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HT16K33 Current Limits
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HT16K33 Current Limits

by dmadison on Fri Jun 16, 2017 5:01 pm

I could use a bit of help understanding the current limits of the HT16K33.

If I'm reading the datasheet right, each 'row' pin can source 25 mA and each 'com' pin can sink 200 mA, with all 'row' segments active at the same time. But if you're lighting all 16 segments, isn't that 400 mA? What am I missing?

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Re: HT16K33 Current Limits

by franklin97355 on Fri Jun 16, 2017 7:17 pm

You can have half of them on at a time (multiplexing)

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Re: HT16K33 Current Limits

by dmadison on Fri Jun 16, 2017 8:05 pm

You're saying that the HT16K33 only enables half of its outputs for a given 'com' at any time?

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Re: HT16K33 Current Limits

by franklin97355 on Fri Jun 16, 2017 10:16 pm

I'm saying you have a 200ma limit that you can use any way you want.

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Re: HT16K33 Current Limits

by dmadison on Fri Jun 16, 2017 11:41 pm

Right, but my question is why doesn't having all 16 outputs simultaneously turned on exceed that limit? Or does it? How does that math work out?

Surely it doesn't exceed the limit, otherwise - if I'm reading the code correctly - the library's demo example for the alphanumeric backpack would exceed the current limits. From what I see the library doesn't lower the duty cycle to reduce load, and it seems odd that I can't find a mention in either Holtek's documentation or yours about being careful with enabling too many LEDs.

It seems like I'm overlooking something simple.

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Re: HT16K33 Current Limits

by adafruit_support_mike on Sat Jun 17, 2017 6:15 am

The 200mA value for each COM pin is spec'd as typical, not a maximum.

It ends up being a statistical thing.. to get more than 200mA through a COM pin, more than 8 ROW pins would have to be active at the same time. The number of alphanumeric patterns where that happens is fairly low, and it isn't a continuous load. Each COM pin is only active for 1ms at most (less if the brightness is set below the maximum value), and gets at least 7ms to rest between pulses. Mosfets are good at handling burst current as long as they have time to cool off between pulses.

It's also possible that the chip has an internal limit on the total current that goes to the ROW pins. It isn't mentioned in the datasheet, and I haven't hooked one up to measure the total current under pathological conditions, but it would be prudent and easy to do.

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Re: HT16K33 Current Limits

by adafruit2 on Sat Jun 17, 2017 12:58 pm

what mike said - the true output is pulsed (multiplexed) so it turns on all the LEDs and then pauses for just a tiny bit to cool off. it works fine :)

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Re: HT16K33 Current Limits

by dmadison on Sat Jun 17, 2017 9:41 pm

Good to know. Thank you all for the help and the reassurance!

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