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LM5140-Q1: ABOUT MOSFET selection

user6204380
Prodigy 20 points
Part Number: LM5140-Q1

LM5140-Q1, datasheet ,page 31,

8.2.2.6 MOSFET selection

equation 47,  Pdhs = (IOUT*IOUT*Rds(on)*Dmax)+1/2*VIN*(tr+tf)*IOUT*fsw

calculate with this equation, I‘ll have a very big Watt value.

consider with the thermal parameter of the MOSFET,this became very hard.

also, at this equation ,1/2*VIN*(tr+tf)*IOUT*fsw  !=  1/2*Rds(on)*IOUT*IOUT*(tr+tf)*fsw.

can you explain this problem?

  • 0
    TI__Expert 4720 points

    Hi,

    equation 47 is correct. 

    If the power loss is big, you need to select another MOSFET, with low Rds(on), and fast switching speed (small tr, tf)

    1/2*VIN*(tr+tf)*IOUT*fsw is correct, it stands for the switching loss. 

    1/2*Rds(on)*IOUT*IOUT*(tr+tf)*fsw doen't make sense.

  • 0 in reply to Neal Zhang
    Prodigy 20 points

    thanks for resolve.

    my design PWM=2.2MHz, Iout=20A, VINmax=16V, Vout=3.3V, with the equation, high side+low side MOSFTE thermal is 8W, this can not accept.

    so i want lower PWM clock.

    Q1:

    from the LM5140-Q1 datesheet,

    page 18, 7.3.4 SYNIN and SYNOUT

    pin SYNIN can only input 374-506KHz for 440KHz clock, 1.87-2.53MHz for 2.2MHz clock

     am i understand right ?

    can i select 1MHz or other frequency input to SYNIN pin?

    Q2:

    pin SYNIN and pin OSC which is more previledge?

    if i set OSC=HIGH(2.2MHz), then SYNIN=1MHz clock , the PWM clock = ?

  • 0 in reply to user6204380
    TI__Expert 4720 points

    Hi, 

    Please send email to me neal-zhang@ti.com