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I designed a board with an ATSAME70 on it, which uses small SI2342DS MOSFETs to drive LEDs. I need to figure out which value gate resistor I need to use for this, or if maybe setting the drive strength to "low" on the pin is already enough. I tried to go the "usual" approach by checking the datasheet for the maximum current an I/O pin can source/sink, but didn't find any information about that. Only the maximum current on all I/O lines is listed as 150 mA in section 58.1 of the datasheet. Does that mean I can go to something like 100 mA if I am confident that I will never switch 2 FETs at the exact same time? If another FET switches a clock cycle later the current on the first FET should already be low enough, so it should be safe?

The datasheet mentions

Each I/O line also embeds a RSERIAL (On-die Serial Resistor), as shown in the following figure. It consists of an internal series resistor termination scheme for impedance matching between the driver output (SAM E70/S70/V70/V71) and the PCB trace impedance preventing signal reflection. The series resistor helps to reduce I/Os switching current (di/dt). thereby reducing in turn, EMI. It also decreases overshoot and undershoot (ringing) due to inductance of interconnect between devices or between boards. Finally, RSERIAL helps diminish signal integrity issues. The following figure illustrates the On-Die Termination (ODT).

In the electrical characteristic section it is mentioned, that this resistor has 26 Ohms.

As I need fast switching speeds, my gate resistor should be as low as possible, but obviously still within a safe operating area for the processor.

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  • \$\begingroup\$ Why do you need fast switching speeds for LEDs? What does ‘fast’ mean to you? Nanoseconds? \$\endgroup\$ Commented Dec 17, 2023 at 18:27
  • \$\begingroup\$ @SpehroPefhany It is a matrix and to reduce flicker it is necessary to switch the LEDs as fast as possible. The shortest possible on time would be about 1000 nanoseconds. That might need some adjustments once it is up and running to see how it works out under real world conditions. \$\endgroup\$
    – Jenny
    Commented Dec 18, 2023 at 17:01

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You can use something like 100Ω and the MOSFET will switch within less than 100ns and the demands on supply bypassing will be relatively modest.

Normally visual flicker is not an issue until you get timing of the order of ten milliseconds (corresponding to about 100Hz) for a display that is physically fixed in position, and 1/5 or 1/10 of that (say 500Hz-1kHz-ish) if there is vibration or motion. Flicker in the 50-100Hz range is also more visible out of the corner of your eye, presumably because the receptors are more optimized for speed over acuity in order to detect movement that is almost out of the field of vision that could be a threat or opportunity.

Using even higher value resistors will slow the switching but will also reduce the EMI.

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    \$\begingroup\$ I don't think EMI is an issue here, as I am basically switching only 24 LEDs at the same time (it is a 8x8 RGB matrix). That also means my switching frequency is higher than the "visual frequency" as I need to drive one row after another. I wanted to do 255 intensities per LED, with my 1 microsecond interval that is 255 microseconds per row, which is about 8*255 uS = 2 milliseconds for one iteration through the whole matrix. That means I would be operating at 500 Hz. for an individual LED. I hope that explains the unusual fast speeds. \$\endgroup\$
    – Jenny
    Commented Dec 18, 2023 at 18:36
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If your LEDs are wired with actual wires, or on long LED strips that may act as antennas, it is better to switch slowly to avoid emitting unnecessary EMI. If your LEDs are on a PCB next to the micro over a ground plane, it's less important.

To avoid visible flicker, your PWM frequency should be above 200Hz. To avoid annoying stroboscopic effects, a higher frequency is necessary. 5kHz seems to be adequate. In any case, switching losses should not be high enough to warrant switching as fast as possible, so a switching time of 100-300ns should be okay. Check the gate charge in the datasheet: gate current divided by gate charge is an approximation for switching time. Considering your gate drive voltage, this gives you a starting point for the resistor value to get that gate current.

Switching slowly also gives you a bonus: if you want to dim your LEDs to a very low brightness, you may find the first few steps on your PWM are actually too bright, basically it lacks resolution at very low brightness. One solution is to increase the counter clock frequency, but there's a limit to that. The other solution is to reduce the switching frequency to get more counter bits, but then it will flicker. The third solution is to switch the FET slower, so it takes a few PWM steps to turn on. This greatly reduces the brightness of the first few PWM steps, and works well for deep dimming.

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