PCA9306: PCA9306 causing the VREF 1 become unstable

Amir Eshraghi

Expert 1270 points

Part Number: PCA9306

Tool/software:

I have asked this question before regarding Vref1 is not unstable, I did below suggestion but non-working properly:

1. Add pull down resistor of <470K to vref1, however that did not solve the issue Entity.

2. Decrease Vref2 series resistor which make the condition worse.

Also you can see on datasheet talk about this issue on 8.1.7. suggest to add pull down and capacitor, which I did

Ven =3.3+0.6 = 3.9
Ibia = (5-3.9)/200k = 0.0000055

Rpul = 3.3/0.0000055 *0.75 = 450K (I have replace C63 to this)

Please let me know what I am doing that cause this issue.

Thanks

7 months ago

0 Clemens Ladisch 7 months ago

Guru 316510 points

Your calculation is correct.

What resistor did you use? How did it change Vref1? Would a smaller resistor (e.g., 100 kΩ or even 10 kΩ) work?

0 Amir Eshraghi 7 months ago in reply to Clemens Ladisch

Expert 1270 points

I end up using 400K to 470K for pull down but still you can see little swings in oscilloscope. Reducing Cref1 resistor to 100K cause more spike in Vref2 voltage. This is even got worse during the conducted immunity when the logic translator is enabled.

THanks

0 Tyler Townsend 7 months ago in reply to Amir Eshraghi

TI__Mastermind 47008 points

Hi Amir,

Is it possible to solder a large PD resistor and decoupling cap on the same SM footprint?

I am wondering if removing C63 in place of the PD resistor is causing more ripple.

Regards,

Tyler

0 Amir Eshraghi 7 months ago in reply to Tyler Townsend

Expert 1270 points

Hi Tyler

I have tried do that and add 0.1uf and 1uf in same place but the result is not that much different.

Thanks

0 Tyler Townsend 7 months ago in reply to Amir Eshraghi

TI__Mastermind 47008 points

Hi Amir,

I will respond by noon CST tomorrow.

Regards,

Tyler

0 Tyler Townsend 7 months ago in reply to Amir Eshraghi

TI__Mastermind 47008 points

Hi Amir,

What is the device you are using to supply the VREF1 voltage?

Regards,

Tyler

0 Amir Eshraghi 7 months ago in reply to Tyler Townsend

Expert 1270 points

Hi Tyler,

I am using NCV8165ML330TBG as my 3.3V regulator

Thanks

+1 Tyler Townsend 7 months ago in reply to Amir Eshraghi

TI__Mastermind 47008 points

Hi Amir,

I am not an expert on LDO's, but the electrical characteristics of the datasheet for NCV8165ML330 look to have IOUT = 1mA unless otherwise noted.

Is there an issue with running the LDO at very LOW current draw? With 100k PD resistor, the LDO will have a load that is pulling in the uA range. What happens to your case when you have a PD resistor = 3300 ohm which would effectively draw 3.3V / 3300 ohm = 1 mA?

Regards,

Tyler

0 Amir Eshraghi 7 months ago in reply to Tyler Townsend

Expert 1270 points

Hi Tyler,

I will test this by adding a drop down resistor right next to regulator

Thanks

0 Tyler Townsend 7 months ago in reply to Amir Eshraghi

TI__Mastermind 47008 points

Hi Amir,

This sounds good. Let me know the results.

Regards,

Tyler

0 Amir Eshraghi 7 months ago in reply to Tyler Townsend

Expert 1270 points

Hi Tyler,

You are correct the load on regulator was not enough to regulate, so by adding resister fix the problem.

Thanks

Interface

Interface forum

PCA9306: PCA9306 causing the VREF 1 become unstable