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What is the minimum Vin vs Vout for TPS40170 ?

Edward Su
Prodigy 40 points
Other Parts Discussed in Thread: TPS40170

Hi All,

I am using TPS40170 as a step down regulator.

Vout is at 24V, 10Amax.

My current design can accept Vin from 33V ~ 27V. The Vout droped and tracking Vin once Vin below 27V.

Since the Vin source is Li-Pol Battery, I hope it can work with Vin as low as possible.

So my question is what is the minimun Vin above Vout for TPS40170 to working properly, I mean keep Vout requlated as design set.

Thanks & Best Regards,

Edward Su

 

  • 0
    TI__Prodigy 80 points

    Hi Edward,

    1.

    For a Buck, the duty cycle should be D = [Vout + Iout*(RLS_DS + RDCR)]/[Vin - Iout*(RHS_DS - RLS_DS)],  where RLS_DS and RHS_DS means the Rdson of the low side and high side FET, and RDCR is the ESR of the inductor.

    For TPS40170, the max duty cycle is about 91%(Fsw = 300K), which you can find it in the datasheet.

    Here I assume that RLS_DS + RDCR = 50mohm and RLS_DS = RHS_DS (using the same FET for HS and LS)

    So we got Vin_min = [Vout + Iout*(RLS_DS + RDCR)]/D = (24 +10*0.05)/0.91 = 26.92V. So I think what you test is normal.

    2.

    If you want a lower Vin, you should:

    A. Use the lowest Fsw, which is low as 100K Hz, and the Dmax = 95%.

    B. Use low ESR inductor.

    C. Use low Rdson FET.

     

     

    Kim Yang