Particular H bridge

[Mauri65]

Hello,
I need to supply DC voltages to an electrical device. These voltages are 4V, 2.5 and 1.2V as well as -4V, -2.5V and -1,2V, positive or negative controlled by a MCU.
It is not a motor so I do not have the problems that come with EMF, diodes etc..The voltage creation is taken care by voltage regulators so everything is ok.
Max current drawn by the device is around 15mA.
The problem I have is in the voltage inversion circuit.
I was thinking about a mosfet H bridge, which is notoriously able to supply voltages in 2 directions.
The MCU output is 5V, 20mA max so I am not sure which kind of design I should use, seen the limited voltages/currents involved. Also the circuit should be the tiniest possible.
Can anyone please help?
Thank you

 

[MikeMl]

Do you have a split power supply, or do you have to do it with just a single supply? A garden variety opamp can drive those voltage/currents. Do you have some sort of DAC available? Are you willing to cobble a PWM type of DAC onto the PIC output port pin?

 

[Mauri65]

Mike, I apologize, I have edited my message hoping to make it clearer.
Power supply is possibly the same, 5V. I had thought about a DAC but since I really only needed step voltages, I guessed I would be fine with 3 adjustable voltage regulators (like the National LP 2980-ADJ) and a circuit that upon command is able to just invert the voltage out of the National volt. regulator.

 

[MikeMl]

I count that you need seven steps, counting zero V, which means three port pins. You have those?

 

[Mauri65]

Mike, my idea was controlling the voltage regulators enable pin with MCU, so it is 3 pins just for that (4V, 2.5V, 1.2V). Then two more MCU pins to control the direction of the voltage (in the supposed H bridge).
I should have enough pins, I can pick a MCU which has as many as I need.

 

[ronv]

If you have a D to A, depending on how precise you need the voltages, you might buffer the D to A and toggle it between an output port.

 

[Mauri65]

If you have a D to A, depending on how precise you need the voltages, you might buffer the D to A and toggle it between an output port.

That could be an idea if I can figure out how to do it, I was leaving the DAC as last solution if I couldnt solve the other way.

 

[MikeMl]

Ok, here is a hack:

Each of the Port Pins, A, B, C can take on three states: High, Low, and floating (TRIS set to input). They are used one at a time; If using A, set B and C to input mode, etc. A low-high makes +1.5V and -1.5V across the load R9,respectively . B makes +2.5V and -2.5V. C makes +4V and -4V. Resistors R1, R2, R3 ratioed against R4 is what makes the specific voltages you wanted (poor man's non-monotonic DAC).

If A, B and C are all floating (set as inputs), the voltage across R9 is zero. R7 and R8 make a Vdd/2 floating ground.

The OpAmp can be any modern CMOS 6V rail-to-rail type which is capable of sourcing/sinking 15mA.

 

[Mauri65]

Thanks a lot Mike.
However, if I set a port to be output (like TRISA=0) there is no way (as far as I know, but I might be wrong) that I am able to provide a float.
I also need to reduce the voltage to 1V instead of 1.25.
I tried this circuit with Multisim, unsuccessfully. I know Multisim is not the most indicated software though.

 

[MikeMl]

When you make a PIC Port pin an input, it just "floats" (unless the weak Pull-Up is enabled).

Here is the mod for +-1V: