24 V to -1.25 V and -12 V Converter

Question

With reference to my first question, (+-5VDC to 4-20mA Converter), as you can see from answer from my first question, I have succeed to create 4-20 mA from 0-5 V with using three TL081 operational amplifiers.

Now, I have to create -1.25 V and -12 V from +24 V supply voltage. I have read all questions and explanations about negative voltage. I know I can pin my -12 V as ground and get over this problem but I cannot do it because of other parts of my system. So I have to invert voltage to -12 V and -1.25 V. I have tried to use ICL7660 but its output voltage is not stable. Also there is a 20-50 mV difference about aimed voltage values.

Also I cannot divide -5 V to -1.25 V accurately. I have tried to use basic voltage divider circuit but voltage drop on resistance is too high that I cannot take a stable -1.25 V. I had to lower resistance values so voltage drop had lowered, but I passed the maximum current value.

Do you have any idea about how I can achieve -12 V and -1.25 V accordingly and stable from +24 V supply?

Answer 1

I'd be tempted to use a switching inverting regulator like this one: -

Then I'd derive -1.25V from the -12V output. The question showed 1A linear regulators so I'm assuming 2A is going to be enough to power the -12V and the -1.25V.

Should a linear regulator be needed on the output (less noise and usually cleaner) then use a "low-drop-out" device and run the switcher with an output of maybe -13V to power the linear regulator.

You can use the same chip to directly produce the -1.25V supply. It expects to see 0.8V on the FBX pin so choose the resistor divider appropriately and see how low this circuit can stably produce a -1.25V output. You might need to raise the inductance of L1A and L1B to more like 6.8uH though.