How to generate negative voltage from PIC

[jakehop]

Hi guys,

I'm new here at the forum, and I've been searching for a solution to my problem without any luck! I need to design a circuit, where I can control a negative voltage with a PIC (in this case, the AT-platform). I've got the PIC's PWM-function, as well as digital potentiometers to control the circuit. However, the PWM is limited from 0-5V+ DC, and the digital potentiometers have certain limits to the voltage at their taps.

The negative voltage should be -25 to -70 Volts DC. -50 Volts will also be OK if -70 is totally out of the question. So far, I've got a opamp working as an inverting amplifier, but even the best rail-to-rail model can only give me -22 volts from the +5V sources that I have. I thought about having the opamp controlling an elevated LM337 to get the -25V to -50V, but I can't get it figured out in my head, as I constantly bump into the limits of the PIC or the digital potentiometers.

Also, linearity is important for me. The PWM allows me a resolution of 8 bits - 256 steps. If this proves a problem, I'll use a DAC to get 1024 steps.

Does anyone have a clue as to how I can solve this?

Kind regards, Jake

[Silent Jack]

There are numerous different models of the PIC MCU and many of them are pretty affordable.  Have you tried using the pic to just perform a switching function, and then using an inductor, caps and a 555 to produce the levels you are trying to obtain?  The PIC should be able to monitor and control process by tracking a sample voltage that is stepped down to levels it can handle.

[jakehop]

Hi Jack, thanks for your reply!

I am having a hard time of conceptualizing your idea. However, the circuit that I am looking to integrate this solution into already has a negative voltage source, that I would just need to attenuate. It's like -80-90V, that I'm looking to get down to -50-25V. I would like to get some linearity. My digital potentiometers can't handle this voltage, and I was hoping to get some kind of solution that made use of the 8bit D/A's in my pic, so I could get a resolution of, say, 0,1V in that 25V of potential.

Is this impossible? I'm currently working with a optoisolator, however it's not nearly as linear as I could hope for, even though the manufacturer claims exactly that!

[Silent Jack]

You could try dividing the voltage and using a capacitor to smooth things our for clearer readings.

[Hero999]

You need a negative supply, even the best rail-to-rail op-amps will only give you 0V, if the negative supply voltage is 0V and there's no load current.

The easiest way is to use a buck-boost converter.

http://en.wikipedia.org/wiki/Buck

[jakehop]

You need a negative supply, even the best rail-to-rail op-amps will only give you 0V, if the negative supply voltage is 0V and there's no load current.

The easiest way is to use a buck-boost converter.

http://en.wikipedia.org/wiki/Buck

[Hero999]

When I was talking about a buck-boost, I meant using the PIC as a controller not buying a pre-made controller-IC but if you already have a negative supply then you don't need a buck-boost converter.

I think you're already going down the right path with op-amps but you're correct in that finding one which will work up to 50V will be a problem and 70V is probably certainly out of the question so a buck-boost might be your only option after all, in which case you should read the Wikipedia article I linked to earlier.

[Kevin Weddle]

The easiest way to generate -50V is from a battery. The PIC output can switch a MOSFET that is in series with the battery and the load. The positive terminal of the battery is connected to ground and the negative terminal is connected to the load.