Designing a constant power circuit

[asp87]

Hi,

I'm trying to design a circuit that can provide constant power from 6mW to 400mW.

The output of multiplier is V=I*R_sense*I*R_load/k
= P*R_sense/k

The problem lies in the range of setpoint required at low and high power.
Ex:
1. Required P= 6mW, R_sense= 20 Ω. If k= 0.75V, setpoint = 0.16 V.
2. Required P= 400mW, R_sense= 20 Ω. If k= 0.75V, setpoint = 10.67 V.

I feel the setpoint input to the opamp in ex. 1 is too low and we are operating near the noise level in the circuit.
In Ex. 2, the setpoint input is high, close to the maximum output voltage swing of any multiplier IC available.

Fiddling around with k will either increase or decrease either setpoints equally.

P.S.: I've drawn the inverting and non-inverting inputs of opamp incorrectly.

 

[crutschow]

You may have to use more than one value of R_sense and select it with a switch to give an adequate dynamic range.

 

[ZekeR]

What's wrong with having a setpoint of 0.16V? Many op-amps have an offset voltage of less than 1mV; with this offset, you can still achieve better than 1% accuracy at 6mW. It's simply a matter of ensuring your op-amp's inputs and the multiplier's ouput include 0V.

If that still doesn't work for you, you might consider using a divider. The input signal (power) can be divided by the measured output current (P/I produces voltage), and this signal can then be fed to the op-amp.