# Temperature conpensated LED Drive

I know that there are many LED Drivers out there but most of them are quite big / complex / expensive and don´t do what I need. (all of them do constant-current)

I want to drive a single coloured LED somewhere between 25mA and 100mA (the maximum current is fixed in the application but should be once configurable by some resistor, for example). The LED should shine "as bright as possible" over the range between 0 and 70 °C. So:

I saw the strong current derating of LEDs with temperature, so I could just use the current that is allowed after derating for +70°C over the complete range, but than I might (e.g.) just use 30mA all the time, where at 25°C temperatures, I could e.g. use 80mA and get out more of the LED.

So what I search for is something like a simple temperature controlled current source.

Any suggestions welcome! have a nice day!

• Current source with a thermistor in the source/emitter/virtual_ground branch. Or a cheap micro capable of temperature measurement with derating curve programmed in. Commented Aug 12, 2016 at 7:00

What you need is for the current to decrease linearly from a maximum at T<= 25°C with a certain slope.

You can do this by adding just a few parts- a temperature sensor, part of a quad op-amp and a couple of resistors to the usual current sink arrangement- see below. The TMP36 temperature sensor (+10mV/°C output) is the only part that isn't extremely cheap and widely available.

simulate this circuit – Schematic created using CircuitLab

RS senses the LED current giving about 800mV drop for 80mA LED current.

R5/R6 set the temperature coefficient current slope given the +10mV/°C output of the TMP36 and Rs.

R7 and R8 are a voltage divider that sets the current at a given temperature.

However there is only flaw in the basic circuit- usually derating does not allow you to increase the current below 25°C ambient, for whatever reason.

The TMP36 saturates at 0°C so the current will continue to increase for ambients between 25°C and 0°C, which may violate the specs. If that is a concern, you can add the optional 3rd amplifier and diode to force the output of the TMP36 to saturate at 250mV = 25°C. V4 (250mV) can be derived by splitting R7 into two resistors or by another divider.

The buffer OA3 op-amp is necessary because the TMP36 is not good at sinking current. However that allows us to easily force the output to 250mV when desired.