DESIGN IDEAS Tiny and Efficient Boost Converter Generates 5V at 3A from 3.3V Bus by Dongyan Zhou Introduction Circuits that require 5V remain popular despite the fact that modern systems commonly supply a 3.3V power bus, not 5V. The tiny LTC1700 is optimized to deliver 5V from the 3.3V bus at very high efficiency, though it can also efficiently boost other voltages. The small MSOP package and 530kHz operation promote small surface mount circuits requiring minimal board space, perfect for the latest portable devices. By taking advantage of the synchronous rectifier driver, the LTC1700 provides up to 95% efficiency. To keep light load efficiency high in portable applications, the LTC1700 draws only 180µA in sleep mode. The LTC1700 features a start-up voltage as low as 0.9V, adding to its versatility. The LTC1700 uses a constant frequency, current mode PWM control scheme. Its No RSENSE™ feature means the current is sensed at the main MOSFET, eliminating the need for a sense resistor. This saves cost, space and improves efficiency at heavy loads. For noise-sensitive applications, Burst Mode operation can be disabled when the SYNC/MODE pin is pulled low or driven by an external clock. The LTC1700 can be synchronized to an external clock ranging from 400kHz to 750kHz. L1 3.2µH 1 470pF 22k 2 SGND ITH 270pF 3 R1 316k 0.1% 5 RUN/SS PGND VFB TG SYNC/MODE VOUT M1 + C4 470µF VOUT 5V/3A 9 6 7 (408) 573-4150 (207) 282-5111 (619) 661-6853 (847) 956-0667 (310) 322-3331 (800) 554-5565 Figure 1. 3.3V to 5V, 3A boost regulator 3.3V Input, 5V/3A Output Boost Regulator Figure 1 shows a 3.3V input to 5V output boost regulator which can supply up to 3A load current. Figure 2 shows that the efficiency is greater than 90% for a load current range of 200mA to 3A and stays above 80% all the way down to a 3mA load. C2 is a tantalum capacitor providing bulk capacitance to compensate for possible long wire connections to the input supply. In applications where the regulator’s input is concontinued on page 35 L1 3.3µH 1 470pF 33k 2 SGND 100pF 470pF 30.1k 1% 52.3k 1% SW ITH BG + M2 10 8 C1 10µF C3 22µF M1 + C2 68µF 6.3V VIN 2V TO 3V VOUT 3.3V/1A C4 330µF 6.3V LTC1700 90 EFFICIENCY (%) 8 C3 22µF VIN 3.3V ±10% DN280 F01 VIN = 3.3V VOUT = 5V 70 M2 10 C1, C3: TAIYO YUDEN CERAMIC JMK325BJ226M C2: AVX TAJB686K006R C4: SANYO POSCAP 6TPB470M L1: SUMIDA CEP1233R2 M1: INTERNATIONAL RECTIFIER IR7811W M2: SILICONIX Si9803 100 80 BG C2 68µF 6.3V LTC1700 470pF 4 R2 100k 1% SW C1 22µF + 4 3 100pF 5 RUN/SS VFB PGND TG SYNC/MODE VOUT 9 6 7 60 DN280 F03 50 40 1 10 100 1000 LOAD CURRENT (mA) 10000 DN280 F02 Figure 2. Efficiency of the circuit in Figure 1 28 C1: C2: C3: C4: L1: M1: M2: TAIYO YUDEN CERAMIC JMK316BJ106ML AVX TAJB686K006R TAIYO YUDEN CERAMIC JMK325BJ226M SANYO POSCAP 6TPB330M MURATA LQN6C SILICONIX Si9804 SILICONIX Si9803 (408) 573-4150 (207) 282-5111 (619) 661-6853 (814) 237-1431 (800) 554-5565 Figure 3. 2-cell to 3.3V, 1A boost regulator Linear Technology Magazine • May 2002 DESIGN IDEAS V2 V2 604Ω 604Ω VO1 V1 R1 1 R2 C V1 6 VOUT 1 600Ω 2 – 600Ω R3 = R1 – + 2 0.1µF 7 + 3 600Ω – – + VO2 0.1µF 150Ω VREF 6 600Ω 3 5 8 V+ V+ 4 0.1µF V– 5 VREF DN194 F03 VO1 = –V1 + 2 • VREF VO2 = –V2 + 2 • VREF VDIFF = VO2 – VO1 = V1 – V2 OUTPUT DC COMMON MODE VOLTAGE, VOCM = 2 • VREF – VINCM V+ 8 V+ 4 V– LT1567 0.1µF DN194 F04 R2 , R3 = R1 R1 VO = GAIN (V2 – V1) + VREF GAIN = Figure 3. A differential input and output buffer/driver f–3dB BANDWIDTH AT VOUT = to one (R1 = R2 = 604Ω and VOUT = V2 – V1) the input referred differential voltage noise density is 9nV/√Hz and differential input signal-to-noise ratio is 80.9dB with 0.1VRMS input signal in a 4MHz noise bandwidth. The input AC common mode rejection depends on the matching of resistors R1 and R3 and the LT1567 inverter gain tolerance (common mode rejection is at least 40dB up to 1MHz with one percent resistors and two percent inverter typical gain tolerance). If the differential input is DC coupled, then VREF must be set equal to input common mode voltage (VINCM) (if VREF is greater than VinCM then a peak volt- 7pF 150Ω LT1567 7 + 7pF IF R1 = R3 = 604Ω, THEN 1 ≤ 5MHz 2 • π • R2 • C R2 604Ω 1.21k 2.43k Vη GAIN 9.0 1 8.4 2 8.1 4 NOISE AT VOUT = GAIN • Vη • √fηBW fηBW = 1.57 • f –3dB Vη IS THE INPUT REFERRED DIFFERENTIAL VOLTAGE NOISE DENSITY IN nV/√Hz Figure 4. A differential input-to-single-ended output amplifier age on Pin 7 may exceed the output voltage swing limit). The DC voltage at the amplifier’s output (VOUT, Pin 1) is VREF. Conclusion With one LT1567 and two or three resistors, it is easy to design low noise, differential circuits for signals up to 5MHz. The LT1567 can also be used to make of low noise second and third order lowpass filters and second order bandpass filters with differential outputs. See www.linear.com for a spreadsheet-based design tool for just this purpose. LTC1700, continued from page 28 nected very close to a low impedance supply, this capacitor is not needed. In digital cameras and other batterypowered devices, the LTC1700 makes for a high efficiency boost regulator in a small package. Figure 3 shows a 2alkaline cell to 3.3V output circuit. This circuit can supply 1A maximum output current. Figure 4 shows the efficiency at different battery voltages. Efficiency of this circuit peaks at 93%. If a lower RDS(ON) MOSFET (such as Si6466) is used for M1, the Linear Technology Magazine • May 2002 VOUT = 3.3V VIN = 3V 90 EFFICIENCY (%) 2-Cell Input, 3.3V/1A Output Regulator 100 VIN = 2.5V 80 VIN = 2V 70 60 50 40 1 100 10 LOAD CURRENT (mA) 1k Figure 4. Efficiency of the circuit in Figure 3 maximum output current can be increased to 1.4A with about a 2% reduction in efficiency due to the increase in gate capacitance. MOSFETs with lower than 2.5V gate threshold voltages are recommended. The LTC1700 is also an ideal device for single cell Li-Ion battery to 5V applications. Conclusion The LTC1700 boost controller brings high efficiency and small size to low voltage applications. Its features are ideally suited to both battery-powered and line-powered applications. 35