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TPS7B67-Q1: Output Spike Issue in Rapid Power Cycle Test

Part Number: TPS7B67-Q1

Hi, Team

My automotive customer is using TPS7B6701-Q1 in their car infotainment system to convert VBATT 14.4V to 5V for CAN and MCU. They find there is spike in output during repid power cycle test. Here are the waveform and schematic. Is there any suggestion to fix this issue?

CH1 (Yellow): Input Voltage of TPS7B6701-Q1, 5V/Div

CH2 (Blue): Output Voltage of TPS7B6701-Q1, 2V/Div

Customer Schematic:

Thanks

Kevin

  • Hi Kevin,

    The overshoot is caused by the dropout recovery of TPS7B67-Q1. While all LDOs have some overshoot when recovering from dropout, TPS7B67-Q1 has a slower transient response than many other LDOs. As a result, the overshoot when recovering from dropout is larger. If you can limit your Vin slew rate to less than 200 mV/ms, the overshoot can be limited to 0.5 V; however, if you are not able to limit the input slew rate, a zener diode on the output can limit the overshoot.

    Additional information can be found under section 10 Power Supply Recommendations of the datasheet.

    Very Respectfully,
    Ryan
  • Hi, Ryan

    Thanks for suggestion.

    Customer Vin slew rate is about 2V/ms, which is bigger than 200mV/ms.

    CH1 (Yellow): Vin

    CH2 (Blue): Vout

    But after customer change the resistor PR214 / PR215 on EN pin from 47k / 47k to 100k / 47k, and cange the resistor PR32 PR112 on FB pin from 10k / 3.3k to 30k / 10k, the issue is fixed and spike is gone.

    Could you suggest why changeing the resister on EN and FB pin can solve the issue?

    Thanks

    Kevin

  • Hi Kevin,

    By changing the resistor divider on EN, you made it so that Vin is at a higher voltage when the LDO is enabled. It is possible that during the Vin transient, you are now disabling the LDO and then re-enabling it after Vin has recovered enough to minimize the overshoot. The output capacitors will still hold Vout high temporarily while the LDO is disabled; therefore, it is difficult to tell from the provided scopeshots if this is what occurred.

    Likely changing the feedback resistor divider did not have much of an impact as the overshoot coming out of dropout is determined by the transient response of the LDO and the Vin slew rate only. Increasing the total resistance in the divider will simply decrease the amount of current from Vout to GND through the divider.

    Very Respectfully,
    Ryan