DESIGN IDEAS Efficient DC/DC Converter Provides Two 15A Outputs from a 3.3V by David Chen Backplane Introduction converter to around 85%. A more efficient solution is to use logic-level MOSFETs, which have very low RDS(ON) but require a 5V supply. The LTC1876 allows the use of logic-level MOSFETs by combining a 1.2MHz boost regulator, which produces a 5V bias supply from a 3.3V input, with two stepdown controllers, which provide the low voltage outputs. By integrating all three regulators in a single IC, the LTC1876 makes for efficient power supplies that can be small and inexpensive. 96 94 OVERALL EFFICIENCY (%) The 3.3V DC bus has become popular for broadband networking systems, where it is tapped for a variety of lower voltages to power DSPs, ASICs and FPGAs. These lower voltages range from 1V to 2.5V and often require high load currents. To maintain high conversion efficiency, power MOSFET conduction losses from the step-down converters must be minimized. The problem is that the 3.3V bus also brings with it frequent use of sub-logic level MOSFETs. Such MOSFETs have a relatively high RDS(ON), limiting the full-load efficiency of a 92 90 88 86 84 0 2 4 8 6 10 12 IOUT1 = IOUT2 (A) 10k 1µF 6.3V 5.6k 1 2 3 20k 4 5 20k 6 0.01µF 0.01µF 7 8 9 10 11 12 0.1µF 6800pF 47k 10k 8.06k 0.1µF 470pF 6800pF 470pF 470pF + SENSE1 TG1 SENSE1– SW1 VOSENSE1 BOOST1 FREQSET VIN STBYMD BG1 FCB EXTVCC ITH1 INTVCC SGND PGND LTC1876 3.3VOUT ITH2 BG2 BOOST2 SW2 13 SENSE2– TG2 14 SENSE2+ 10.2k 16 30.9k 17 D4 CMDSH-3 PGOOD RUN/SS1 18 RUN/SS2 AUXSGND AUXSD AUXVFB 330µF 6V ×3 10Ω VOSENSE2 15 47k VIN 3.3V 12Ω + 0.01µF AUXVIN AUXSW3 AUXPGND AUXSW3 AUXGND D1 BAT54A 36 0.47µF 34 L1 0.6µH CDEP134-0R6-H 33 32 31 D2 UPS840 Q2 Si4838 30 C17 2.2µF 10V 29 28 + C16 10µF 10V VOUT1 0.002Ω + 220µF 2.5V 4V AT ×3 15A + 330µF 1.8V 2.5V AT ×3 15A 0.47µF 27 1µF 6.3V 26 Q3 Si4838 25 L2 0.6µH CDEP134-0R6-H 24 0.002Ω VOUT2 23 1k 21 C22 1µF 6.3V + C21 10µF 10V Q4 Si4838 D2 UPS840 20 19 470pF L3 4.7µH FSLB2520-4R7M 1µF 6.3V Q1 Si4838 35 22 15 Figure 2. High efficiency of the design in Figure 1 12Ω 1000pF 14 C27 1µF 6.3V C36 1µF 6.3V 0.1µF 12Ω 1000pF 12Ω 10Ω VOSENSE2 OPTIONAL REMOTE SENSE 8.25k 5V 470pF 470pF 17.4k 10Ω VOSENSE1 Figure 1. An LTC1876 design converts 3.3V to 2.5V at 15A and 1.8V at 15A 32 Linear Technology Magazine • May 2002 DESIGN IDEAS capacitors. This significantly reduces the power loss associated with the ESR of input capacitors. Figure 3 shows detailed current waveforms of this operation. CURRENT THROUGH Q1 5A/DIV CURRENT THROUGH Q3 5A/DIV Conclusion INPUT CURRENT FROM 3.3V SUPPLY 5A/DIV 1.25µs/DIV Figure 3. Each switcher has 5A peak current, but the total ripple at the input is still only 5A, minimizing CIN requirements. Design Example Figure 1 shows a design that provides 2.5V/15A and 1.8V/15A from a 3.3V input. Because the LTC1876 provides a 5V bias for MOSFET gate drive, a very low RDS(ON) MOSFET Si4838 (2.4mΩ typical) can be used to achieve high efficiency. Figure 2 shows that the overall efficiency is above 90% over a wide range of loads. Figure 2 also shows that the light load efficiency of this design is more than 84%. This is a direct benefit of the Burst Mode operation of the LTC1876. Further efficiency improvements come from operating the two step-down channels out-of-phase. The top MOSFET of the first channel is fired 180° out of phase from that of the second channel, thus minimizing the RMS current through the input The LTC1876 uses three techniques to efficiently power low voltage DSPs, ASICs and FPGAs from a low input voltage. The first technique uses an internal boost regulator to provide a separate 5V for the MOSFET gate drive. Secondly, its Burst Mode operation achieves high efficiency at light loads. Lastly is the out-of-phase technique which minimizes input RMS losses and reduces input noise. Complete regulator circuits are kept small and inexpensive, because all three switchers (one step-up regulator and two step-down controllers) are integrated into a single IC. For systems where a separate 5V is available or the input supply is greater than 5V, the internal boost regulator can be used to provide a third step-up output with up to 1A switch current. 5.25 100 5.2 90 5.15 80 5.1 70 VIN = 36V 5.05 VIN = 48V 5 4.95 VIN = 72V EFFICIENCY (%) OUTPUT VOLTAGE LT1725, continued from page 30 VIN = 72V 60 VIN = 48V 50 40 4.9 30 4.85 20 4.8 10 4.75 VIN = 36V 0 0 500 1000 1500 2000 OUTPUT CURRENT (mA) 2500 Figure 2. LT1725 regulation off, the current that had been flowing in the primary of the transformer begins to flow in the secondary. The voltage on the drain of M1 rises to a level determined by the transformer turns ratio and the output voltage. Similarly, the voltage on the feedback winding rises to a level set by the output voltage. The LT1725 reads the voltage on the feedback winding durLinear Technology Magazine • May 2002 0 500 1000 1500 2000 OUTPUT CURRENT (mA) 2500 Figure 3. Efficiency vs output current for the circuit in Figure 1 ing the flyback pulse using a proprietary sampling technique. This sampled voltage is then compared a precision internal reference and current is added to or subtracted from the capacitor on the VC pin. This has the effect of modifying the M1 turn-off current in such a way as to regulate the output voltage. An important benefit of this sampling technique is that output voltage information arrives at the controller about a microsecond after the switching cycle is terminated. In a conventional optocoupler-based design. Delays of tens to hundreds of microseconds occur in the optocoupler alone, severely limiting the converters transient response. Additionally the LT1725 features internal slope compensation. This suppresses sub-harmonic oscillations that can occur with less sophisticated current mode controllers. Sub-harmonic oscillations increase output voltage ripple and increase switching stress. Conclusion The LT1725 isolated flyback controller greatly simplifies the design of isolated flyback converters. Compared to traditional opto-isolated designs, an LT1725 based circuit has far fewer components, superior transient response and is easier to stabilize. 33