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LED driver dimming
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Tantalum:
Hello

I wanted to dim the led drove by a PT4115 controller. The chip has a DIM input for that purpose.
https://people.xiph.org/~xiphmont/thinkpad/PT4115E.pdf

According to the datasheet it could be done by using a PWM signal or a DC voltage (0-2.5V).

--- Quote ---dedicated DIM input accepts either a DC voltage or a
wide range of pulsed dimming. Applying a voltage of
0.3V or lower to the DIM pin turns the output off
--- End quote ---

The pin is internally pulled-up:

--- Quote ---The DIM pin is pulled up to the internal regulator (5V)
by a 200KΩ resistor. It can be floated at normal
working.
--- End quote ---








Using a PWM (OC) is ok.

Question:

How can a DC voltage/signal regulate the brightness of the led, when the DIM pin is internally pulled-up to 5V???




Kasper:
It is pulled up through a 200kohm resistor. That means it is only lightly pulled up. You can overcome that and pull it down in a variety of ways.

If you try connecting various resistors between dim and gnd, you'll see. Resistors between 0 ohm and 200 kohm should work.
james_s:
Driving it directly with a microcontroller PWM pin works fine too. 200k is not going to require much current at all, it's just pulled up so you can leave it floating if you don't need dimming.
Tantalum:
Yes ok I know, but want to UNDERSTAND how it works in DC mode although it has an internal pull-up restistor to 5V.
paulca:

--- Quote from: Tantalum on August 05, 2019, 09:30:18 am ---Yes ok I know, but want to UNDERSTAND how it works in DC mode although it has an internal pull-up restistor to 5V.

--- End quote ---

If you put 2.5V on that pin your circuit will sink a tiny bit of current to hold the pin down at 2.5V.  The other 2.5V will drop across the pull up resistor.
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