TLC5940 PWM control with LED drivers (ZXLD1360 / MT7201)

[danrogers]

Hi all. I am working on something that is really starting to frustrate me.

I have a Arduino hooked up running a TLC5940 works very nicely. I want to be able to use the PWM control of the TLC5940 to feed into the ADJ pins on the drivers, to allow me to dim them.

If I ground the ADJ pin on the drivers, the LED's go off as expected.

I currently have the following circuit hooked up, using a BC327.

It does work, but sending a write of 4095 to the TLC5940 should turn the lamp fully off but it does not. It seems that the transistor is not being driven hard enough? I have tried both a 10k (connected to 12v) and a 4.7k pulling up to 5v) but still the same results. I have also tried setting the TLC5940 to sink 10 & 20mA constant currents.

Also the PWM range does not seem sensible i.e. a write of anything <3500 appears to give no dimming effect, when approx 2000 should in theory be half brightness.

If anyone has some suggestions on how I can get this working I would be very appreciative

https://www.electro-tech-online.com/custompdfs/2013/01/ZXLD1360.pdf
https://www.electro-tech-online.com/custompdfs/2013/01/slvs515c.pdf

 

[ronv]

Try removing the diode in the emitter.

 

[danrogers]

Hi Ron, thanks I will give that a go. I think it was included as there is a bridge rectifier between the lamp supply pins and the regulator chip, so didn't want to drive the ADJ pin below the local ground of the regulator chip? Do you think the voltage drop over the diode is causing the problem then?

Attached is photos of the lamp's driver PCB

 

[ronv]

Yes, I think so. To turn the adjust pin off you need a voltage of 0.2 volts. With the diode the best you can probably do is about 0.7 volts. The ground feom the bridge and the chip ground should be the same.