Ok, so I hope the title was some what descriptive. I've got a project I'm working on and I was hoping to get some insight on this because I've looked through the LED drivers manual and it doesn't make too much sense and it's got to be something I don't understand.
I'm driving 16 SMD (1206) LED's (3.3V,20mA) from a TI TLC5928DBQR. I've got the driver Vcc at 3.3V (can take 3-5v) and then the LED bus, or as in the data sheet "VLED" is also 3.3V. I'm feeding both the LED and driver from a 3.3V 800mA regulator, which is fed by a 1A 5V regulator for other IC. Now, I'm getting this odd behavior that I can have the first 15 LED's on but as soon as I turn on the 16th, it goes into a funky mode which sporadic LED's going off. What I noticed when I took my DMM to it was the the power was dropping from 3.3V to about 1.9V and bouncing back up to the timing of the shifting from the driver.
So some ideas I have to correct this is to place 5V on the line and get rid of the 3.3V regulator. So finally my question. If my LED's have a Vf of 3.3V, and the driver is constant current regulating to 20mA, what is happening to the "extra" 1.7V on the bus side of the LED or as the data sheet calls it "VLED". In case it matters I've got a 1uF cap for a bypass on the driver. I hope the schematic helps understand more. Just for reference, ROUT is currently 3.3V and I wanted to bump to 5V.
In case the photo didn't work right the schematic can be view HERE
Thanks all in advance!
I'm driving 16 SMD (1206) LED's (3.3V,20mA) from a TI TLC5928DBQR. I've got the driver Vcc at 3.3V (can take 3-5v) and then the LED bus, or as in the data sheet "VLED" is also 3.3V. I'm feeding both the LED and driver from a 3.3V 800mA regulator, which is fed by a 1A 5V regulator for other IC. Now, I'm getting this odd behavior that I can have the first 15 LED's on but as soon as I turn on the 16th, it goes into a funky mode which sporadic LED's going off. What I noticed when I took my DMM to it was the the power was dropping from 3.3V to about 1.9V and bouncing back up to the timing of the shifting from the driver.
So some ideas I have to correct this is to place 5V on the line and get rid of the 3.3V regulator. So finally my question. If my LED's have a Vf of 3.3V, and the driver is constant current regulating to 20mA, what is happening to the "extra" 1.7V on the bus side of the LED or as the data sheet calls it "VLED". In case it matters I've got a 1uF cap for a bypass on the driver. I hope the schematic helps understand more. Just for reference, ROUT is currently 3.3V and I wanted to bump to 5V.
In case the photo didn't work right the schematic can be view HERE
Thanks all in advance!