TPS3808: How to estimate the time for discharge of Ct

Noriyuki Takahashi

Part Number: TPS3808
Other Parts Discussed in Thread: TPS3840

Hi, team

My customer is trying to use TPS3808 and wants to set the delay time to 10sec. Then, according to equation 1 in the datasheet, Ct is needed to be about 1.8usec. Then they have concerns about the discharge time after /RESET is asserted. Could you tell how to estimate the discharge time?

Regards,

Noriyuki Takahashi

over 3 years ago

0 Abhinav.Sharma over 3 years ago

TI__Genius 9375 points

Hi Takahashi-san,

This spec is described as High-to-low level RESET delay which is 20us.

Thanks,

Abhinav.

0 Noriyuki Takahashi over 3 years ago in reply to Abhinav.Sharma

TI__Genius 9245 points

Abhinav,

I think my question was not conveyed correctly. Please let me ask you again.

How long does it take to discharge Ct completely after /RESET is asserted? They have concerns about this, because their Ct can be 1.8uF and they think the time for discharging Ct can be long time.

Regards,

Noriyuki Takahashi

0 Noriyuki Takahashi over 3 years ago in reply to Noriyuki Takahashi

TI__Genius 9245 points

Abhinav

This is kind reminder.

Could you give any comments on that?

Regards,

Noriyuki Takahashi

0 Abhinav.Sharma over 3 years ago in reply to Noriyuki Takahashi

TI__Genius 9375 points

Hi Takahashi-san,

The discharge of the cap should be according to the time constant. That being said, there should be no concern with re-detection of the capacitor or false detection with consecutive faults.

Thanks,

Abhinav.

0 Noriyuki Takahashi over 3 years ago in reply to Abhinav.Sharma

TI__Genius 9245 points

Abhinav

Thanks a lot. Please let me make sure my understanding.

- After /RESET is asserted at the 10sec delay time, the charge accumulated in the Ct will be discharged. Is this understanding correct?

- You mention that High-to-low level RESET delay is always 20us. Is this true in case of any delay time(1.25ms~10sec)?

- The 20us of High-to-low level RESET delay is typical value. But, can you tell how much max value they can consider as High-to-low level RESET delay? They want to know this value can be upto 100us, for example. Could you give any comment on this?

Regards,

Noriyuki Takahashi

0 Abhinav.Sharma over 3 years ago in reply to Noriyuki Takahashi

TI__Genius 9375 points

Hi Takahashi-san,

The CT pin should begin discharging as soon as the RESET is fully asserted. The High-to-low SENSE to RESET delay time is always 20us, regardless of delay time. Additionally, we do not spec the variation of the propagation delay value, as this spec was not tested for a min-max range on this device. For example, the TPS3840 has this characterized.

Thanks,
Abhinav.

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TPS3808: How to estimate the time for discharge of Ct