Temperature compensation in circuit?

[Conrad Hoffman]

No it can't. IMHO, the circuit is inherently flawed because it doesn't address the TC problem in the right place. This has to be done where the current is sensed and converted to a control signal. You could buy special high TC resistors, or add components to try and compensate it, but those schemes are inherently troublesome. All transistors have basically the same temperature dependency, so changing that isn't the answer. I've used the simple op-amp circuit for exactly what you describe (stabilized LEDs) in scientific instruments, and it works well with almost zero TC. The trick is to use a modern op-amp that operates near ground on both input and output, then you have a wide range of adjustment without using another supply. Or, use one of the discrete circuits described earlier that are typically used as current sources in audio amps. Their bandwidth will be better than the op-amp circuit, but their accuracy, though ok, won't be quite as good.

[stef1777]

Thanks for the advice. In fact, the circuit is done and used. I can live with it. I should only take care of the temp drift, aka take care of the temp in the box before selecting the Rsense.

For next boards, I'll use the AMC7135 cheaper and efficient (but with higher dropout).

Do you have some op-amp ref which will be good to use for an opamp/mosfet version of the linear regulator? I've some small mosfet in stock and can try to build a prototype.

[Conrad Hoffman]

It depends on how much current you need, the value of the sense resistor, the adjustment range, the available supply, and the bandwidth. I usually use LF411/412, TL07x, OP-27. It should be something unity gain stable for least trouble. You only need special op-amps if you want to use a very low supply voltage, or if the MOSFET turns on with a very low gate voltage. Also, if you want to use a low value of sense resistor and a low current setting. Just look at the voltage on the sense resistor at the desired current setting, and make sure the op-amp inputs are ok at that voltage. No numbers off the top of my head, but any amp that advertises that the input CMR includes ground is a good bet. If you use dual supplies, none of that matters at all, but I assume most people want to go single.

[hailteflon]

If you are using an LM334 as a CCS on the base of the transistor you have chosen a temperature sensor. The current varies with temp.

Use a TL431 CCS, rock solid.

datasheetarchive.com will have the datasheet.