BQ78350-R1: About the difference between the set value of the ASCD delay time and the measured value

Hi,

How are you measuring the delay? Do you have a scope capture you can share? The ASCD Delay Time should set the timing, but there are other things to consider. Check the RC time constant of the SRP/SRN input filter. Look at the time the CHG/DSG pins switch vs. the Vgs of the FETs.

Matt

Thank you for contacting
The delay confirms the drain-source voltage of the discharge FET.
We take a constant current pulse load and check if the discharge FET turns off. Therefore, I don't care about the time from when the signal comes to when it turns off completely.
This is because the customer's inrush current is 1 msec.
I want to prevent the protection from working due to the inrush current.
Currently, ASCD is detected in 1.4 msec, so it meets the requirements.
I would like to know the reason why the inrush current of 1msec does not enter even if it is mass-produced.
I will send a circuit that shows the RC time constant. Do you know anything about this?
Please let me know if you have a calculation formula.

capture.pdf

If ASCD is not triggering even with 1 ms, I think that there may be an issue with the hardware. I do not think the RC time constant of the input filter would cause this delay. You can try changing R36 and R37 to 100 ohms (this is what is used on the datasheet example schematic and on the EVM) to see if it makes a difference.

What are the values of the Cf and Rf components? (see Figure 13 / Figure 14 of the BQ769x0 datasheet https://www.ti.com/lit/ds/symlink/bq76940.pdf). The recommended values are 1uF and 1k. Using smaller values may result in the caps being discharged during a short circuit event - this would cause the BQ769x0 to lose power which disables the timer for ASCD. It is possible that this is happening which might explain what you are seeing.

Regards,

Matt

Thank you for your prompt reply
I will check the cases of R36 and 37.
I'm calibrating the measuring instrument now, so I think the report will be delayed.
The values of Cf and Rf are 10μF and 1.1kΩ.
Does the large value have any effect?

The Cf and Rf values you are using are correct.