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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

schematic

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

schematic

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?

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  • \$\begingroup\$ I don't think I could justify the LZ7. It does have a strong (1W) blacklight LED. The Luxeon Color C line have a very small package (2.0 x 2.0), are reasonably priced, and have good color mixing. You could fit 16 Luexon C in the same area of a LZ7. Luexon C provides flexibility in layout and color selection. \$\endgroup\$
    – Misunderstood
    Commented Aug 21, 2018 at 19:14
  • \$\begingroup\$ Those look interesting but does bring up question of best way of getting the equivalent flux output(s), as they appear to operate at "base" current of 350mA and give roughly half the flux, of a LZ4/7 at that current, and while can operate at 700mA ("base" for LZ4/7) seem to give ~1.75 times there base flux at that current. I also cant find thermal derating information on there spreadsheet. \$\endgroup\$
    – fireblade
    Commented Aug 22, 2018 at 15:39
  • \$\begingroup\$ Not only 350 mA vs. 700 mA but 85° vs 25°. Looking at min flux for red Luxeon C = 35 lm adjusted for temp => 58 lm, then adjust for current => 104 lm vs. LZ7's min flux of 60 lm which equates to Luxeon C red having 58% better performance than LZ7. Then adjust for efficacy Luxeon C red min Vf = 1.75 and LZ7 min Vf = 2.1 so add another 20% for Luxeon C when comparing lm/watt. \$\endgroup\$
    – Misunderstood
    Commented Aug 22, 2018 at 18:04

1 Answer 1

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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 Radj, a single thermistor, and feed the IADJ of all the drivers.

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

enter image description here

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.

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.

enter image description here

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