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TPS54620: Condition of 100% duty cycle

Kuramochi Tadahiko
Guru 28715 points
Part Number: TPS54620

Hi,

Follows is described on the datasheet.

"the device is designed to operate at 100% duty cycle"

Could you tell me the condition to enter into 100% duty cycle?

In other words, how does the device detect VIN < Vout ?

100% duty cycle and normal duty cycle are repeated continuously on my customer's circuit as attached.

I would like to get information to guess the cause of this behavior.

TPS54620 100% duty cycle.xlsx

Best Regards,

Kuramochi

  • 0
    TI__Guru**** 191445 points
    TPS54620 does not use any special function to operate at 100% duty cycle. As the input voltage falls towards the output voltage the required duty cycle for regulation will increase. If the required on time is greater than the switching period (T = 1/Fsw), the high side FET can remain on longer the the period T so long as there is enough charge on the BOOT capacitor to keep teh BOOT voltage relative to SW above the BOOT UVLO of approximately 2.1 V. When BOOT falls below the UVLO, the high side FET is turned off and the low side is turned on to refresh the BOOT capacitor charge.

    I need to see the actual waveforms to tell you what is happening. In your case you should only be operating at about 95 % duty cycle anyway, not 100 %. It may be that you have a dynamic load change at that frequency. Mace sure to include PH node, BOOT node and VSENSE node voltages with your waveforms. Inductor current waveform would be helpful as well. Please provide the "big picture" that shows your 6 kHz event as well as zoom ins of the start and stop transitions into 100%v duty cycle operation. Zoom in enough to see individual switching pulses.