Multiple Parallel LED Drivers with Single Thermal Derating NTC Thermistor

I am looking at powering Some multi color High power LEDs e.g LED engin LZ4 or LZ7 (probably a string of five) using a tps92691 based Circuit

these being high power leds it seems wise to include thermal derating circuitry

the tps92691 suggests using a voltage divider with a NTC thermistor from its on voltage regulator e.g

^{simulate this circuit – Schematic created using CircuitLab}

The LEDs however are either 4 or 7 LEDs in one die so using multiple PTC seems redundant and complicates track layout near the LEDs . I am therefore looking for the best way to connect multiple drivers to a single NTC thermistor.

The simplest circuit would be

^{simulate this circuit}

but that makes assumptions about the TPS92691's 7.5V LDO Regulator and that they wont "fight" with each other.

and external regulator is possible but is that really necessary?

is this a sensible way of doing this or is there a better way?

Answer

A single thermistor is located near the LED's thermal pad.

Your circuit should work, but you only need one. Just use the VCC output from one driver, one R_adj, a single thermistor, and feed the IADJ of all the drivers.

Because the I_adj is an unbiased input (can be modulated by external voltage source from 0 V to 2.25V), one circuit should work for all the I_adj inputs.

This is assuming that all drivers are set to use the same output current. If you need separate current adjustments, then you should consider using a driver with a separate thermal foldback pin like the TPS92510.

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TI's LM3414 datasheet has a thermal foldback circuit that will work for you.

The app note for the LM3414 eval board has a detailed explanation for the circuit values. The LM3414's reference voltage is 1.225V where the TPS92691 is 2.25V. Use one of the TPS92691's VCC output (pin 16) to power this circuit.