# Need help understanding part of circuit

#### Teik

Apr 22, 2017
9
I would like to understand how this part of circuit works. CF on the left is input in microcontroller. CF on the right goes logical high and low. Datasheet states that R1 is 1K, R2 should be 4K or higher and C3 depends on application. What does R2 and C3 do? To me this circuit looks like low pass filter but how does it affect communication between CF pins?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,652
Show us more of the circuit or give us a link to the datasheet/app-note from where you took this image.
CF on the right goes logical high and low.
CF on the right looks like it is directly connected to a point labeled GND. How is it supposed to go high? Or are these case outlines, not connections?
What is the supposed function of these pins?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,652
CF is an output (open drain) of the driver IC. R1/C3 form a low pass filter. This filter will suppress any high frequency spikes on the CF output which can arise due to the high power switching operation of the IC.
R2 is required to generate a high signal to the µC when CF is inactive. The ratio of R1/(R1+R2) needs to be such that for a low output at CF of the controller IC (assume 0 V) the voltage at the CF input of the µC is still recognized as low.
Thinking in TTL levels this means that V(CF_µCin) needs to be <= 0.8 V. With R1 = 1 kΩ and R2 = 4 kΩ this voltage will be V(CF_µCin) = 5 V × 1 / (1 + 4) = 1 V which is to much for a reliable TTL level. In this case R2 needs to be higher. I recommend 10 kΩ.
Thinking in CMOS levels this means that V(CF_µCin) needs to be <= 1.5 V (assuming the µC is powered by 5 V), so here 1 k and 4 k would be o.k.
You need to study the datasheet of the µC you are using to find out which situation you have. It is always a good idea to leave some safety margin between the low level input threshold of the µC and the max. low level voltage at C3. Use my above calculations as a starting point for your own.

#### Teik

Apr 22, 2017
9
R2 is required to generate a high signal to the µC when CF is inactive.

Why? Is it meant to be like a pull-up thing?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,652
Is it meant to be like a pull-up thing?
Of course it is. The output of the VBG08H is open drain, as I said.

#### bertus

Moderator
Nov 8, 2019
3,269
Hello,

As the block diagram shows , the CF output is open drain as @Harald Kapp said:

Open drain requires that a load is connected to the powersupply to have effect.

Bertus

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