MC34063 as voltage inverter

I am building a small bench PSU that will feature a negative voltage rail. Whatever voltage (+) I adjust as the output, will be “negatively mirrored” on the negative output.

Since my entire project is based on a notebook PSU, that only has positive voltage, I decide to use a MC34063 on its inverting configuration, and this is where I need your help. The examples in the datasheet and other application notes only mention it being used as an inverting adjustable regulator, with fixed voltage output. What I need, however, is to configure it so that Vout = -Vin.

Reading its datasheet, I found this circuit (page 9, Fig. 7):

Based on the above, my questions are:

  • If I understood correctly, this circuit will invert any input voltage (Vout = - Vin), is that correct?
  • I can go either way, using a PNP or NPN transistor, but are there advantages of one over another? NPN seems a bit simpler (only uses 1 resistor).
  • What are proper transistors to be used?
  • The resistor on the left, between pins 6 and 7, is Rsc, and should be around 0.22 Ohms. How to I get that value?
  • I figure the inductor can be around 100uH, based on other schematics in the datasheed, and the schottky diode is a 1N5819. What about the 2 resistors that are connected to the base of the transistor? What are suitable values for them?
    (or the value for the single resistor in the NPN configuration).

Thanks in advance and sorry if these questions are too basic.

What is your input voltage? how much output current?

1N5819 is fast schottky diode and not zener diode, only good for 1A current.

Please read this it will show you how to get those value of resistors

BillHo, Vin will vary from around 2V to 17V. I believe I don't neet more than 700mA on the output of the MC34063.

Yes, I was mistaken about 1N5819 being a zener (rather than a schottky diode), but it is still the one the datasheet specifies.

I think the datasheet does not show a voltage mirror schematic. Sorry, but they show normal DC-DC converter schematics and some have a negative output. The negative output does not follow the input voltage.

The most common way is perhaps using a OpAmp with a power output.
When I search for this : "dc-dc" "voltage mirror" - Google Search
then I mostly find books, but no practical schematics.