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[on:]

Anyone know of an 8-channel constant current chip (driver or sink) capable of more than 300 mA per channel? Highest I've been able to find is an 8-channel sink chip capable of 150 mA per channel.

[Reply #1 on:]

Combine two channels on that chip...?

[Reply #2 on:]

Combine two channels on that chip...?

Thought of that, but have gotten conflicting answers as to whether that would work or not. Doing it with one chip would also be a bit easier.

[Reply #3 on:]

Plus, finding anyone who sells that 150 mA chip (Mos-Tech MT508) has proven to be difficult. If anyone knows of a dealer who might carry these ...

[Reply #4 on:]

Take your pick:-
http://uk.farnell.com/jsp/search/browse.jsp?N=2031+202729&Ntk=gensearch&Ntt=constant+curent&Ntx=mode+matchallpartial

[Reply #5 on:]

Combine two channels on that chip...?

Thought of that, but have gotten conflicting answers as to whether that would work or not. Doing it with one chip would also be a bit easier.

You can definitely safely combine channels with the Texas Instruments TLC series (e.g. TLC5916, TLC5940...). Here's a white paper from TI that uses that technique:

http://www.ti.com/lit/an/slva346/slva346.pdf

Good luck!

[Reply #6 on:]

Combine two channels on that chip...?

Thought of that, but have gotten conflicting answers as to whether that would work or not. Doing it with one chip would also be a bit easier.

It's hard to give guarantees ... but I can't think of a commonly used circuit where it wouldn't work.

[Reply #7 on:]

Plus, finding anyone who sells that 150 mA chip (Mos-Tech MT508) has proven to be difficult. If anyone knows of a dealer who might carry these ...

I'm sure it can be done with common parts but it would be nice to know how many you need, any size restrictions, budget restrictions, etc.

Are you going to control it with am Arduino? Most LED drivers need an external chip to initialize them.

Do you need PWM control?

etc., etc.

[Reply #8 on:]

It's hard to give guarantees ... but I can't think of a commonly used circuit where it wouldn't work.

Basically if the output uses transistors they won't share, but if they use FETs they will.

It is all to do with the different temperature coefficients of the two devices. With a FET the hotter it gets the higher is it's resistance, a positive coefficients, so if you have two FETs and one is taking more current the on resistance increases and it takes less current so they share.

With a transistor it has a negative coefficient, the hotter it gets the lower is the resistance. Therefore when one transistor gets more current it gets hotter and so draws more current. Having the transistors closely coupled thermally can help mitigate this to some extent.

[Reply #9 on:]

With a transistor it has a negative coefficient, the hotter it gets the lower is the resistance. Therefore when one transistor gets more current it gets hotter and so draws more current.

But...how can it draw more current if it's in a constant current circuit?

[Reply #10 on:]

But...how can it draw more current if it's in a constant current circuit?

That is the mechanism where by they do or do not share current. You are talking about two constant current supplies in parallel. Hence it is taking more current than just one.

[Reply #11 on:]

Thanks for the link, MIke - I've bookmarked it for future reference. Looks like all the solutions at that current range require one chip per channel, so that may be the way I go.

To answer your question, fungus, I'm planning an 8x8 direct drive LED matrix with LEDs that individually run on either 100 mA or 300 mA, full power (no dimming). Only one LED will be lit at a time, so total current draw at any time from all the LEDs will be that 100/300 mA figure. Will control it with an Arduino.

[Reply #12 on:]

To answer your question, fungus, I'm planning an 8x8 direct drive LED matrix with LEDs that individually run on either 100 mA or 300 mA, full power (no dimming). Only one LED will be lit at a time, so total current draw at any time from all the LEDs will be that 100/300 mA figure. Will control it with an Arduino.

Does that mean you need 8x300mA current sinks?

The TLC5916 looks good to me. If you use 3 pins per LED (100mA per pin) then 3 chips can do 8 LEDs and it's switchable between 100/200/300mA in software (depending on how many pins you enable per LED).

Watch the total power dissipation of the chips. Use external resistors to drop the voltage arriving at the chip to less than 1V then the chip should be OK with the rest.

1V x 100mA x 8 channels is 0.8W - on the limit. Aim for less than that, eg 0.5V.

[Reply #13 on:]

Thanks for the inputs. I'm going to chicken out, and use 8 separate constant current sinks (CAT4101). Rated to 1 A, so power dissipation shouldn't be a problem, and up to 6V continuous LED voltage (25V transient). Thermal protection mode and "zero" current shutdown mode. Even I shouldn't be able to blow these up  .

Will control each channel with a separate digital out from the Arduino; that way, programming will be easier than it would be with three shift register chips. They'll be turned fully on/off, so won't need PWM control (though they support PWM).

[Reply #14 on:]

With constant current, if it rated at one amp you get one amp. You need it rated at the current you want to use.