Minimalistic switcher

21.02.2007 11:38

Taking a bit of a break from old computers...

Here's an interesting step-down switching power supply I found in a car charger for one of the older Ericsson mobile phones.

It really has a bare minimum of components. If you take out the dual op-amp, which is there only for enhancing user experience (it changes the color of the LED if the charger is in use), you end up only with two transistors, a simple shunt regulator AN1431, a coil and a couple of other passive elements.

AN1431 is used as a comparator here. When the voltage between the REF and A pins drops below 2.5V, no current flows into K and IC1 switch is open. If on the other hand voltage on REF raises above 2.5V, this chip acts as a short circuit and IC1 closes.

So basically the output voltage is determined with the voltage divider R7, R8, R9 and R10 (8.4V in this case). R6 and R5 form a positive feed-back that gives some hysteresis to this comparator so that the switching frequency doesn't get too high.

Posted by Tomaž | Categories: Analog


I know this is an old page, but after randomly hitting this site from a Google search I thought you'd like to see this simple SMPS design:

Posted by jammit

Aargh. Forgot email address. It's almost like forgetting to give a handshake.

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