I am using TLC5940 in our design. I noticed if the load is not connected then the chip using more current than if the load is connected. We are using 8 TLC5940 chips and I measured the following current by just changing the output current required (No load connected)
Chip blank active : 0.44A
0% current : 0.45A
10% current : 0.25A
50% current : 0.45A
100% current : 0.63A
The above current is for the board but the only parameter changed is the output current for all 8 chips.
If I connect the load to the chips then the current comparison make sense and no problem.
Chip blank active : 0.3A
0% current : 0.3A
10% current : 0.45A
50% current : 1.3A
100% current : 2.6A
Is this mean the TLC5940 chip is not efficient if there is no load.
The TLC5940 is operating properly. If you have the output programmed to sink current, but the output is open, the IC trys harder and harder to turn on the internal FETs which results in higher input current to the IC. This will not damage the IC. If you want to reduce power consumption, program unused outputs to sink 0mA.
In reply to Matthew Day:
In the lab, I have a bank of up to 16 daisy-chained 5941s but have a connector of only 64 LEDs for test purposes. I move the connector from one group of 4 chips to the next in order to test various functionality. During this process, I've noticed that sometimes one of the unconnected 5941s becomes extremely hot -- I think due to the phenomenon described above. It seems to always be the first chip in the chain. Are you certain that the disconnected drive currents are low enough to avoid damaging the IC? This CPU has high (40mA) drive IOs so is it possible that there is an issue with some input IO actually being driven?
Is there any mode where the drive currents could go much higher? Would "marginal" chips (i.e. ones that have been exposed to engineering rework and experimentation) be more likely to get abnormally hot?
In reply to Andrew Stone:
As long as the outputs are programmed to sink current, the IC will try everything it can to sink this current. So in general I recommend programming the output off when there is no load connected.
As long as you make sure that there is no ground issue or leakage the part should not get damaged if the load is disconnected, but I recommend programming the output to off state.
In reply to Brigitte:
What specifically do you mean by ground issue or leakage?
Sorry for the misleading comment. In general it is important to make sure that the GND paths are good (meaning that there is the same GND potential on the IC as well as on the VLED supply).
So at the end the layout is important. Sometimes these parts are supplied on a breadboard and then with the relatively fast communication and the high power, different issues can occur.
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