Low-Cost Step-Up/Step-Down Converter Accepts 2V to 16V Inputs
Abstract: Step-up/step-down DC-DC converter converts 2V to 16V input to a 9V output at up 500mA. The circuit employs the MAX761 internal-switch boost DC-DC converter and a Zetex FZT749 PNP transistor. The topology is boost followed by LDO.
The circuit shown in Figure 1 is a low-cost step-up/step-down DC-DC converter. By definition, its input voltage can range above and below the regulated output voltage. The circuit includes a simple switch-mode boost converter (IC1) that contains a comparator, normally used to detect low battery voltage. In this example, the comparator controls an external, low-cost pnp transistor operating as a linear regulator.
Figure 1. This step-up/step-down converter maintains a regulated output (set to 9V for the R3/R4 values shown) as the input voltage varies between 2V and 16V.
IC1 steps up VIN (2V min) to the level of VX as determined by the jumper block JU1. A 2-3 jumper selects the internal divider, producing VX = 12V, and a 2-1 jumper selects feedback resistors R1 and R2, producing VX = 1.5V(R1 + R2)/R2. VX should be set 1V to 2V above the desired output voltage.
Linear regulator Q1 steps down VX to an output level set by R3 and R4:
VOUT = 1.5V(R3 + R4)/R4
where VX>VOUT.
When VIN>VX, the switching regulator stops and the linear regulator alone controls VOUT. C6 reduces output ripple. This circuit allows a wide range of input and output voltages, and delivers output currents up to 500mA (Figure 2).
Figure 2. For VOUT = 9V, the maximum output current in Figure 1 varies with input voltage, as shown.
A similar idea appeared in the 7/16/98 issue of Electronic Design.
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