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TPS3838-Q1: TPS3838L30 doesn't perform expected reset output

Part Number: TPS3838-Q1
Other Parts Discussed in Thread: TPS3838, TPS3837, TPS3836

HI,

My customer is facing that TPS3838L30 sometime doesn't perform correct reset out under certain condition.

it happened that very first time power cycle (12V ramp then ACC S/W ON).

It happened that power cycle again by ACC S/W ON-> OFF -> ON many times.

Please find attached file which shows OK case and NG case waveforms.

Also circuit and power tree.

Could you please suggest which is an problem point or possible cause?

Please suggest where I need to check around TPS3838L30.

INPUT.zip

Regards,

Kasuya

  • Kasuya,

    Is there a reason why there is a resistor divider on the /RESET output? This is not normal configuration and typically only a pull-up resistor is used.

    -Michael

  • Hi,

    As for pull-down resistor, Customer wanted to suppress spike during power ramping.

    They removed pull down to check behavior again but result was the same as before.

    Please check VDD and other signal ramp timing waveform.

    VDD ramp first and then MR, CT.

    It happened first power cycle w/ 100%.

    Before power ramp, around 600mV voltage is present on all the input pins.

    What can cause this?

    Regards,

    Kasuya

     

  • If VDD ramps above the minimum operating voltage but /MR is still low, the device will not function and will remain in reset. Also, if the CT pin ramps slower than VDD, the timing will be off until the CT pin is correctly detected. I don't see this happening in the above captures.. and if there is voltage on the input pins while VDD is off, then I think there is a ground noise issue.



    -Michael
  • Hi,

    Thank you for your comments.

    Do you have any possible cause (idea) of this happening (RESET out is released VDD is reaching threshold)?

    As you can see input waveform, MR is ramping almost but RESET out is released at certain VDD level (almost threshold).

    It's no big difference between OK case and NG case but it always happens for the first power cycle.

    Regards,

    Kasuya

  • Kasuya,

    Another concern is the direct short from CT to VDD with a 0 ohm resistor. Try a 50kohm pull-up resistor so that the CT pin can charge up to VDD rather than being shorted to VDD. See if this fixes the timing issue.

    -Michael

  • Hi,

    Could you please elaborate the concern to me?

    You expected some delay time on CT?

    They did below experiments but still saw the issue.

    Removed R1277 and populate R1276.

    It gives 10ms delay time.

    As for pull-up as original circuit, it gives 200ms delay time.

    Regards,

    Kasuya

  • Kasuya,

    The issue is shorting CT pin to VDD. Please remove R1276 and populate R1277 with a resistor value >10kohm. Before, you had R1277 = 0 ohm which is a short.

    -Michael

  • Hi,

    Thank you for prompt response.

    We tried with 50kOhm for R1277 after removing R1276 but still saw the issue.

    What we have to do as next step?

    Any check point or need some modifications to fix?

    Regards,

    Kasuya

  • Kasuya,

    The only other cause of issue I can see is the resistor network on the /RESET pin. Can you increase R1275 to 10kohm, remove R1408, and short (or set to 0 ohm) R1278. This is the typical application circuit. If the error still exists, then it could be the device itself is damaged. In this case, please replace with another known working device and see if the issue is still there.

    -Michael

  • Hi,

    Thank you for your support.

    Our customer tried to do experiment that you suggested.

    They have not seen the issue after the modifications.

    Could you please explain what caused and problem mechanism?

    Regards,

    Kasuya

  • Kasuya,

    The /RESET output is an open-drain type output which means the /RESET output requires a pull-up resistor in order to pull-up to the "high" state. When R1408 is connected, this acts as a pull-down resistor or a level shifting resistor which alters the normal /RESET operation. Also, when you add R1278 at the output, the additional resistance and parasitic capacitance can alter the delay from what is expected. It is best practice to only have a single pull-up resistor of at least 10kohm on the open-drain /RESET output in order for the output to function correctly in terms of operation and timing thus only R1275 is needed. I hope this helps! Please let me know if there are any more questions. Thanks!

    -Michael

  • Hi,

    Thank you for the prompt response.

    You mean that SIMPLE pull-up with 10kohm is ”typical application circuit.” right?

    Why 10kohm is needed instead of 1kohm?

    Regards,

    Kasuya

  • Kasuya,

    Because of the output type for this device (open-drain output type), this means a pull-up resistor and only a pull-up resistor is required on the output. The pull-up resistor prevents the pull-up voltage from being directly shorted to GND through the internal MOSFET which is shown in the block diagram in the datasheet. The resistor value is important in limiting the amount of current that can flow through the internal MOSFET while maintaining proper output voltage levels when "high" and when "low" and also maintaining the expected transition delay timing on the output. I hope this helps your understanding. Please let me know if anything is unclear or if you have any other questions. Thank you!

    -Michael

  • Hi,

    Sorry for late response.

    I want to make sure which is MOSFET in the block diagram of DS?

    I don't see any MOSFET in the DS.

    Please let me know which one you meant as MOSFET in DS.

    Regards,

    Kasuya

  • Kasuya,

    The MOSFET I am referring to is in the "Reset Logic and Timer" block in the "functional block diagram" on page 5 so you can't actually see it. The push-pull version (TPS3837, TPS3836) use two internal MOSFESTs in a CMOS configuration thus do not require a pull-up resistor. The open-drain version (TPS3838) uses a single internal MOSFET which requires a pull-up resistor to operate. Please refer to the figure below. I took this figure from another device just to show you the Open-drain vs Push-pull output types. All devices that have the single MOSFET are "open-drain" and require a pull-up resistor to operate.

    Please let me know if your understanding is clear or if you have any more questions. Thank you!

    -Michael

  • Hi,

    Thank you for your explanation.

    I fully got it.

    I don't have any more questions.

    Kasuya