What is the purpose of a zener diode on the drain of a load-switching PFET?

Gary Leutheuser

Hi,

I have been reviewing the following reference design for an LED matrix: www.ti.com/.../tida-00161.

There is one component that perplexes me: the zener diode that separates the 100 ohm resistors on the drain side of the PFETS from ground.
I believe I understand the point of the 100 ohm resistors - to quickly discharge the load, preventing ghosting of the LEDs when multiplexing the LED matrix at high frequencies (confirmation/explanation of this idea would also be appreciated.)

Thanks,

Gary

over 8 years ago

0 Brett Barr1 over 8 years ago

TI__Mastermind 18200 points

Gary,
Can you provide a screenshot of the schematic where this diode appears? I don't believe our FET's are on this reference design, but I can take it to our applications team for a brief explanation.

0 Gary Leutheuser over 8 years ago in reply to Brett Barr1

Prodigy 20 points

Hi Brett,

Yes, you are correct, there are no TI FETs in the design. Also, I am only a student with a purely educational interest, so hopefully this question is an okay use of this forum (if not, please let me know).

Here is the requested screen-shot, at any rate:

0 Brett Barr1 over 8 years ago in reply to Gary Leutheuser

TI__Mastermind 18200 points

Gary,

No problem. I sent this to our apps team so lets see what they come back with.

0 Brett Barr1 over 8 years ago in reply to Brett Barr1

TI__Mastermind 18200 points

Response from the apps guys is the diode is there to prevent (clamp) excess voltage stress across the LEDs where the 100ohm resistor will limit the current.

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What is the purpose of a zener diode on the drain of a load-switching PFET?