Adjusting the MC34063A regulator with an external voltage?

I am trying to think if it is possible to adjust the MC34063A SMPS controller (or a similar controller) with an external voltage or MCU control. I could use a digital potentiometer but those are not cheap.

Is there some analog solution I can use to change the feedback pin voltage? It doesn't matter too much if it's not a linear response as the MCU can correct for that.

I was thinking of using JFETs because I've made 1:16 variable resistors with them before, but I wanted to know if anyone has any better idea.

Thanks,
Tom.

Re: Adjusting the MC34063A regulator with an external voltag

Hi

You can use PWM signal created in a microcontroller Timer followed by an LPF (low pass filter) that will produce DC voltage you can inject into the feedback pin

To fully control the regulator you have to sample the output voltage, process the result and set the PWM signal

All the best

Bobi

The microcontroller specialist

Re: Adjusting the MC34063A regulator with an external voltag

Since the FB pin requires around 1.25 volts, how would I modulate the voltage? If the output reached the goal voltage, would I put FB at 1.25 volts?

I could try and implement some kind of software voltage divider; that might work.

Re: Adjusting the MC34063A regulator with an external voltag

A linear adjustment is usually done by adding a voltage (respectively a current) to the voltage feedback divider. There's no particular sense of using a programmable voltage divider in my opinion.

Re: Adjusting the MC34063A regulator with an external voltag

I came up with a rather elegant solution (in my opinion ).

I am using a small set of NMOS transistors to switch in various resistors. By doing this with a binary input of say 4 bits, I can choose between 16 levels. This shows the voltage going from 1.25 volts to 28.3 volts. In my application, I use a SMPS to generate a voltage slightly above (2 volts or so) the output voltage set for my linear regulator. This way, the regulator drops only 2 volts or so and at 5 amps it only dissipates 10 watts (plus various inefficiencies), making it easier to cool, and more efficient; I get an efficient (more than 80% at full load) power supply with the low noise/ripple, easy adjustability and lack of overshoot of linear, with the high efficiency of switched-mode power supplies.

A circuit simulation on Falstad's Circuit Simulator is shown below. Note that there is a linear regulator instead of a MC34063A, that is because the simulator does not have a SMPS chip, but this is only controlling the FB input so it doesn't matter too much.

Note you can use NPNs if you want, but not with the MC34063A, because they drop more than half a volt in operation; you'll need either higher resistance values or a higher reference voltage. Also note the resistors are all standard E12, making the output slightly nonlinear. Replace with precision resistors if your application demands, but in my case I'll just use a PIC micro to select the range.

It's a good solution, if you only need a few voltage steps (e.g. up to 4 or 5 bit resolution). Instead of discrete transistors, you can also use open drain outputs of a uP or programmable logic device.

If you want to control the regulator with an existing DAC or other control voltage, you can add the control voltage through a resistor network, as suggested before.