DESIGN IDEAS Monolithic Buck-Boost Converter Provides 1A at 3.3V by Mark Jordan without Schottky Diodes Introduction Inside the LTC3441 The LTC3441 patented control technique provides smooth and continuous transfer from buck, buck-boost and boost modes while maintaining a constant frequency at no load. The operating frequency is factory set to 1MHz and can be synchronized up to 1.7MHz. For light loads, the part offers user controlled Burst Mode operation to maximize battery life, drawing only 25µA of quiescent current. To limit inrush current at start-up, an external RC network can be connected to the SHDN/SS pin to control output voltage rise time. The LTC3441 is available in a small 3mm by 4mm low thermal resistance 12-lead DFN package. L1 4.7µH 4 9 2.7V TO 4.2V 1 32 * CIN 10µF Li-Ion 7 3 SW2 5 340k 8 VOUT 12 LTC3441 VIN FB 11 SHDN/SS VC 2 MODE/SYNC GND 6 PGND PGND PVIN *1 = Burst Mode OPERATION 0 = FIXED FREQUENCY COUT 22µF 15k 1.5nF 200k CIN: TAIYO YUDEN JMK212BJ106MG COUT: TAIYO YUDEN JMK325BJ226MM L1: TOKO A916CY-4R7M Figure 1. Li-Ion to 3.3V at 1A boost converter response. For applications requiring optimum transient response an additional pole/zero pair to broaden the loop will achieve the desired results. Figure 2 shows that the converter can achieve 95% peak efficiency without the use of Schottky diodes. 100 90 80 Not Just a Buck-Boost 4 9 10 1M Li-Ion 0.047µF C1 10µF 1 * 70 60 VIN = 4.2V 50 VIN = 2.7V 40 VIN = 3.6V 30 10 VOUT = 3.3V 0 0.1 1 7 3 *1 = Burst Mode OPERATION 0 = FIXED FREQUENCY SW1 10 IOUT (mA) 100 1000 Figure 2. Efficiency curves for the converter in Figure 1 The Schottky diode limits the voltage spikes on the SW2 pin. L1 4.7µH 2.5V TO 4.2V Burst Mode OPERATION 20 The LTC3441 can also be configured as a boost converter with output disconnect as shown in Figure 3. The 5V at 600mA converter from a Lithium-Ion battery demonstrates peak efficiencies of over 94%. Input current at start-up is also controlled by the LTC3441, reducing the load burden on the battery. Single Inductor Li-Ion to 3.3V/1A Converter Figure 1 shows a 3.3W converter powered from a single Lithium-Ion battery. The single inductor topology of the LTC3441, along with all ceramic capacitors, minimizes critical board real estate. Dominant pole compensation is shown as a simple means to compensate the converter’s transient 10 SW1 VOUT 3.3V 1A EFFICIENCY (%) The power density and small form factor of lithium-Ion batteries makes them the power source of choice for many portable devices. A SEPIC converter topology is a popular way to provide a regulated bus voltage that falls within the 2.7V to 4.2V battery range, but a SEPIC converter has some flaws. It offers mediocre efficiency and requires coupled inductors and a high current flyback capacitor. The LTC3441 1A buck-boost converter offers a compact and efficient alternative that requires only a single inductor and very few external components. D1 SW2 VOUT 5V 600mA 5 8 VOUT 12 LTC3441 VIN FB 11 SHDN/SS VC 2 MODE/SYNC GND 6 PGND PGND R1 619k PVIN COUT 22µF R3 15k C4 1.5nF R2 200k C1: TAIYO YUDEN JMK212BJ106MG C2: TAIYO YUDEN JMK325BJ226MM D1: MBRM120LT3 L1: TOKO A916CY-4R7M Figure 3. Li-Ion to 5V at 600mA boost converter with output disconnect Linear Technology Magazine • September 2003