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TPS92515-Q1: Coff and SW did not synchronize

Koji Hayashi
Intellectual 955 points
Part Number: TPS92515-Q1

Hi All,

My customer created and evaluated a circuit similar to the one in Figure 3 on the next link.

As result, Coff and SW did not synchronize.

http://www.ti.com/lit/ug/tiducq5/tiducq5.pdf

Although the circuit constants are almost the same, R1 is not connected.

Because, there was no place to place it.

This phenomenon occurs when SHUNT FET is OFF.

This is not synchronize even if IADJ is changed.

Is it due to not attaching R1?

Please tell me why Coff and SW are not synchronized.

Best Regards
Koji Hayashi

 

  • 0
    TI__Mastermind 28655 points
    Hello Koji,

    I'm not sure what you mean by there was no place for R1? R1 is part of the current source to charge the Coff capacitor. Even without R1 it should work though the off-time will be different (larger).

    I don't understand 'As result, Coff and SW did not synchronize'. If SW is on Coff is not charging, when SW turns off Coff starts charging (it determines the amplitude of the inductor current down slope (or peak to peak ripple). They should be synchronized 180 degrees out of phase, when SW is off Coff is charging. When the VCoff threshold is met the SW turns on, and at the same time Vcoff is pulled low. The only way I know of this not working is if there's a fault or the Boot cap is not charged high enough to turn SW on.

    Between VCoff rising (controlling SW off-time) to its trip threshold and when SW is on, charging the inductor current to its peak trip threshold they have to be in sync. Coff controls when SW turns on.

    Regards,