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Q: TLC5944, Vup connection if not used by 1090226
"We are looking at changing from a TLC5941 to a TLC5944 as it provides a more flexible serial bus for our application. We do not need the Pre-charge capability of the TLC5944 but we have concerns about how to connect the Vup pin that is involved with the pre-charge function. We have the evaluation board and this seems to imply that it is actually OK to leave Vup floating, something that we did not get from the data-sheet. However, the evaluation board is using the same 5V PSU for all the LEDS. For out application we need to allow for a mixture of 6.25Volt and 12Volt PSUs being used for different length stings of LEDs. Is it still OK to leave Vup floating with these multiple LED PSUs? Are there protection diodes on the current sink outputs? These will be reversed biased where connected to the LEDs on the lower supply voltages. The reverse bias will only be a few volts but will this cause any damage or unwanted operation? Are there any other reasons not to leave Vup floating?
A: Re: TLC5944, Vup connection if not used by 10449
If you are not using the TLC5944 precharge function, you can leave VUP open. The internal precharge FET will still turn on and off, but it will not have any affect on LED performance because it is only connecting an open circuit to the OUT pins. If you have different voltages powering different LEDs that are connected to the same IC, the higher supply voltage will be present on the OUT pins connected to the LEDs with the lower supply voltage. Let's use an example where LED0 is powered from 12V and LED1 is powered from 5V. When BLANK goes high, both LEDs turn off. The 12V supply will go through LED0 and put 11V on OUT0. During this time, the precharge FET is on, so VUP will be 11V. This puts 11V on OUT1. LED1 will be reversed biased with 11V on the cathode and 5V on the anode. When BLANK goes low and the LEDs are turned on, the TLC5944 sinks the proper current from each LED. If one LED is turned on and the other is turned off, the TLC5944 still functions properly.
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