DESIGN IDEAS High Efficiency PolyPhase Converter Combines Power from Multiple Inputs by Wei Chen and Craig Varga Introduction A Typical Application As more functions are integrated into one IC, the power drawn by a single IC can easily exceed the capability of a single input power source. One solution is to use several available power sources to obtain the required output power, drawing some percentage of the total power from each source. The LTC1929 PolyPhase™ controller provides a simple solution to this problem. Design Details The LTC1929 is a PolyPhase dual, current mode controller. It is capable of driving two synchronous buck channels 180 degrees out of phase to reduce output switching ripple cur- rent and voltage. One buck stage receives its input power from the 12V input and the other receives its power from the 5V input. In a 2-phase design, as the inductor current in the 5V circuit increases, the inductor current in the 12V circuit decreases. This results in a smaller net ripple current flowing into the output capacitor. Since there are two intervals in one switching period where ripple cancellation takes place, the output ripple voltage of the 2-phase design is much smaller than that of a single-phase design and fewer output capacitors can be used. The currents available from a PCI connector are limited to 2A for the 5V supply and 1A for the 12V supply. In the example shown here, the load can be as high as 6A or 16.8W at 2.8V. Neither the 5V nor the 12V source is capable of providing this power. Hence, it is desirable to design a power supply that can draw currents from two power sources and whose maximum input currents from each source will not exceed the corresponding limit. With only one IC, two SO-8 MOSFETs and two small inductors, a high efficiency, low noise power supply can be built. continued on page 36 C1 1000pF C2 L1, L2: SUMIDA CEE125-7R0 (847) 956-0666 Q1, Q2: FAIRCHILD FDS6690A (207) 775-4502 C4: SANYO 16MV470AX (619) 661-6835 C9, C14: SANYO 6MV1500AX 2 3 5 7 1 8 4 10 9 11 12 13 C13 (OPT) C11 0.1µF 14 C12 1200 pF R1 10Ω D1 BAT54A 6 R3 (OPT) 1µF SENSE1+ VIN SENSE1– TG1 PLLFLTR BOOST1 PLLIN LTC1929 NC NC SW1 RUN/SS BG1 ITH EXTVCC EAIN INTVCC VDIFFOUT SGND VOS– VOS+ SENSE2– SENSE2+ PGND TG2 BOOST2 SW2 BG2 AMPMD 24 27 28 C5 L1 7µH/4A 26 0.22µF 23 C4 470µF 16V RTN + C9 1500µF 6.3V RTN R2 0.007Ω Q1B 22 21 C6 20 1µF 16 + C7 10µF 10V Ta 5VIN+ C8 1µF Q2A 18 17 C10 0.22µF L2 7µH/4A 19 15 Q2B R4 0.007Ω + C14 1500µF 6.3V R5 49.9k 1% R6 20k 1% C3 1µF Q1A 25 12VIN+ + C15 1µF VOUT+ 2.8V/6A VOUT– RTN C16 1000pF Figure 1. LTC1929 PCI-bus powered, dual-input PolyPhase power supply 28 Linear Technology Magazine • November 1999 CONTINUATIONS Conclusion LT1461, continued from page 5 This is pretty hard to determine (read impossible) if the peak-to-peak output noise is larger than this number. As a practical matter the best laboratory reference available has long-term drift of 1.5µV/mo. This performance is only available from the very best subsurface Zener references using specialized heating techniques. The LT1461 long-term drift data was taken with parts that were soldered onto PC boards as in a “real world” application. The boards were then placed in a constant-temperature oven with TA = 30°C and their outputs were scanned regularly and measured with an 8.5 digit DVM. Figure 4 shows the long-term drift of three typical LT1461S8-2.5s soldered into a PC board. This is the best performance we have measured on an IC voltage reference that is not based on a subsurface Zener. The LT1461 series reference meets the growing need for low power, high accuracy and low temperature coefficient, while simultaneously serving micropower precision regulator applications. This new bandgap reference comes in the 8-lead SO package. It is available in 2.5V and will be available in 4.096V, 5.0V and 10V options. 250 LT1461S8-2.5 3 TYPICAL PARTS SOLDERED ONTO PCB TA = 30°C 200 ppm 150 for the latest information on LTC products, visit www.linear-tech.com 100 50 0 –50 0 200 400 600 800 1000 HOURS 1200 1400 1600 1800 2000 Figure 4. Long-term drift LTC1929, continued from page 28 2.0 VIN1 = 5V VIN2 = 12V VOUT = 2.8V fS = 300kHz 1.8 INPUT CURRENT (A) 1.6 1.4 12V BUCK INDUCTOR CURRENT 1A/DIV 5V INPUT CURRENT 1.2 1.0 0.8 5V BUCK INDUCTOR CURRENT 1A/DIV 0.6 0.4 12V INPUT CURRENT 0.2 0 0 1 2 3 4 5 LOAD CURRENT (A) 6 7 Figure 2. Input currents vs load current for Figure 1’s circuit Figure 1 shows the schematic diagram of the complete power supply. The switching frequency is about 300kHz per-channel for an effective output ripple frequency of 600kHz. The inductors in both stages are 7µH. The current sense resistor is 0.007Ω for each channel. OUTPUT RIPPLE VOLTAGE 50mV/DIV 1µs/DIV Figure 3. Ripple current and voltage waveforms 12V sources are 1.66A and 0.84A, respectively, which are well below the PCI connector’s current limits. Figure 3 shows the waveforms of the inductor ripple currents and output ripple voltages. Note the ripple cancellation phenomenon. The peakto-peak switching ripple voltage at Test Results the output terminal is only 50mVP-P The overall efficiency is above 90% with one 1500µF/6.3V aluminum from 0.5A to 6A. Figure 2 shows the electrolytic capacitor. If two buck cirdistribution of two input currents as cuits are synchronized in phase, the the load current varies. The maxi- ripple voltage will be 70mVP-P, almost mum input currents for the 5V and a 50% increase. 36 Conclusion The PolyPhase technique reduces the output ripple voltage without increasing the switching frequency. High efficiency can be obtained for low output voltage applications. The LTC1929 PolyPhase controller provides a small, low cost solution for multi-input applications. If more than two inputs are needed, use the LTC1629 rather than the LTC1929. Multiple LTC1629s can be configured for 3-, 4-, 6- or even 12-phase operation. Linear Technology Magazine • November 1999