High Efficiency 100 mA Synchronous Buck Converter with Wide Input Range from 4 V to 150 V

Linear Technology LTC3639

Charlie Zhao, Linear Technology

Design Note 532

Introduction

The wide 4 V to 150 V input range of the LTC3639 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 100 mA output current and features a programmable peak current limit. Its output voltage range is also broad, from 0.8 V 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.

4 V to 150 V Input to 3.3 V Output, 100 mA Buck Converter

High Efficiency 100 mA Synchronous Buck Converter with Wide Input Range from 4 V to 150 V
Figure 1. 4 V to 150 V Input to 3.3 V Output, 100 mA
Synchronous Buck Converter.

Figure 1 shows a 3.3 V output, 100 mA maximum load current synchronous buck converter, with wide input range from 4 V to 150 V. The LTC3639 has three programmable fixed output voltages 1.8 V, 3.3 V and 5 V. These fixed outputs use an internal feedback resistor divider and can be simply selected with the VPRG1 and VPRG2 pins. For 3.3 V 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.

High Efficiency 100 mA Synchronous Buck Converter with Wide Input Range from 4 V to 150 V
Figure 2. Efficiency Curves of the Converter in Figure 1.

36 V to 72 V Input to 24 V Output, 100 mA Buck Converter

The LTC3639 has a very wide output voltage range, from 0.8 V 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 24 V output, 100 mA synchronous step-down converter. The input voltage range is controlled from 36 V to 72 V, with the 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.

High Efficiency 100 mA Synchronous Buck Converter with Wide Input Range from 4 V to 150 V
Figure 3. 36 V to 72 V Input to 24 V, 100 mA Buck Converter
with Input Overvoltage and Undervoltage Lockout.

Negative Output Voltage Applications

High Efficiency 100 mA Synchronous Buck Converter with Wide Input Range from 4 V to 150 V
Figure 4. 4 V to 135 V Input to –15 V Output
Positive-to-Negative Regulator.

An additional useful application for the LTC3639 is to generate a negative voltage from a positive one. The part’s wide voltage range makes even large negative output voltages realizable. Figure 4 shows the implementation of a –15 V output regulator from a 4 V to 135 V input. Since the output is connected to the ground pin, the maximum input voltage is limited to the sum of 150 V and the –15 V regulated output, or 135 V. The maximum output current for the LTC3639 in this positive-to-negative configuration is about

Conclusion

The LTC3639 has a very wide input voltage range, very wide output voltage range, integrated power MOSFETs, low quiescent current (1.4 μA in shutdown and 12 μA 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.

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