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TPS3850-Q1: Start up from watchdog disable

Part Number: TPS3850-Q1
Other Parts Discussed in Thread: TPS3850

Hi Experts,

Customer want to use below start up sequence of TPS3850. It is start from watchod disabled. The SET0 is connected to VDD, and SET1 is pull-down to GND in initial state.

1) Customer chage SET0 to 0, and SET1 to 1 to enable the watchdog as below. To make the timing latched and effective, is there any need to reset the VDD? Or the timing can be effective immediately without reset VDD?

2)Can SET0 and SET1 change at the same time to enable the watchdog? Datasheet shows 500us is requried between SET0 and SET1 change but customer's test results shows that SET0 and SET1 can change at the same time. Could you please help to clarify it?

3) What's the watchdog status in stage S1, S2, S3 and S4?

Arie

  • Hi Arie,

    Please see my answers below in red:

    1) Customer chage SET0 to 0, and SET1 to 1 to enable the watchdog as below. To make the timing latched and effective, is there any need to reset the VDD? Or the timing can be effective immediately without reset VDD?

    There is no need to reset VDD if the SET pins are being toggled.  Be aware that if the SET pins are toggled, the device will take around 150uS (tWD-SETUP) to respond to changes on WDI after being enabled.  Also, you will need 500uS (tSET) when changing between SET0 and SET1.  See below:


    2)Can SET0 and SET1 change at the same time to enable the watchdog? Datasheet shows 500us is requried between SET0 and SET1 change but customer's test results shows that SET0 and SET1 can change at the same time. Could you please help to clarify it?

    Nothing prevents SET0 and SET1 to change at the same time but we do not guarantee the performance and functionality of the part.  So please follow the datasheet specs.

    3) What's the watchdog status in stage S1, S2, S3 and S4?

    S1 = part is not turned on (WD disabled and output is high-z)

    S2 = WD disabled and output is high-z

    S3 = WD disabled and output is high-z

    S4 = WD disabled and output is high-z


    Ben

  • Hi Ben,

    Thank you very much for your quick reply. I’m still a little confused.

    1.  If customer add delay time between changing SET0 and SET1, the startup sequence would change to below. So what the actual watchdog ratio? 1:8 or 3:4? Will the 1:8 ratio take effect? Is there any need to reset VDD as below to make the 3:4 ratio effective? Customer only want 3:4 ratio here.

    2.  What's the minimum and maximum value of tSET? Datasheet only shows the typical value, which is 500us. Customer need the min and max value to determine what delay time they should set.

  • Hi Arie,

    1.  If customer add delay time between changing SET0 and SET1, the startup sequence would change to below. So what the actual watchdog ratio? 1:8 or 3:4? Will the 1:8 ratio take effect? Is there any need to reset VDD as below to make the 3:4 ratio effective? Customer only want 3:4 ratio here.

    In order to get to the 3:4 ratio you will need to have a /RESET event to change the ratio.  This can be done by reducing VDD to trigger a /RESET event.  See below:  


    2.  What's the minimum and maximum value of tSET? Datasheet only shows the typical value, which is 500us. Customer need the min and max value to determine what delay time they should set.

    We do not have that data.  Since the customer will need to do a /RESET event, the time tSET will be added to tRST-Del and tRST.

    Ben