Open Drain circuit

Thread Starter

Matthias98

Joined Mar 14, 2020
12
Hi
I am working on a LED array controller based on PAM2863ECR LED driver IC controlled by a microcontroller atmega328p (datasheet: https://datasheet.lcsc.com/szlcsc/1811151536_Diodes-Incorporated-PAM2863ECR_C80312.pdf).

My problem is that the driver IC requires an "[Drive with] PWM signal from open-collector or open-drain transistor, to adjust output current. Adjustment range 1% to 100% of IOUTnom for f < 500Hz" (quote from datasheet). My problem is that i haven't heard anything about "open collector/open drain".
The results of my research so far is that I cant simply connect it to a PWM pin of the microcontroller. The solution I came up with is to place a cheap MOSFET in the date line as shown in the picture.
Unbenannt.JPG
Is this a suitable solution?



Thank you for your help.
Matthias
 

MrChips

Joined Oct 2, 2009
21,074
It depends on the current requirement of the LED and the current sink capacitor of the open collector/open drain output.

Open collector/open drain means that the output pin has no dedicated load attached.
You can connect
+POWER -> LOAD -> OPEN COLLECTOR/OPEN DRAIN PIN

The open collector/open drain pin will sink current into GND.

1589816815604.png

If the pin is not capable of sinking the desired load current then you resort to an external driver transistor such as 2N7000 as you have shown. In such a case you still need to give the open collector/open drain pin a load (aka pull up resistor).

1589816936563.png
 

MrChips

Joined Oct 2, 2009
21,074
No. I am referring to the MCU output.
If the MCU is ATmega328 then you do not need pullups.

I will assume that your problem is with regards to PAM2863ECR LED driver chip which I would have to study before I can understand your problem.
 

Thread Starter

Matthias98

Joined Mar 14, 2020
12
So basically in this configuration?
To pull the IC Pin either to ground or to leave it floating right?
I am not sure if I really need the pull down resistor.
Unbenannt3.JPG
 

n1ist

Joined Mar 8, 2009
184
The pulldown keeps the FET off until the micro has a chance to reconfigure the pin as an output. During reset, all GPIOs are set as inputs so the FET gate would float.
 
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