Converting 5V to ±12V
I would like to convert 5V rail I have on PCB to ±12V. Existing solution is using boost converter 5V to 12V, and a buck for 5V to -12V. This feels bulky for what it's needed - here are the requirements:
- +12V and -12V rail shall serve for powering opamp only
- Ripple of the ±12V rails is not of a concern, as the opamp is used in comparator mode(something I would like to swap with the comparator in the future but keeping it for now in the design)
- +12V and -12V rail should be within ±5% ie. acceptable voltage range of rail is (11.4V, 12.6V) both for positive and negative
- ±12V rails shall be capable of providing no more than 5mA of current
- There is a 3V3 powered uC on board with available PWM output which could be used for charge pumps
Everything mentioned so far asks for a charge pump, the way how I see it.
Some options I found:
- Maxim/Analog have +5V to ±10V converter IC MAX680/681, with VCC max of 6.2V and V+/V- being +12V/-12V in absolute maximum ratings, I feel like stretching it + it is not within acceptable voltage range
- Do a DIY charge pump using PWM uC output - haven't done this earlier so if someone could explain a concept with jellybean components that would be a useful answer as well.
- Use a standalone 5V to ±12V converter like a Murata one
- Use 2x dedicated charge pumps - 5V to 12V(to be found) and 12V to -12V(TC1044S)
Questions:
- Could charge pump 5V to ±12V be implemented with 1xPWM output from the uC, how would the circuit look like and what would be the component values(capacitor size, diodes, etc.)?
- Are there some other way how to do this which I have not covered?
2 answers
You say the only thing that will be powered by the ±12 V supplies is a comparator. The first thing I would look at is whether you really need the comparator to run from that voltage range.
Presumably you are asking for ±12 V because that is the range of the signals being compared. Use resistors to get the signals into the 0 to 5 V range, then run the comparator off the existing 5 V supply.
I need comparator output to be +-12V, not 0/5V.
Then you need to explain more about your overall system. Where does this -12 V to +12 V signal go if there are no such supply voltages anywhere? This smells like an X-Y problem.
I only address this point:
Are there some other way how to do this which I have not covered?
Yes. Instead of using boost and buck converters, you can use integrated charge pumps. They don't use inductors but only capacitors.
The only one i could find (in my very short search) that has 2 symmetric outputs that can deliver 12V was the LT1026IS8 IC. But it only double the voltage. Maybe you could cascade 2 ICs to generate more. But you have to check if it really works the way you want it.
Honestly, i don't see the advantage over boost converters. What stops you from using a boost converter? When you need more than double your voltage, a boost converter maybe the better option over charge pumps.
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