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I am looking for a non-inverting LED driver. The main idea is to connect 20 x LEDs (4 series of 5 LEDs each) to 12V source (car battery: car is running so there will be some interferences -> I use 12V voltage regulator). LEDs should be controlled via PIC uC, so a driver is needed and I need help.

I try this circuit, but is inverted (uC pin is HIGH -> LEDs are turned OFF). Is any method to obtain: output pin is HIGH -> LEDs are turned ON?

Note: LEDs will be placed in the right side, replacing ammeter and R6 resistor.

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    \$\begingroup\$ Why all the complexity? You could connect your MCU directly to the FET gate, but we don't know what is the purpose of this circuit, how much current or how quickly the LEDs must be able to blink, do you intend to use PWM or why there is a FET gate driver? \$\endgroup\$
    – Justme
    Commented Dec 4, 2023 at 19:17
  • \$\begingroup\$ Yes, you are right i miss some information: the output is a PWM signal, I want to control the brightness of LEDs in different situation. Sorry for confusion. LEDs use 20mA each, so I need like 0.1A (20mA x 5 series) from FET. Regarding how quickly, I did not calculate or think about frequency, but should be fast enought to avoid flickering. \$\endgroup\$
    – SkePsis
    Commented Dec 4, 2023 at 19:40
  • \$\begingroup\$ OK, then add that to the question. But still, why can't your MCU be directly driving the gate with PWM, why isn't it compatible? What PWM frequency you will use? \$\endgroup\$
    – Justme
    Commented Dec 4, 2023 at 19:43
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    \$\begingroup\$ @SkePsis Filtering is almost certainly not needed. It's possible that your MCU can't drive enough current, but unless the PWM frequency is very high you don't need a whole lot of current to drive the gate of a tiny FET like a 2N7000. \$\endgroup\$
    – Hearth
    Commented Dec 4, 2023 at 19:48
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    \$\begingroup\$ One single lowside MOSFET will do what you need. \$\endgroup\$
    – winny
    Commented Dec 4, 2023 at 20:19

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You can directly drive a 2N7000 gate from a 5V MCU pin at a reasonable PWM frequency such as a few kHz. So GPIO pin high = 'on'.

Worst case MOSFET Rds(on) with 4.5V drive is 5.3Ω @25°C. Probably 50% more when hot, so more like 8Ω. That corresponds to dissipation of 80mW at full power. Not bad at all.

There are much lower Rds(on) logic level parts such as AO3400A (SOT23) - about 5x the gate charge though so they are harder to drive fast. At 1kHz or even 10kHz it will be significantly better than a 2N7000, assuming a 100Ω output resistance of the MCU GPIO.

One potential downside is that in case of a failure of the MOSFET in the form of a gate-drain short, the MCU will likely be scuppered if it's directly connected.

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  • \$\begingroup\$ Thank you for detalis. Make sense to create some protection in case of failure? Another question: how can I reduce the dissipation? Is to much that 80mW? \$\endgroup\$
    – SkePsis
    Commented Dec 6, 2023 at 15:43

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