DESIGN IDEAS Low Voltage, High Current DC/DC Power Supply with Load Sharing and Redundancy by Henry J. Zhang and Wei Chen Introduction As computer and networking systems get larger and faster, their supply currents continue to rise and their supply voltages continue to drop. Load currents are high enough to require that power supply designers use several power supply modules in parallel. High performance power supplies for data-processing and communication equipment must also provide exceptional reliability and fault tolerance. For example, power systems for mission-critical data processing systems are must be functional better than 99.999% of the time. To satisfy the needs of these systems, the power management solution must provide load sharing, fault tolerance and redundancy. This article presents a power management solution that offers all of these features in a relatively simple circuit that uses the LTC3729 PolyPhase® controller and LTC4350 hot swappable load-share controller. About the LTC3729 PolyPhase Controller The LTC3729 dual current mode PolyPhase controller provides the performance and reliability required by low voltage, high current computer and network systems. The Polyphase technique interleaves the clock signal of several paralleled power stages, thus reducing the input and output ripple current so less capacitance is required. Reduced ripple currents significantly improve the reliability and lifetime of the input and output capacitors. The accurate current sensing scheme of LTC3729 provides additional reliability. Current sharing amongst phases is excellent, making for a uniform thermal distribution, thus ensuring the reliability of power semiconductors and output induc34 VIN(BUS) LTC3729 DC/DC MODULE LTC4350 HOT SWAP & LOAD SHARE • • • • • • LTC3729 DC/DC MODULE LTC4350 HOT SWAP & LOAD SHARE Q2 LOAD SHARE BUS Q2 Q1 VOUT(BUS) • • • Q1 Figure 1. System block diagram of the DC/DC power supplies with load sharing and redundancy tors. Other advanced features of LTC3729 include true remote sensing, integrated high current MOSFET drivers, overvoltage protection, foldback current limit, and optional overcurrent latch-off. All of this adds up to a reliable and high performance low voltage, high current supply. Adding the LTC4350 Hot Swappable Load Share Controller To further improve system reliability, add the LTC4350 hot swappable load share controller after the LTC3729. The LTC4350 allows paralleled power supplies to share the load with fault tolerance and redundancy. To share the load amongst redundant supplies, the LTC4350 adjusts the output voltage of each supply until the current of each supply matches the value set by the share bus. The LTC4350 also isolates failed supplies by turning off the series output MOSFETs and identifies failed supplies to the system. The failed supply can then be removed and replaced with a new unit without turning off the system power. The LTC4350 improves system efficiency by allowing the use of low RDS(ON) output MOSFETs instead of ORing diodes. The LTC4350 is a universal load share controller that works with any DC/DC controller, such as the LTC1628, LTC3728, LTC1629 and LTC1778. 3.3V/40A Output Power Supply with Load Sharing and Redundancy Figures 2a and 2b show a 3.3V/40A output power supply with load sharing and redundancy. Figure 2a shows the first part of the circuit: the LTC3729 controller in a 2-phase, synchronize buck DC/DC converter that provides 3.3V/40A output from a 5V– 12V bus. The converter only requires one IC, eight tiny SO-8 size MOSFETs and two 1µH, low profile, surface mount inductors. Efficiency is 91%– 93% over the full input voltage range with a 3.3V/40A output. Figure 3 shows the efficiency of the supply over a wide 2A to 40A load range. Figure 2b shows the LTC4350 load sharing and hot swap circuit. The load current of each supply is determined by the share bus voltage. For each channel, the IOUT pin of LTC4350 is connected to the voltage feedback Linear Technology Magazine • August 2002 Linear Technology Magazine • August 2002 R2 2.7k C11 4.7nF R3 10k 470pF R6 8.66k C15 470pF R5 4.7k C10 100pF C9 0.01µF C7 0.1µF R7 25.5k SENSE2– 100Ω C17 1000pF C1 1000pF 100Ω SENSE1 – SENSE2+ 100Ω 14 13 12 11 10 9 8 7 6 5 4 3 2 1 SW1 PGOOD SENSE2 + SGND PGND TG2 SW2 BOOST2 BG2 PGND SENSE2 – VOS + VOS – VDIFFOUT SGND INTVCC ITH BG1 EXTVCC PLLIN NC VIN BOOST1 PLLFLTR EAIN SENSE1 TG1 – CLKOUT SENSE1 + U1 LTC3729EG RUN/SS 100Ω SENSE1+ CIN: COUT: L1, L2: Q1–Q8: 15 16 17 18 19 20 21 22 23 24 25 26 27 28 C2 1µF C8 0.47µF 3 2 C16 0.47µF D1 BAT54A C7 1µF (619) 661-6835 (714) 373-7334 (847) 956-0667 (800) 554-5565 C14 4.7µF 1 OS-CON 16SP100M PANASONIC EEFUE0G18R 180µF 4V SUMIDA CEP125-1R0 SILICONIX Si7440DP C12 1µF R1 10Ω EXTVCC Q7 Q5 Q3 Q1 R10 100Ω + C22 1µF ×2 Q4 Q2 SENSE1– R8 0.002Ω R4 0.002Ω SENSE1+ C24 10µF SENSE2– SENSE2+ L2 1µH D1 B540C L1 1µH D2 B540C CIN 100µF 16V ×4 COUT 180µF 4V ×6 Q8 Q6 + VSENSE (FROM LTC4350) VOUT (LOCAL) 3.3V/40A VIN 5V TO 12V DESIGN IDEAS Figure 2a. DC/DC converter portion of the redundant, load sharing power supply 35 DESIGN IDEAS VOUT (LOCAL) Q10 Q9 Q11 0.002Ω 1% 1W + 0.002Ω 1% 1W INTVCC COUT 180µF 4V ×3 VOUT (BUS) 3.3V 40A 21.5k 1% 100Ω 5% 0.22µF X5R 0 VSENSE (TO LTC3729) 100Ω 5% OPTIONAL 86.6k 1% 200k 1% 30.1K 1% VCC GATE IOUT RSET R+ R– FB 12.4k 1% 0.1µF X7R 51k STATUS SHARE BUS (SB) STATUS GAIN SB LTC4350 GND TIMER 0.1µF X7R UV 0.1µF X7R 150k 1% OV 12.1K 1% COMP1 COMP2 1000pF X7R 150Ω 5% 1µF X7R Q9, Q10, Q11: SILICONIX Si7866DP (800) 554-5565 COUT: PANASONIC EEFUE0G18R 180µF 4V (714) 373-7334 Figure 2b. Load sharing and Hot Swap portion of the power supply solution 100 VIN = 5V 95 VIN = 12V EFFICIENCY (%) 90 85 80 75 70 VOUT = 3.3V 65 0 10 20 30 LOAD CURRENT (A) 40 Figure 3. Measured efficiency of the LTC3729 circuit failure to the system through the STATUS pin. In this design, to simplify the circuit, single-direction MOSFETs are used in each module since the LTC3729 also has output overvoltage and short circuit protection functions. Figure 4 shows the pulsed load current waveforms of two paralleled power supplies with load sharing. The waveform shows that the two supplies have good current sharing from no load to heavy load, 40A. Figure 5 shows the hot swapping resistor R10 of the LTC3729. Therefore, the local output voltage VOUT of LTC3729 can be adjusted until the current of each supply matches the value set by the share bus. The LTC4350 monitors the local output voltage VOUT of each supply at the UV (undervoltage) and OV (overvoltage) pins. Low, high and open circuit faults are detected in this way by the LTC4350, which turns off the series output MOSFET to isolate faulty supplies. The LTC4350 also provides an open-drain signal to report the local continued on page 38 Module 1 Output Current Module 2 Output Current VOUT 1 (3.3V) VOUT 1 (3.3V) VOUT 2 (3.3V) VOUT 2 (3.3V) 40A 40A IOUT 1 ~5.5A/DIV IOUT 1 20A 0A 200µs/DIV LOAD CURRENT STEP = 0A TO 40A Figure 4. Pulsed load current of two paralleled LTC3729 power supplies with LTC4350 36 IOUT 2 IOUT 2 1ms/DIV 1ms/DIV Figure 5a. Swapping in module 2 Figure 5b. Swapping out module 2 0A Figure 5. Hot swapping waveform of two paralleled LTC3729 supplies with LTC4350 Linear Technology Magazine • August 2002 NEW DEVICE CAMEOS MSOP-8 packaged LT1961 are a high efficiency 1.5A switch and all the control circuitry required for a complete current mode buck converter. A patented anti-slope circuit maintains the 1.5A maximum switch current limit over all duty cycles. Low switch resistance maintains high efficiency at a high switching frequency over the 35V maximum switch voltage range. A low dropout internal regulator ensures consistent performance over the part’s entire 2.7V to 30V input range. The accurate shutdown threshold, which reduces quiescent current to 6µA, can be used as a precise undervoltage lockout. Synchronization allows an external logic level signal to increase the internal oscillator frequency from 1.4MHz to 2MHz. LT6550 and LT6551: 3.3V Triple and Quad Video Amplifiers The LT6550 and LT6551 are triple and quad video amplifiers designed to operate from a single 3.3V supply. These voltage feedback amplifiers drive double-terminated 50Ω or 75Ω cables and are configured for a fixed gain of 2, eliminating either six or eight external gain setting resistors. The LT6551 quad is designed for single supply operation and performance is fully specified on single 3.3V and 5V supplies. The LT6550 triple can be used on either single or split supplies of ±5V. The LT6550 and LT6551 both feature 110MHz –3dB bandwidth, 340V/µs slew rate and 3% settling time in 20ns, making them ideal for RGB video processing with a maximum screen resolution of 1024 x 768 on a single 3.3V supply. On a single 3.3V supply, the input voltage range extends from ground to 1.55V and the output swings to within 400mV of the supply voltage while driving a 150Ω load. These features, combined with the ability to accept RGB video signals without the need for AC coupling or level shifting of the incoming signals, make the LT6550 and LT6551 ideal choices for low voltage video applications. Both the LT6550 and LT6551 are available in a small 10-pin MSOP package and utilize a flow-thru pin out. Each device is available in both commercial and industrial temperature range versions. LTC3729/LTC4350, continued from page 36 waveforms of two paralleled modules with a total of 40A output current. The settling time for load transients and hot swap load currents can be adjusted via the compensation resistors and capacitors on the COMP1 and COMP2 pins of the LTC4350. See the LTC4350 data sheet for details. VIN(BUS) VOUT(BUS A) LTC4350 LTC3728 DC/DC SB (A) VOUT(BUS B) LTC4350 SB (B) Redundancy for Multiple Output Applications Figure 6 shows a simple and robust redundant power supply system with three outputs. In this example, three LTC3728 (similar to the LTC3729, but with two outputs) dual output PolyPhase controllers provide voltage control, and six LTC4350s provide hot swappable load sharing. Each LTC3728 regulates two outputs which are switched 180-degrees out of phase to minimize the number of input capacitors. Conclusion The LTC3729 PolyPhase current mode controller and dual output LTC3728 provide high performance, minimize component count and increase the reliability of low voltage, high current power supplies. These parts, com38 LTC4350 LTC3728 DC/DC SB (A) LTC4350 SB (C) SB (B) LTC4350 SB (C) LTC4350 LTC3728 DC/DC VOUT(BUS C) Figure 6. Block diagram of a redundant multiple voltage output system bined with the L TC4350 hot swappable load share controller, make for easy-to-design fault tolerant redundant power supply systems that are suitable for mission-critical applications. Linear Technology Magazine • August 2002