A switching regulator with an adjustable output voltage is a handy weapon in the battle to reduce heat dissipation in circuits. The step-down converter described here can be used with input voltages from 7.5 V up to 25 V and can deliver an output current of up to 1.5 A. At its minimum output voltage of 3.3 V the circuit can even operate from a 5 V input. The circuit is built around a Linear Technology LT1376 and is mostly based on the design given in the device’s datasheet [1]. The minimum inductance and current rating of choke L1 depend on the maximum output current desired from the circuit. For example, at 0.6 A an inductance value of 5 μH is suitable; at 1 A the minimum value is 10 μH, and at 1.5 A a 20 μH inductor is required. In the circuit diagram, we show a value of 22 μH.
The maximum possible output voltage obtainable depends on the input voltage available and the required output current: the datasheet contains all the relevant information. The adjustment range of R2 should be sufficiently large in all cases to allow output voltages from 3.3 V to 14.5 V to be obtained. With the component values shown the expected typical characteristics of the circuit are as follows.
The maximum available output current is also highly dependent on the quality of choke L1 and electrolytic capacitor C5. For the inductor, the DC current and resistance ratings are critical in addition to its inductance. The current rating should be at least double the maximum desired output current of the circuit and the DC resistance as low as possible. For the capacitor, a low ESR is essential. The author has designed a small printed circuit board (measuring 31 mm by 52 mm) for assembling the circuit: the board also has space for a protection fuse. The layout and schematic files are available for free download in Eagle format at [2].
Internet Links
[1] www.linear.com/product/LT1376
(datasheet)
[2] www.elektor.com/110286
(downloads)
