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TPS43350-Q1: over Max Duty condition

Part Number: TPS43350-Q1

Hi,

We got a question from customer about TPS43350-Q1.
Could you help us?

[Question]
If the IC exceeds the maximum duty 98.75%, how is the behavior of the IC?

- condition -
Vin=8V to 16V
Vout=8V
fsw=400kHz *synchronized to external clock

They want non pulse skip operation, even if the output voltage falls. Does the IC meet to their requirement?

Best Regards,
tateo

  • Hi Tateo,

    The device will operate in continuous mode as long as the sync pin is held high or provided with a clock.

    When the input voltage drops, the device will increase the duty cycle to maintain the same output voltage, and can maintain a 100% duty cycle for three clock cycles before it needs to recharge the bootstrap capacitor. This means that the device is effectively operating at 100% duty cycle for 3 normal switching periods, and 95% on the fourth period, leading to an output waveform 1/4 of the normal clock frequency, and appearing as a 98.75% duty cycle.

    If the buck cannot sustain higher than 7.44V (93% of 8V), the PG pin will simply be pulled low.

    Best Regards,
    Rick S.
  • Hi Tateo,

    as Rick explained, the part can be forced into continuous PWM-mode. However, there are cases where it will skip pulses even in this mode:

    - in case the minimum on-time of the HS is violated (no risk in your application as minimum dutycycle is 8V/16V=50%)

    - drop-out-mode, which is the case if Vin approaches Vout, linek in your 8V in / 8V out case.

    If they can not accept the boot-recharge, they may want to consider using th eTPS43330(A)-Q1, which offers a pre-boost. If this is set to 10V or 11V, the buck-supply voltage will always be higher than the output. However, this comes with more external components and - owing to the two-stage approach - lower total efficiency.

    Best regards,

    Frank