Problem with 7135 in 2 cell light

[ImA4Wheelr]

@Flashy Mike

Yes, the board is zenermodded.

I also believe it’s the stepdown, but why does it stepdown with 5x 7135 at 100% PWM and not with 9x 7135 at about 65% PWM? Both variants pull a similar current, and the heat on the board should be not that different.
And if every single 7135 decides to stepdown due to the individual heat on the chip, why only the 380 mA versions on the slave and not the 350 mA version on the FET+1.

Ok, perhaps it’s just the 30 mA more on the 380 version which makes the difference.
Did anyone else see this behaviour?

I think this is the thread where I first learned about the 7135’s thermal fold back feature. Since then, I have witnessed it several times. I think I had a data sheet that had some info on it, but I lost a bunch of files a ways back.

AMC7135 - Thermal Fold-Back

I suspected but I wasn’t sure… the AMC7135 has a self-recovering thermal fold-back circuit that drops the output to about 250ma in a solid step. I was running 2x 7135’s on a brass strip opposite an XM-L. With the circuit was running at 700ma and with 4v input, it would fold back fairly quickly. A simple touch with a finger on the 7135’s would cool them enough to recover almost immediately. A 3.6V input lowered the occurrence. Nice thing to know.

Regarding your first question above. I’m guessing 5 chips at 100% duty cycle will get very hot if there is a big difference between Battery voltage and emitter Vf. Where as, 9 chips at 65% duty cycle will be much cooler. Just a guess.

Regarding the second question: The single 7135 can dissipate its heat directly into the driver and host. It is probably mounted on the ground ring of the driver which makes direct contact with the host.