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BQ25970EVM-893: The feature of SYNCOUT

Ryker She
Intellectual 2050 points
Part Number: BQ25970EVM-893
Other Parts Discussed in Thread: BQ25970, BQ2597

Hi

I have some questions about the feature of SYNCOUT:

  • What is the benefit of SYNCOUT except reducing output voltage ripple?
  • Does the phase difference be held 90° by SYNCOUT between master BQ25970 and slave BQ25970?

  • We have tested the output voltage of two BQ25970 work parallelly (one is master and the other one is standalone). We found the output voltage ripple is smallest when the phase difference between master BQ25970 and standalone BQ2597 near 180°, and the output voltage ripple is largest when the phase difference between master BQ25970 and standalone BQ2597 near 0°. What causes this phenomenon? And why don't set the phase difference be held 90° by SYNCOUT between master BQ25970 and slave BQ25970?

Thank you very much.

Best regards,

Ryker

  • 0
    TI__Mastermind 37105 points

    Hello Ryker,

    • The feature of SYNCOUT is to prevent beat frequency in the audio range caused by variation in the switching frequency.
    • I have checked, and the master does indeed lead the slave by 90 degrees.
    • It is not recommended to use dual charger without using the SYNC pin as you cannot control who will be leading/lagging.

    Regards,

    Mike Emanuel

  • 0 in reply to Michael Emanuel35
    TI__Intellectual 2050 points

    Hi Mike,

    Thanks for your kindly reply. I have two questions in details.

    • Why is 90 degrees? 
    • We have tested the output voltage of two BQ25970 work parallelly (one is master and the other one is standalone). We found the output voltage ripple is smallest when the phase difference between master BQ25970 and standalone BQ2597 near 180°, and the output voltage ripple is largest when the phase difference between master BQ25970 and standalone BQ2597 near 0°. What causes this phenomenon? And why don't set the phase difference be held 90° by SYNCOUT between master BQ25970 and slave BQ25970?

    Best regards,

    Ryker

  • +1 in reply to Ryker She
    TI__Mastermind 37105 points

    Hello Ryker,

    • This is the way the part was designed. 
    • When the chargers are out of sync, beat frequency noise injection can occur. Again, it is not recommended to use dual charger without using the SYNC pin as you cannot control who will be leading/lagging. When using master and standalone they are not synced together.

    Regards,

    Mike Emanuel