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BQ25798: SW, BTST undershoot

Yoshinobu Kagawa
Expert 2030 points
Part Number: BQ25798

The undershoot of the SW and BTST terminals is lower than the absolute maximum ratings, but is it correct that this regulation is for externally applied voltage and does not apply to the undershoot that occurs during switching?
I have attached the measurement results, so please check them.

Results
　 Measurement Point Measured Value [V] Specified Value [V] Result
1 SW1 -3.67(14ns) -2(50ns) NG
2 BTST1 -1.88 -0.3 NG

BQ25798_undershoot_20230928.xlsx

Regards,
Kagawa

  • 0
    TI__Guru**** 166996 points

    Hi Kagawa,

    Did you measure the undershoot with a ground spring like below:

    .

    If not, the measurement might be a bit exaggerate due to long ground return pick up.  Even then, the datasheet abs max are tighter than what the IC can withstand.  If you are concerned, you can add a Schottky diode from SWx to ground to clamp that node to 0.3V below ground.  Those diodes will also boost efficiency by up to 0.5%.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Expert 2030 points

    Hi Jeff-san,

    Does it mean that the maximum rating of the SW pins exceeds the MIN, but the regulations are strictly set so there is no problem?
    How about the BTST1 pin?

    Regards,
    Kagawa

  • 0 in reply to Yoshinobu Kagawa
    TI__Guru**** 166996 points

    Hi Kagawa,

    The short duration undershoot < abs max(min) rating on SWx and BTSTx should not cause long term damage.  Did you follow the recommended PCB layout in the datasheet and EVM user guide?

    Regards,

    Jeff