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Common Reference for individual references

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This devices uses a resistor to set the constant current value. I am using up to 24 of these things all tied together. I'd like to have one common reference for all. (to be controlled by a pot to set the maximum current for a dimming effect)

Ultimately what would be nice is if I could just tie them all to the same resistor but I do not know if this will work? Maybe i'd have to use 24x the resistance? I imagine they simply measure the current going through the resistor and use that as the current(maybe current mirrors?)

I'm not sure though if this would be a valid method though? It definitely would solve my problem though.

I believe the formula for setting the current is

I = 41*1.20 = 49.2/R_IREF as it shows on page 4.

so I'd just have to increase R_IREF by 24x(48k should get 24.5mA per IC).

Anyone see any problems doing it this way? (ultimately I probably will use an active resistor so I can control it digitally)

Reply to
Jon Slaughter
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It is unlikely that a common resistor would do what you want; the data sheet quotes the reference voltage as around 1.2V, and it seems likely that one of your up-to-24 packages would have a higher Vref than the others, and would supply the bulk of the current going through the resistor, so that one block of LEDs would end up brighter than the rest.

Not in a way that you would like.

What you could use would be 24 parallel current sinks; 24 NPN transistors, each with its collector tied to an Iref pin, each with its emitter returned to 0V through a 620R resistor, and all the bases connected to a common, nominally 1.2V control voltage that you could turn down to dim all your LEDs at once.

If all the transistors were BC107 or BC109 for which Vbe is specified to be in the range 0.55V to 0.7V at 2mA and 25C, each transistor would sink between 0.8 and 1.05mA for a 1.2V control voltage, which you could then turn down by reducing the control voltage,

The tolerance is based on worst case specifications; actual transistors from the same manufacting batch tend to cluster more tightly, and you could do even better by sorting your transistors into batches by Vbe-on.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

I'd suggest a separate 47K resistor per chip, with all the free ends connected together, then to a single pot to ground.

Or separate 47K resistors per chip, with the free ends connected together, driven by a voltage source that goes from 1.2 volts (leds off) to ground (max bright). Brightness would be linear on the applied voltage (or actually on 1.2-V).

John

Reply to
John Larkin

huh? that would limit the current way too much! I assume you mean something like 2k. I also don't see the point of doing it this way if you have a common connection except maybe that the resistors can be close to the IC.

I'd rather have one pot and one inline resistor(to limit the max current).

I'm not too concerned about linearity as I will have a lookup table for the RGB to PWM colors. (it will be done emperically since I don't need an infinite number of colors... or maybe I'll map it to a function and use that)

I'm just curious as to why you say 47k resistors per chip? What is the purpose of seperating them? Surely if I have a common point for all of them the effect of just using one will be the same? Or are you saying maybe there will be "back flow" into each chip? If so I guess it's better 1 resistor per chip but I was hoping to skimp on the resistors if possible ;/ (not that it's a big deal but problem is that if the resistors are not accurate then some led's might be brighter than others and I would have to deal with that somehow. I could fix it in the software but I'd rather take care of it up front.

Reply to
Jon Slaughter

I'd expect very bad current sharing if you do that.

I thought that was the nominal full-bright value for each chip. If not, use the right value.

What is the

Current sharing.

Surely if I have a common point for all of them

If you know it all already, why ask for advice?

John

Reply to
John Larkin

Well!

Voltage on the IREF pin is +1.2V.

Output compliance goes down to +0.5V.

Take one TLC5947 and use it to reference 24 other TLC5947's... feed outputs of first into IREF pins of the 24 others.

R_IREF will have to scale up an additional 41X. HOPEFULLY that regulator can cope with the low current.

...Jim Thompson

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Reply to
Jim Thompson

nice idea, but I'm not sure it would work, the output of the TLC5947 is PWM.

It might give some strange effects with one PWM referencing other PWMs all running on slightly different 4MHz internal oscillators.

I'd think the most straight forward solution would be a 1k6 resistor for each TLC5947 driven by 0-1.2V. (reference, pot,buffer) but it depends on the parts behind resonably matched. Don't know if the max 4% channel-2-channel and 7% device-2- device is too much

-Lasse

-Lasse

Reply to
langwadt

if you tie all the "Iref" inputs together they'll all get the same voltage, but not neccessarily the same current, and it's the current that's important.

Figure out what the minimum size of resistor that you would ever want to use is (eg: 1K6) , and use 24 of that size (one from each chip to the wiper of your pot adjustment pot) that should share the current well enough.

(if it doesn't you'll need to build a 24 output current mirror... but from skimming the datasheet I expect the resistors will work for you.)

your combined adjuster should be about 1/24th the resistance of the individual adjuster, so, if you were going to use a 20K variable resistor on one chip you can use a 1k one on the group and get about the same adjustment range.

Bye. Jasen

Reply to
Jasen Betts

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