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LM5150-Q1: LM5150 parameter calculation

tao niu
Prodigy 130 points
Part Number: LM5150-Q1

Dear supporter,

we used LM5150 for some time and it worked well.

recently our customer reviewed our design and pointed out some uncertain point, as attached ppt.

we want to confirm the affect of these parameter.

if we don't want to change, dose it matter?

i have to admit that we did not calculate so carefully in the beggining, we checked with LM5150 calculator before we use current design. but our TI FAE had reviewed the design and think it is ok.

.LM5150 problem.pptx

  • 0
    Prodigy 130 points

    LM5150 quick start 09282017-7500.xlsx

    attach is calculator result. please help check 

  • 0 in reply to tao niu
    TI__Guru* 77035 points

    Hi Tao,

    We will review this in some detail and get back to you by COB Thu this week.

    Thanks,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Prodigy 130 points

    Dear xi,

    any update?

  • +1 in reply to tao niu
    TI__Guru* 77035 points

    Hi Tao,

    I reviewed your circuit and also your version of the design calculator.  Generally, your overall circuit look okay to me.

    Your calculator sets Vout=7.5V but your customer uses 8.5V.  This would cause some results differences.  Below are some comments.

    • By the calculator, an Rsl of 750 Ohm is recommended if VIN can go down to 2.5V. However, it is only needed for Vin below 4.5V. Without it, you may see some subharmonic oscillation but the output would still be regulated, just a switch duty cycle become one small and one big pulses, not affecting the output regulation.  Also, the start-stop operation is a transient and the input voltage will recover in short time, therefore, you may survive without adding the Rsl.  But, I agree with your customer that it is a good practice to add the recommended Rsl.  
    • Changing the load current from 2.94A to 2.3A does not change the fact of above point.
    • RR is not a very critical parameter in design.  The recommended 30 to 70% was based on experience that the overall performance could usually be optimized if RR in that range for full power,  but RR would always become higher in any design with reduced load, and the circuit should work.  So, I do not think reducing RR (or increasing Lm) is necessary in this case.  To make the design robust, it seems adding Rsl is an easier and cheaper approach if your PCB reserves the position. 
    • Regarding your loop compensation, your selection of 3.9k of Rcomp simply moves the crossover frequency to be higher than your targeted 2.27kHz (you can see it in the calculator when plugging in your design selections).  I am not sure about your actual ESR.  However, either 9mOhm or 32mOhm,  it seems the loop is still stable with slightly reduced GM with 9mOhm ESR.  
    • Either the calculator or the datasheet equations are just provided to help your design to get a good start point,  and we always recommend experimental verification.  Fine tune as needed to get optimal performance. 

    Thanks,

    Youhao

  • +1 in reply to Youhao Xi
    Prodigy 130 points

    Dear xi,

    thank you for your kindly help. we will discuss with our customer.

    BR,