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TPS54260: The length of CBOOT discharging period - slva547a

Ella KIM
Expert 4212 points
Part Number: TPS54260

Hello,

I'm looking into slva547a and have a question on the basic operation of TPS54260.

In figure 6, I wonder how the length of CBOOT discharging period is determined. As far as I understand, the length is determined by turn-on time of HS FET in the normal CCM operation. But HS FET is off during CBOOT discharging period in the figure 6. Could you help me for understanding?

Thanks,

  • 0
    TI__Mastermind 33765 points

    Hi Ella,

    This is for low drop light load condition.

    The HS should turn-on at maximum duty cycle at low drop-out, but the Cboot voltage is low and it will drop below 2.1V after it drive the HS ON for a short time, then the HS has to be turn-off, that's why you see the BOOT is a narrow pulse.  

    at low drop heavy load condition, the inductor current peak is high enough to pull the PH to ground level, then the Cboot can be fully charged up by VIN, the HS can turn-on at max duty cycle in this case.

    B R

    Andy 

  • 0 in reply to Andy Chen
    TI__Expert 4212 points

    Hi Andy,

    Thank you ffor your explanation. 

    Andy Chen said:
    The HS should turn-on at maximum duty cycle at low drop-out, but the Cboot voltage is low and it will drop below 2.1V after it drive the HS ON for a short time, then the HS has to be turn-off, that's why you see the BOOT is a narrow pulse

    Then I believe PH node should also appear on the level of Vin (8V) during the narrow pulse period, because HS FET will turn on during the period. But I cannot see it on PH node. Could you help me to understand?

  • +1 in reply to Ella KIM
    TI__Mastermind 33765 points

    Hi Ella,

    The Vin is 6V, output is 5V, device starts switching once BT-PH reach 2.1V, so the BT voltage will jump to Vin+2.1V=8.1V, but PH equal to Vin=6V at HS turn-on.

    As BT-PH is only 2.1V level and also decrease quickly, the HS will not be turn-on ideally, so on last cycle, the inductor has no forward current to pull PH to zero.

    B R

    Andy