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J
JPZ
Associate
May 14, 2026
Question

STUSB4761 FB pin

  • May 14, 2026
  • 5 replies
  • 174 views

If I have only 5V input. what to do with the FB pin?

5 replies

EThom.3
EThom.3
Senior II
May 14, 2026

The way I read the datasheet, this device is designed to regulate the VBUS voltage through an optical connection to a circuitry sitting on the primary side of a transformer:

EThom3_1-1778768329271.png

This means that the STUSB4761 acts as part of an adjustable voltage regulator. The resistor and capacitor connected to FB act as the compensation network for this regulation. It does this not matter which voltage it needs to output, so the compensation network will be needed regardless of the VBUS voltage.

If you only need to generate 5 V, then this device may be overly complex.

J
JPZAuthor
Associate
May 14, 2026
Hi,
thank You very much.
If I only have 5V and I need 5A. Can I leave this pin open?
EThom.3
EThom.3
Senior II
May 14, 2026

If you use this device, I don't think that you can leave this pin open. But I am not sure, as I haven't seen your circuit.

What do you mean by having only 5 V? Do you have a 5 V power supply that you use as power source for the STUSB4761 and VBUS? If this is the case, what is the purpose of the STUSB4761?

EThom.3
EThom.3
Senior II
May 14, 2026

Now, if you don't intend to use the STUSC4761 for regulating the VBUS voltage, then I assume that you aren't going to connect Opto_DRV to anything either, and the device will not have any influence on the supply voltage. In this case, I don't think that you need to connect FB to anything.

The question is if the device will accept that the 5 V supply is good, or if will shut down the output because it finds the voltage (that it cannot regulate) to be wrong for some reason. That I don't know the answer to.

One thing you need to keep in mind is that the STUSB4761 is becoming obsolete. So if you are designing something that is to go in production, the STUSB4761 may be an impractical choice.

J
JPZAuthor
Associate
May 15, 2026
hi,
we only need max 1000 devices.
yes i will use sw3561 thank you
J
JPZAuthor
Associate
May 15, 2026

I tryed for more the 80 hours to make this stusb4761 work.
Even programming with nucleo board is not posible.
I will have to focus on chinese chips. 
This chip is very unstable. 
i will go for sw3516 or sw3518 this looks much more easy.
I also changed from st chip to esp32 works much better and easyer.
sorry to tell you this.
But I think the future is in china if usa and europe do not change.

EThom.3
EThom.3
Senior II
May 15, 2026

Any suggestions as to what exactly we need to change?

J
JPZAuthor
Associate
May 16, 2026

The programming is too complicated.

The manual is incomplete.

More information would be helpful.

Good, short YouTube videos would be helpful.

Test circuits and better explanations about the chip would also be helpful.

J
JPZAuthor
Associate
May 19, 2026

Hi 

i will tray to make a pcb with stusb4761 just for testing.
I can not unterstand why it is not working.
this is my schematic. And ist is not working.

JPZ_0-1779221411220.png

Now a made a testing pcb

JPZ_1-1779221480496.png

i hope this will work

 

 

 

EThom.3
EThom.3
Senior II
May 23, 2026

Regarding the first schematic

I don't quite understand what it is you wish to achieve with this. The STUSB4761 is designed to regulate a supply voltage, according to the request of the conected device, and to monitor and limit the current. In your design, it doesn't get the chance to regulate anything.

Voltage regulation is already done by U1. And as it cannot regulate anything, it shouldn't matter whether or not you connect C17 and R28 to the FB pin.

While I haven't read every single word in the datasheet, I do believe that the STUSB4761 chip isn't designed to work this way. As you haven't said anything about in what way it isn't working, I will have to guess. My best guess is that the STUSB4761 senses that the 5 V supply from U1 is slightly off, either high or low. It tries to compensate for this by adjusting the pulse width and/or frequency on the OPTO_DRV pin. This, however, doesn't do anything, and the chip ends up in a fault situation, and doesn't turn on U4/U5.

Regarding the second schematic

Here you apparently haven't connected VDD to the supply voltage. Also, I don't understand why R14 is connected between TSENSE and OPTO_DRV. These may be simple mistakes, though.

But even if these are fixed, I don't give this a better chance at working. You still don't use STUSB4761 as a voltage regulator. OPTO_DRV now just drives a yellow LED, which in itself doesn't do anything useful.

My conclusion is that you are using the wrong device. The STUSB4761 is not designed for what you are trying to do, and (as mentioned before) it has also been discontinued. This means that it is no longer in production, and using it in a new design is not recommendable.

In other words: To achieve what you are trying to do, you will need to find a different part.

 

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