Understanding MOSFET driver circuit.

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daba1955

Joined Apr 27, 2019
130
Need some help to understand a MOSFET driver circuit I have elected (so far) to use, but need help understanding it.

The picture shows a couple of the 8 drivers for the Anodes of a bunch of LEDs. It's actually an 8x8x8 RGB LED cube, so each anode driver will be supplying power to the anodes of 64 RGB LEDs. The cathodes use TLC5916IN shift registers with constant-current sinking outputs, the current being set with an external resistor to give 20mA per sinking output. I will likely be modifying the values to get a better balance for R G and B LEDs, but they are what they are, for now.

Potentially (no pun intended !) that means there could be a whole layer of 64x3 (192) LEDs "on" if "white" is commanded, giving a total current per anode of 192 x 0.02 = 3.84 A. White will never be a colour selected for long periods, for obvious reasons.

Hopefully I have provided enough information, and my question is "What is the purpose of the 100R additional load ?" Is it there to ensure the MOSFET shuts off when not driven ?

Anode Drivers.jpg
 
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wayneh

Joined Sep 9, 2010
17,153
Hopefully I have provided enough information, and my question is "What is the purpose of the 100R additional load ?" Is it there to ensure the MOSFET shuts off when not driven ?
It would not have that effect as far as I can see. But I’m clueless what its purpose is. Unless it helps make sure the LED turns hard off and doesn’t glow for some reason. But 100ohms?
 
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Papabravo

Joined Feb 24, 2006
16,830
It would be helpful to know the value of the LED supply voltage VD. The HC device will not quite get all the way to Vcc. If VD is higher than Vcc it could damage 74HC595. OTOH the 10K resistor will will limit the current. Maybe you are hoping the MOSFET will limit the current to the LED. I would not bet on that.
 
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