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LED Drivers/LCD Bias
LED Drivers/LCD Bias Forum
TLC5943 - spurious internal reset ?
I'm using the TLC5943 in a multi-channel rgb module - 6 drivers for 2 channels of 4:1 scan rgb leds.
The Vled is 12V with voltage dropping series resistors inline with each led column. 5V for Vcc.
I seem to be getting a reset of the 5943 drivers with noise spikes external to the system - ac line side.
These are getting through the power supply and filtering on the module itself.
All I see on the scope is a 8 ns dip to 4.49 V. I'm not thinking this should be enough to reset the internals of the driver.
But when I load the brightness register back with valid data - they resume normal operation. Same operation as expected at startup.
This is on my test bench - the external event is turning on/off my magnifying lens. It has happened with other events in test, but this
reliably causes the apparent reset/clr of the brightness register/chip.
Scope results taken right at pin 28 with a short spring clip to gnd at the gnd side of the bypass cap (0.1uF).
Looking for ideas as to if I'm right about the internal reset and what other things I might look at or try to absorb this event or similar ones.
I have added a clc pi filter, and increased bulk capacitance to 1000uF - still happens. I have thought about a common mode choke - but not sure how I'm going to add this to the input power path on the board. Total draw for the 5V on the board is ~70mA. Also this event seems to have no affect on my omap cpu board on the same 5V rail.
Any thoughts - anyone?
Could you please check the VCC voltage with GND connected close to the IC GND pin instead of the filter cap? The IC just sees the voltage difference between it's own pins and therefore it might be that you see a different voltage there.
In addition it would be helpful to get a schematic of your system as well as a measurement of VCC to IC GND and measurements of the digital input pins with respect to the IC GND pin.
Would you please as well describe a little more in detail what happens? You say you are getting a reset of the TLC5943, but what exactly are you observing and what makes you feel that it is a driver reset?
The scope reading using the gnd pin was not significantly different.
I don't see why I can't send you a pdf of my schematic. Just need to know where to send it.
When the event happens, the display/leds flicker - some go to a random intensity - many just go off.
The part that leads me to believe the 5943 is getting reset internally, is that if I update the brightness value to what it was in all of the drivers - everything, operation wise, appears to be back to working as normal.
Anyone else have a thought/idea/question?
Brigitte, in-case you don't have access, for some reason, to my email addr, you may contact me: m p o p e 1@ g m a i l. c o m
I will then forward the schematics to you. I will not post them on a forum. Thx.
You mention that you have to reload the same data after the problem occurred and then the system works properly again.
What happens if you are not reloading the data at all and just turn BLANK off and on without reseting anything else in the system?
Is the problem maybe just happening when the GS value is higher than 50 (decimal)?
I wonder if maybe the auto-off function of the IC generates the behavior. Please check page 28 of the datasheet for this function.
Shielding is for sure as well good practice if the problem is gone then.
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