Hi,
I’m building a LED panel and I’m going to use TLC5926 which is a very simple IC with 16-channels,
serial input and (shift register) and individual channels for constant current on each LED.
As it is very simple there is no embedded circuitry to perform the brightness control.
Brightness is controlled then with an external resistor.
For one device, I would simply use a single digital potentiometer as possible solution.
But once I’m using a daisy-chain of 6 ICs, I would need some different solution to control the global brightness.
Any ideas how to control the brightness with digital output from MCU ?
I could use other ICs with internal config register, but once I only need global control, I’m really inclined to keep with this IC, and figure out how to change the brightness for all the 6 nodes.
How about a current mirror setup ? I forget why I had this circuit simulated but w/a little adaptation it shows one way to change all the control currents in lockstep. In the model V3 and V4 are two of the TLC5926s, pin “R-EXT”. You can see that Ir3 and Ir4 are the same as the “DAC” voltage varies, even though the voltage at the two “R-EXT” pins is different. Of course the 2 resistors (R3, R4) must be matched as well as the two transistors (3904s in this case). Matching Rs isn’t hard and you can buy naked transistor arrays on an IC, 6 or 8 per chip, that are also well matched.
Of course the “DAC voltage” can come from a DAC, or a digital pot or a well filtered PWM line.
Given the TLC5926’s current multiplier (Iled vs Iref) and internal circuitry and your LED choice, you may have to change the nominal value of R3/R4 to get the range of LED current you want.
http://www.fairchildsemi.com/ds/FF/FFB3904.pdf
http://www.fairchildsemi.com/ds/FF/FFB2222A.pdf