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Original question:

TPS61089: Function issue for TPS61089

TPS61089: TPS61089x operates at a quasi-constant frequency pulse width modulation (PWM) in moderate to heavy load condition?

Gordon Xu
Prodigy 440 points
Part Number: TPS61089

Hi team,

My customer is considering the TPS61089. Currently, my csutomer have some confuses about the operation principle, which constant off-time peak current mode control.

1.why the TPS61089 SW PIN direct connect with the Vout pin? And please help me explain the principle of the Vin and 1/K below BD.

2.Below picture 10 and picture 11,you could find that the switch frequency maintain constant. it seems the switch off-time became more large for maintainning the const frequency. So I wanna konw if it means the switch frequency will not change in the(ps: conflict with constant off-time? ) peak current mode. And is there a sub-harmonic oscillation when D>0.5? 

Regards,

Gordon Xu

  • +1
    TI__Expert 4760 points

    Hi Gordon,

    1. The SW pin is not connected directly to VOUT pin. They are connected through high-side MOS.

    For CCM Boost,Toff=VIN/VOUT/fsw. For frequency setting, it uses a current source proportional of VOUT to charge a capacitor through R3, when the capacitor voltage is charged to a voltage VIN/K, the capacitor is discharged at once and the cycle is reset. So Toff=R3*C*VIN/VOUT and f is almost constant as 1/(R3*C).

    2. The switch frequency is basically constant but it actually changes a little according to working condition. And that is why there is no sub-harmonic oscillation issue.

  • +1 in reply to Yichen Xu
    TI__Prodigy 440 points

    Hi Yichen,

    The datasheet refer constant off-time peak current mode control. But in the DCM mode, I find Toff will change to maintain const switch frequency, So is there a conflict with the formula Toff=R3*C*VIN/VOUT(ps:constant off-time)?

  • 0 in reply to Gordon Xu
    TI__Expert 4760 points

    Hi Gordon,

    The practical circuit is more complex at light load condition. Larger Toff is needed so there is other circuit to achieve this.