EP53A8LQI- Noise from VFB pin

[Shifali15]

Hi,

I am using EP53A8LQI in my design. I have designed it for Vout of 1.2V using Resistor divider network. But im not getting the desired output voltage of 1.2V also there is a noise coming from VFB pin. I the pin is pressed the noise disappears. 

Attached is the schematics for reference. Please suggest me a solution

[Shifali15]

Hi Mostafa,

Currently I have rigged up the circuit in general purpose PCB and testing it. And Im finding the instability issue. 

Reason for doing it in general purpose is there is no option to isolate this voltage from FPGA. This instability should not cause any problem to FPGA. Currently its been tested in general purpose.

[Shifali15]

Hi Mostafa,

The instability issue is solved by mounting  22pF on top of R118. Could you please let me know the significance of this capacitor. 

Also Thankyou so much for your support so far.

Now the FA analysis is not required right.

[Mostafa_Intel_AE]

Hi Shifali,

Thanks a lot for testing inserting 22pF on top of R118.

This 22pF is a feedforward cap and it helps to adjust the control loop bandwidth and Phase margin.

Basically this device has an internal feedforward cap which is connected to the Vsense pin, but based on your layout i noticed that you directly connected that Vsense to the Vout pin underneath the package, and this is not the correct connection where the Vsense required to be connected after the latest capacitor in the control loop.

I requested the FA where i though you only have one or two boards that it has that issue.

So i will close the FA request from the system, since we know the root cause.

Thanks,

Mostafa