DESIGN IDEAS Triple, High Output Current Supply Requires only 3.3V Input and Minimal Input Capacitance by San-Hwa Chee Introduction trollers allow off-the-shelf inductors to be used instead of bulky, customwound transformers. The PolyPhase™ architecture of the step-down controllers minimizes input capacitance requirements, reducing overall system cost and footprint. A built-in boost regulator provides a third output. 0.1µF 1 2 42.2k 1% 1000pF 3 20k 1% 4 5 0.01µF 6 220pF 7 8 6.8k 470pF 9 220pF 3.3VOUT 10 470pF 11 6.8k 12 20k 1% 25.5k 1% 13 14 15 10k VOUT3 5V 400mA 16 M1–M4: L1, L2: L3: C1: C2: SENSE1 + TG1 SENSE1 – SW1 VOSENSE1 BOOST1 FREQSET VIN STBYMD BG1 FCB EXTVCC LTC1876 ITH1 INTVCC SGND PGND 3.3VOUT BG2 BOOST2 ITH2 VOSENSE2 SW2 SENSE2 – TG2 SENSE2 + RUN/SS2 100k 36 35 L1 2µH PGOOD 34 0.1µF 33 M1 17 AUXSD AUXSGND C2 47µF 6.3V 10Ω 31 + C1 33µF, 6.3V 30 D3 29 1µF D4 4.7µF 0.1µF + + C2 47µF 6.3V 27 26 M3 AUXVIN AUXVFB AUXSW AUXPGND AUXSW AUXPGND M4 RSENSE 0.008Ω 24 L2 2µH 23 VOUT2 1.8V 5A SHUTDOWN 21 20 D2 0.1µF 25 22 D1 M2 32 28 VOUT1 2.5V 4A RSENSE 0.008Ω 6.8µF 6.3V VIN 3.3V D5 18 10µF 16V X5R PGOOD Figure 1 shows a low input voltage application with the input supply at 3.3V. The boost regulator is set up to provide 5V and is used to power the control circuitry of the step-down controllers and to provide gate drive 0.1µF 1000pF 31.6k 1% RUN/SS1 3.3V Input, 1.8V, 2.5V and 5V Outputs + The LTC1876 is ideally suited for traditional system power supplies, where outputs of 3.3V, 5V and 12V are required from an input ranging from 4.5V to 24V. Another possible configuration allows the LTC1876 to operate from a low 3.3V input supply. The two out-of-phase step-down con- 19 L3, 5.4µH + 10µF 20V FAIRCHILD FDS6912A (408) 822-2126 SUMIDA CEP123-2RO (847) 956-0667 SUMIDA CDRH5D18 PANASONIC EEFCDOJ330R (201) 392-4511 PANASONIC EEFCD0J470R (800) 282-9855 D1, D2: ON SEMICONDUCTOR BAT54A D3, D4: ON SEMICONDUCTOR MBRM140T3 D5: ON SEMICONDUCTOR MBR0520 THICK TRACES = HIGH CURRENT PATH Figure 1. Low voltage 3.3V to 1.8V and 2.5V power supply Linear Technology Magazine • November 2000 29 DESIGN IDEAS 100 Conclusion 95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 50 0.01 0.1 1.0 LOAD CURRENT (A) 10.0 Figure 2. Overall efficiency vs load current for Figure 1’s circuit; load current is kept the same for the 1.8V and 2.5V outputs. voltage to the N-channel MOSFETs. This allows standard logic-level MOSFETs to be used. In addition, the 5V output can also be used for other light loads. The maximum output current that the 5V output can provide is 400mA, including gate-charge currents. The 3.3V input is converted to 2.5V and 1.8V by the high efficiency controllers. The N-channel MOSFETs, FDS6912As, were selected for both their low gate charge and low RDS(ON) resistance. Due to the use of an outof-phase topology for the step-down controllers, the ripple current require- ment for the input capacitance is minimized. Ceramic capacitors are used to further reduce the ESR, thus reducing ripple voltage and losses. Since the controllers and the boost regulator operate independently, the 5V output can be used to power keepalive circuitry while the step-down controllers are shut down. Figure 2 shows the efficiency of Figure 1’s circuit. The curve is plotted with the 1.8V and 2.5V outputs loaded with the same amount of current. Figure 3 shows the output voltage ripple for all the outputs, with the 1.8V and 2.5V outputs loaded at 4A. By using the LTC1876 boost regulator to power the control circuitry and provide the gate drives to its stepdown controllers, high efficiency is obtained with a low input voltage supply. Ideally suited for applications that require three different supply voltages, the LTC1876 provides high performance both in low input voltage applications and traditional system power supplies. With its narrow 36-pin SSOP package and its multiphase technology, the LTC1876 provides high performance power supply solutions in a small board space. 1.8V OUTPUT AC COUPLED 2.5V OUTPUT AC COUPLED 5V OUTPUT AC COUPLED Figure 3. Output voltage ripple for Figure 1’s circuit SCAD III, continued from page 24 that it creates in the switching waveform. This shows why trace lengths should be minimized in a real circuit. The switch voltage rating can be exceeded if the trace length and the subsequent inductance are ignored. Figure 6 demonstrates the available parasitic characteristics of a typical capacitor. Diodes and inductors also have parasitic elements that can be used to enhance the accuracy of a design. However, setting these parameters increases both the number of calculations performed by SCAD III and the overall simulation time. 30 Conclusion SCAD III represents the state-of-theart in schematic capture/SPICE software. It provides power supply circuit simulation and allows designers of all levels to model switching regulator performance. SCAD III includes, for the first time, black-box switching regulator models that dramatically decrease design and simulation time. Notes: 1 Completed in 12 seconds. All times are for a 400MHz Pentium® II PC with 128MB of RAM, under Windows® 98. 2 34 seconds to complete. 3 1 minute, 44 seconds to complete Pentium is a registered trademark of Intel Corp. Windows is a registered trademark of Microsoft Corp. For more information on parts featured in this issue, see http://www.linear-tech.com/go/ltmag Linear Technology Magazine • November 2000