Electronics > Beginners

Led current sense indicator help

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Ian.M:
Don't bother.  Km's probably just a spammer running up their post count.  IMHO Km's less relevant than a Markov bot! Edit: our moderators concur - Km's just been banned.  :-DD

Back to detecting a load on the uP9616PDC8.  Its datasheet says its switching frequency is typ. 125KHz and that its quiescent current is typ. 1mA when its not switching.   Therefore detecting when its active is simply a matter of detecting when its Lx pin is switching.   A charge pump with a 220pF coupling capacitor could deliver 2.6mA at 8V in, rising to 10.5mA at 32V in, without significantly loading Lx, which is already driving the boost capacitor.   That would be plenty for a high efficiency LED.

kentfielddude:
Thanks for the suggestion. I've been doing some research on charge pumps online but I'm a bit confused on how they work. Could you please provide a schematic and edumacate me?
Thanks,
- Jason

Ian.M:
It looks like I fumbled the numbers above.   :-[ To get enough LED current, you need a charge pump with a coupling capacitor in the nF range. 

Provided you aren't running it right at its abs.max. input voltage, tapping in at the earthy end of the existing snubber should work OK.  It only needs three extra components + the LED, one BAT54S dual (series) Schottky diode, and a resistor and capacitor to smooth out the LED current a bit.   

*DON'T* use a LED with a high Vf (so not blue or white) as the extra voltage on C7 directly increases the stress on the Lx pn by decreasing the effectiveness of the snubber.

LTspice sim attached.

kentfielddude:
Thanks for the help. :clap: :clap: I added the led to my circuit below.

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