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LM3401 DIM pin

Other Parts Discussed in Thread: LM3401

Hello.

i tried to make demo board just as described in datasheet. when i turn it on led is working .... ok

now i try to make use of dimming pin. i connect dim pin to my avr microcontroler.

first i want to just turn led off. i read in documentation that when dim pin is low, HG drive is off.

so i set my avr to output 0 on this pin. i also added pull down resistor just in case.

--> led is still on

just in case i try to output 1 on this pin ...

--> led is still on

what am i doing wrong ? shouldnt it work that way ? when DIM is low ... (connected to ground), led should be off ... 

??

thank you for your help.

  • Hi Peter,
    As you said, When you ground the DIM pin, the LED must turn off. I suggest you to monitor the HG pin when you ground the DIM pin.
    The voltage must be high on it, almost 1 V less than Vin. If that is the case, the LM3401 is working fine. Next check whether the MOSFET you are using is PMOS or NMOS. It should be a PMOS.