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Shown in the schematic below is a typical MOSFET driven by a BJT. I know the purpose of almost everything here, except for R4. I just can't figure out what the resistor on the MOSFET's drain is for Is it some sort of pull-down? And why might it be needed here?

schematic image

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  • \$\begingroup\$ What is the point of C8 to ground? \$\endgroup\$
    – sherrellbc
    Commented Mar 14, 2015 at 5:14
  • \$\begingroup\$ @sherrellbc: It's a reservoir cap used to provide a lo-Z voltage source for the load when Q2 first switches ON. \$\endgroup\$
    – EM Fields
    Commented Mar 14, 2015 at 5:29
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    \$\begingroup\$ For the love of FSM, please use the freaking ground symbol for ground connections, and the power symbol for power connections. They exist for a reason, and you're horribly abusing net-labels instead, for no good reason. \$\endgroup\$
    – Connor Wolf
    Commented Mar 14, 2015 at 6:07
  • \$\begingroup\$ @ConnorWolf It's not my schematic, someone else made it. \$\endgroup\$
    – Techjar
    Commented Mar 14, 2015 at 7:40

2 Answers 2

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Given that you have said that the output line goes to an array of LEDs, I think that R4 is there to quickly discharge the LED capacitance when the FET is turned OFF. This would minimize ghosting if the LED array is multiplexed quickly.

I used to run into that problem with some of the large LED arrays we used to build many years ago. Adding a low-value resistor to quickly discharge the charge stored in the LEDs reduced or eliminated the ghosting problem.

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    \$\begingroup\$ Bingo! +1 for you. \$\endgroup\$
    – EM Fields
    Commented Mar 14, 2015 at 6:20
  • \$\begingroup\$ I didn't even think about the stray capacitance, that actually makes a lot of sense. The LEDs are actually being multiplexed at a quite high frequency. \$\endgroup\$
    – Techjar
    Commented Mar 14, 2015 at 7:43
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Q2 is a P-channel MOSFET, so yes, R4 is a pulldown and part of the load Q2 is working against.

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  • \$\begingroup\$ What is the drain pull-down necessary for though? Fast switching or something of that sort? \$\endgroup\$
    – Techjar
    Commented Mar 14, 2015 at 5:05
  • \$\begingroup\$ It serves the same purpose as does R3, which is to provide a voltage drop when charge flows through it. \$\endgroup\$
    – EM Fields
    Commented Mar 14, 2015 at 5:17
  • \$\begingroup\$ It seems to me like R4 is used to limit the on-state current through the MOSFET, as well as provide the node where the voltage is measured. \$\endgroup\$
    – sherrellbc
    Commented Mar 14, 2015 at 5:17
  • \$\begingroup\$ @sherrellbc: With an Id rated at -17 amperes, I doubt it. \$\endgroup\$
    – EM Fields
    Commented Mar 14, 2015 at 5:34
  • \$\begingroup\$ The line labeled ANODE_LEVEL_0 goes to a large array of LEDs, so R4 isn't for current limiting. \$\endgroup\$
    – Techjar
    Commented Mar 14, 2015 at 5:40

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