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TAS5624A Recommended Bootstrap Capacitor

Sadanori Kato
Guru 36540 points


Hi,
Could you please tell us the concern if increasing the capacitor value more than 0.033uF although the value which you recommend is mentioned on page 14 of data sheet regarding the bootstrap capacitor ? 
Best regards,
Kato
  • 0
    TI__Genius 9175 points
    Hello Kato-san,

    The bootstrap capacitor value is a balancing act between charge storage and charge time.

    If the bootstrap cap is made too large, the charge time will be slower so that it won’t charge to the full value of GVDD_X when OUT_X is low. Then when the OUT_X transitions high, it cannot keep the high side FET on and the FET prematurely shuts off.

    If the bootstrap cap is made too small it will now quickly charge to the full GVDD_X voltage when OUT_X is low. However, when OUT_X is high, It discharges too fast when driving the gate of the high side FET and the FET again shuts off prematurely.

    33nF has been found to be the best value for this device.

    Best Regards,
    Matt
  • 0 in reply to Matthew Beardsworth
    Guru 36540 points
    Matt-san,
    Thank you for explaining so politely.
    I understood and will inform our customer of your information.
    So, I have an additional question.
    Do BST_X ports normally start the switching after applying the power supply if using 0.051uF as the bootstrap cap ?
    Best regards,
    Kato
  • 0 in reply to Sadanori Kato
    TI__Genius 9175 points

    Hello Kato-san,

    The bootstrap capacitors are slaves to the outputs since they ride on the output nodes. As soon as the outputs begin switching the bootstrap cap voltage will switch will the output nodes. 

    If using larger bootstrap caps, the customer needs to test that for drop out issues at the maximum output duty cycle due to a lack of charge on the capacitors as mentioned in the previous post.

    Best Regards,

    Matt

  • 0 in reply to Matthew Beardsworth
    Guru 36540 points

    Matt-san,

    Thank you for explaining so politely.
    I understood that I should change to more smaller bootstrap capacitors or change to more lower modulation limit if occurring drop out issues at the maximum output duty cycle.
    So, I will inform our customer of this information.

    Best regards,
    Kato