TPS3851: Power management forum

Si Yoon Kim

Intellectual 710 points

Part Number: TPS3851

Dear Team,

Thanks for your reply.

Link: https://e2e.ti.com/support/power-management/f/196/t/977770

I thought the issue was resolved in the link above, but there was a part that I didn't understand.

Please see my answer in blue below:

Hi,

Please see my answer in red below:

As a result, SET1 is asserted as Low, but /WDO is output as High.

When SET1 is asserted low, the watchdog timer is disabled and /WDO becomes high-Z state.

On the datasheet, isn't /WDO going to high-Z state when a RESET not SET1 is asserted low?

May I ask you about that?
When SET1 is Low, is the part related to /WDO status on the data sheet?

Could it be that SET1 high output for several seconds during initial booting, could be the cause of making /WDO high??

No, /WDO is high because the SET1 pin is pulled low. When SET1 is pulled low, any input to the WDI pin is ignored.

"No, /WDO is high because the SET1 pin is pulled low." <- This is also the case.

Please check, and your answers are helpful to me.

over 4 years ago

0 Ben C over 4 years ago

TI__Guru 56345 points

Hi,

On the datasheet, isn't /WDO going to high-Z state when a RESET not SET1 is asserted low?

When SET1 is low, the watchdog function is disabled. Therefore, the /WDO pin will go to a high-Z state allowing the user to still use the TPS3851 as a supervisor. Also, /WDO only asserts if the /RESET output is not asserted. If /RESET is asserted, the /WDO state is high-Z state.

May I ask you about that? When SET1 is Low, is the part related to /WDO status on the data sheet?

No, when SET1 is low, the watchdog feature is turned off and /WDO will become high-Z.

In your application, the schematic shows SET=1 and CWD=1uF. This means that the initial tWD time is 77.455seconds. See below:

In your scope photos, /WDO is high. This explains why the /WDO stays high in the beginning. The initial setup for tWD is 77.455secs and then SET1 is pulled low which maintains /WDO to be high.

I hope my explanation have answered all of your questions.

Ben