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.