Isolated 1-10V (or 0-10V) output

Question

I need to control an LED driver with analog 0(1)-10V control input.

This control device should be very cheap and compact so until now I made a schematic which uses the fact that those drivers have 100 uA current source at its dimming input. Switching capacitor made this cheap, small and relatively easy.

But now I need to adapt this solution for a driver which has "400...550 uA current at control pin". This is a problem for several reasons:

• I need to drive about 5...10-times more current in the primary circuit which becomes 4-5 mA (I don't have so much current)
• Switching capacitance schematic gives a voltage depending on a current. So if this current isn't precise - my voltage will be not precise as well

I'd like to avoid any isolated DC-DC with opamps solutions as this will be big and expensive.

Is there any better solutions?

Answer 1

I'm not sure I'm following what all of your constraints are, but my first thought would be something like this:

^{simulate this circuit – Schematic created using CircuitLab}

This is similar to the previous idea, but it now uses an LMV431 shunt regulator to deal with the variability of the current source. If the switching frequency is zero, the control voltage will be 1.25V, and as you increase the switching frequency, the control voltage will rise.

Answer 2

I initially just wanted to comment, but I don't have enough rep. So I have to go with a full-blown answer.

You could have a look to the devices like ADuM5240. They are digital isolators (two I/Os) that also integrate an isolated DC-DC converter in a single device. So you send both the PWM and the 5V supply to the other side. Then you can do pretty much what you want.

Here is what I came up with. It goes from almost 0V (VCEsat of NPN) to 10V (it could even go above it with a different R2/R3 divider configuration), and its behavior is totally independant of the current source value. The total cost of this may not be more expensive than Dave Tweed's solution, depending on what you use for the "U1 isolation/SW1/SW2" blocks. Note that I think any basic opamp can do the job here. (Maybe you could even use an open-collector comparator instead, and getting rid of the transistor, provided that the comparator can handle 10V at its output with only a 5V supply, and that it has some linear region - which is not usually what a comparator is for, but...)