High Efficiency 100mA Synchronous Buck Converter with Wide Input Range from 4V to 150V Design Note 532 Charlie Zhao 4V to 150V Input to 3.3V Output, 100mA Buck Converter Figure 1 shows a 3.3V output, 100mA maximum load current synchronous buck converter, with wide input range from 4V to 150V. The LTC3639 has three programmable fixed output voltages 1.8V, 3.3V and 5V. These fixed outputs use an internal feedback resistor divider and can be simply selected with the VPRG1 and VPRG2 pins. For 3.3V output, just connect VPRG1 to ground and tie VPRG2 to the SS pin. The VFB pin is directly connected to the output without using an external resistor divider. Efficiency curves with different input voltages are shown in Figure 2. Introduction The wide 4V to 150V input range of the LTC ®3639 stepdown DC/DC converter allows automotive, avionics and distributed power systems to: • Cover extensive transient requirements • Enable multiple wide-ranging input power sources, such as a high voltage DC bus with a low voltage battery backup • Create a universal power supply to cover a variety of input sources, thereby reducing inventory and the cost of design and manufacture The LTC3639 converts at high efficiency with internal high side and low side power MOSFETs. It can support up to 100mA output current and features a programmable peak current limit. Its output voltage range is also broad, from 0.8V to the input voltage. The integrated high side MOSFET can work at 100% duty cycle for low dropout operation. High efficiency is achieved across the wide input and output voltage ranges. Burst Mode ® operation and low quiescent current are also features of this synchronous buck converter. The loop is inherently stable without compensation because of the hysteretic nature of the control architecture, resulting in a simple easy to use application circuit. VIN 4V TO 150V 1µF 200V 36V to 72V Input to 24V Output, 100mA Buck Converter The LTC3639 has a very wide output voltage range, from 0.8V to the input voltage. Besides the selectable three fixed output voltages, an adjustable output voltage can be set with an external resistor divider. Figure 3 shows an application example of a 24V output, 100mA synchronous step-down converter. The input voltage range is controlled from 36V to 72V, with the L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. TDK SLF12555T-102MR34 1000µH SW VIN LTC3639 RUN VFB OVLO VPRG1 VOUT 3.3V 10µF 100mA SS VPRG2 GND dn532 F01 Figure 1. 4V to 150V Input to 3.3V Output, 100mA Synchronous Buck Converter 11/14/532 part’s wide voltage range makes even large negative output voltages realizable. Figure 4 shows the implementation of a –15V output regulator from a 4V to 135V input. Since the output is connected to the ground pin, the maximum input voltage is limited to the sum of 150V and the –15V regulated output, or 135V. The maximum output current for the LTC3639 in this positive-to-negative configuration is about 100mA • VIN/(VIN+|VOUT|). Effciency vs Load Current, VOUT = 3.3V 100 90 ISET OPEN EFFICIENCY (%) 80 70 60 50 40 30 VIN = 12V VIN = 36V VIN = 72V VIN = 150V 20 10 0 0.1 VIN 4V TO 135V 100 10 1 LOAD CURRENT (mA) dn532 F02 Figure 2. Efficiency Curves of the Converter in Figure 1 220µH VIN 36V TO 72V SW VIN LTC3639 2.2M 187k VOUT 24V 100mA FBO 62k ISET 1µF OVLO 30.9k 10µF SS VPRG1 VPRG2 GND 49.9k dn532 F03 Figure 3. 36V to 72V Input to 24V, 100mA Buck Converter with Input Overvoltage and Undervoltage Lockout overvoltage lockout and undervoltage lockout features of the LTC3639. The input operating range is easily set with a resistor divider from the VIN to the RUN pin and the OVLO pin, as shown in Figure 3. Negative Output Voltage Applications An additional useful application for the LTC3639 is to generate a negative voltage from a positive one. The Data Sheet Download www.linear.com/LTC3639 Linear Technology Corporation CIN 1µF 200V VIN SW LTC3639 RUN VFB SS FBO VPRG1 ISET VPRG2 OVLO GND 200k 102k dn532 F04 COUT 10µF 25V VOUT –15V Figure 4. 4V to 135V Input to –15V Output Positive-to-Negative Regulator VFB RUN L1 220µH Conclusion The LTC3639 has a very wide input voltage range, very wide output voltage range, integrated power MOSFETs, low quiescent current (1.4µA in shutdown and 12µA in sleep mode) and high efficiency across a wide load current range. Rich features include programmable or adjustable output, adjustable current limit, no compensation required, internal or external soft-start, programmable overvoltage and undervoltage lockout. Additionally, the thermally enhanced small MSE package and simple application circuit offer a high performance, small and cost effective DC/DC converter solution for automotive systems, avionics, distributed power systems, medical devices and industrial control supplies. For applications help, call (408) 432-1900, Ext. 3258 dn532f LT/AP 1114 111K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2014