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Today I tried to build a small circuit which basically consists of three NUD4001 LED driver, three n-Channel FETs and a RGB Power LED from SEOUL. I don't have the exact specs ready but the main specs are as follows:

  • NUD4001 (OnSemi)
  • FET: 2.2A I(DS)
  • RGB Power LED (Uf(Red) = 2.5V, Imax = 400mA)

I tried to build the schematics very close to the Figure 9 - Device Configuration for PWM, yet, instead of using a BJT I used a n-FET. I also didn't populate Rext2 since I do not understand its purpose. I am only powering the circuit from 5V and am using a 2R resistor for Rext1. This yields a Vdrop below 2.2V and the Power Dissipation should be fine. The PWM signal is created by a AT32UC3B and its frequency is rather high (some kHz).

I added the somewhat simplified schematics: https://www.circuitlab.com/circuit/8bde88/nud4001_fet/ (Disregard the AT32UC3, it is actually the EVK1101 from Atmel, I just wanted to display where the logic signal is coming from)

Now for the problem: It appears that the PWM doesn't produce a PWM result. I have an 8bit PWM with 255 being the maximum value (always on) and 0 being always off. I also checked the logic signal with a scope and it looks as expected (I can set a variety of duty cycles and the signal changes).

Yet, the LED doesn't seem to notice the change in the logic signal (I drive the FET gate directly from the GPIO Pin which is configured as an output).

I can switch the brightness from 255 down to 20 and the brightness of the LED seems to be the same. If I turn it fully off, the LED goes off as well.

Now, do you have any idea what I am doing wrong. I can think of the following things:

  • The PWM frequency is too high, so the NUD4001 is not able to work as expected
  • I made an error connecting the FET (What good is Rext2 for?)
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Alright, I investigated this today a little and found out that it works as expected if I turn down the PWM frequency to some good 100Hz. Now I just need to fine tune the visual perception and I'm good to go. Still, if anyone knows what good Rext2 is for (or has any other improvements), I'd be happy to know.

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