazine • May 2000 0 5 10 15 20 25 30 35 LOAD CURRENT (A) 40 45 Figure 2. Efficiency vs load current for Figure 1’s circuit 10µs/DIV Figure 3. Load transient waveforms at 40A step and 30A/µs slew rate output to meet this requirement. The maximum output voltage variations during the load transients are less than 200mVP-P. Active voltage positioning was employed in this design to keep the number of output capacitors at six (refer to Linear Technology Design Solutions 10 for more details on active voltage positioning). R4 and R6 provide the output voltage positioning with no loss of efficiency. If OSCON caps are used, four 1200µF/ 2.5V (2R51200M) capacitors will be sufficient. Conclusion fan commutation ripple current. This, however, is complex and requires customization for the characteristics of specific brands of fans. The LTC1695 offers a simple and effective solution through the use of a boost-start timer. By setting the Boost-Start Enable bit high via the system controller, the LTC1695 outputs 5V for 250ms to the fan before lowering the voltage to its programmed value (see Figure 2 for the start-up voltage profile). During a system controller Read command, bits 6 and 7 in the data byte code are defined as the Thermal Shutdown Status (THE) and the Overcurrent Fault (OCF), respectively. The rest of the data byte’s register (bits 0 to 5) are set low during host read back. The LTC1695 shuts down its PMOS pass transistor and sets the THE bit high if die junction temperature exceeds 155°C. During an overcurrent fault, the LTC1695’s overcurrent detector sets the OCF bit high and actively clamps the output cur- rent to 330mA. This protects the LTC1695’s PMOS pass transistor. Under dead short conditions (VOUT = 0), although the LTC1695 clamps the output current, the large amount of power dissipated on the chip will force the LTC1695 into thermal shutdown. These LTC1695 dual protection features protect the IC and the fan and, more importantly, alerts the host to system thermal management faults. During a fault condition, the SMBus logic continues to operate so that the host can poll the fault status data. The LTC1709 based, low voltage, high current power supply described above achieves high efficiency and small size simultaneously. The savings in the input and output capacitors, inductors and heat sinks help minimize the cost of the overall power supply. This LTC1709 circuit, with a few modifications, is also suitable for VRM9.0 applications. Refer to Linear Technology Application Note 77 for more information on the PolyPhase technique. Conclusion The LTC1695 improves battery run times and reduces acoustic noise in portable equipment. In addition, it provides important performance and protection features by controlling the operation of the equipment’s cooling fan. It comes in a SOT-23 package and is easily programmed via the SMBus interface. 25