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Related: What is the purpose of the resistor on this MOSFET's drain?

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This is a circuit taken from a TI reference design (http://www.ti.com/lit/df/tidr687/tidr687.pdf) to drive an LED matrix - it is used to switch on each row of the display in succession. The loads (the LEDs, driven by a current-sink driver), are attached at L15/L16.

From the related question, I can see the point of the 100 ohm resistor, but what benefit does the zener add?

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  • \$\begingroup\$ Where on L15/L16? Before the 100 ohms, right? \$\endgroup\$
    – TylerH
    Commented Apr 27, 2015 at 3:28
  • \$\begingroup\$ Right, between drain and 100 ohm. \$\endgroup\$
    – Gary
    Commented Apr 27, 2015 at 3:29
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    \$\begingroup\$ looks like it's a voltage clamp for the entire rail \$\endgroup\$
    – KyranF
    Commented Apr 27, 2015 at 4:09

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It stops voltage spikes from travelling down that line the cathode is attached to. Just like a gate-source protection diode, once the voltage on L15 or L16 goes past Vzener, the zener clips it off.

Basically stops you from overvoltaging whatever's on that line by the cathode.

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  • \$\begingroup\$ @ARMATV Does it have anything to do the driver being for RGB LEDs, due to the difference in forward voltages of different color LEDs? Although, I don't know why it would be an issue as each color has its own constant current source in the driver... \$\endgroup\$
    – Gary
    Commented Apr 27, 2015 at 17:14
  • \$\begingroup\$ @Gary I have no idea what the requirements are for multicolor LEDs, but the only purpose this diode will serve is to keep that line's voltage less than the zener voltage. You can probably extrapolate whether or not that has anything to do with forward voltage drop. I doubt it. It just looks like it's there to stop you from giving a 5V LED a 10V spike or whatever. Hard to say without the whole circuit. \$\endgroup\$
    – ARMATAV
    Commented Apr 27, 2015 at 18:56

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