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LM25148-Q1: What is the requirement of MOS?

Jiawei Hu1
Expert 5385 points
Part Number: LM25148-Q1
Other Parts Discussed in Thread: CSD18511Q5A, LM25149-Q1EVM-2100, CSD18503Q5A, LM25148, LM25149, LM25149-Q1

Hi team,

As the title, could you kindly tell me what is the requirement of MOS? Do we have some recommended mos? Thanks!

BR

Jiawei

  • 0
    TI__Mastermind 22299 points

    Hi Jiawei,

    For any MOSFET you definitely need to make sure the threshold voltage VGS(th) is less than the gate driver voltage.

    The gate drive of LM25148-Q1 is VCC or 5V.

    For = high-frequency designs you want to minimize switching losses by minimizing gate charges Qg, without sacrificing too much RDSON for conduction losses. 

    The LM25149-Q1EVM-2100 uses IAUC60N04S6L039. You can also use CSD18503Q5A and CSD18511Q5A.

    You can use WEBENCH to generate designs and recommended MOSFETs.

    Hope this helps,

    -Orlando

  • 0 in reply to Orlando Murray
    TI__Expert 5385 points

    Hi Orlando,

    Thanks for your reply! Pls help review attached schematic, thanks!

    DC-DC-LM25148.pdf

    BR

    Jiawei

  • 0 in reply to Jiawei Hu1
    TI__Mastermind 22299 points

    Hi Jiawei,

    15A on a single phase could cause larger temp rises, what is your ambient temperature?

    5mΩ ISNS resistor will set a peak current limit of 12A and will not get 15A, you should use a 3mΩ shunt to set a peak current limit of 20A.  

    You can use a 0.1µF capacitor for VDDA.

    LM25148 will have internal boot diode so you wont need the external diode unless you want it.

    Be sure C1799, C1800, and C1801 have sufficient ESR to dampen input filter resonance.

    I cant find that inductor but be sure it does not saturate.

    Everything else looks good to me!

    -Orlando

  • 0 in reply to Orlando Murray
    TI__Expert 5385 points

    Hi Orlando,

    Thanks for your reply! Customer plan to use SiR422DP mosfet, I check its datasheet and think it's ok, but want to double check with you, thanks!

    sir422dp.pdf

    BR

    Jiawei

  • 0 in reply to Jiawei Hu1
    TI__Mastermind 22299 points

    Hi Jiawei,

    That MOSFET looks good! See quickstart calculator attached.

    LM25148-Q1 Quickstart Calculator_SiR422DP.xlsm

    -Orlando

  • 0 in reply to Orlando Murray
    TI__Expert 5385 points

    Hi Orlando,

    I tested LM25149 EVM with SiR422DP mos, and changed switching frequency from 2.1M to 440kHz, but I founded that if output current was larger than 6A, like 7A, the output voltage wasn't 3.3V, only 2.8V. The higher output current, the lower output voltage.

    Can you explain it? Is the COMP network not suitable? And how to judge one mos can fit 2.1M frequency or not? Thanks!

    BR
    Jiawei

  • 0 in reply to Jiawei Hu1
    TI__Expert 5385 points

    Also, if we want LM25149 can output 15A current, where need to be modify in EVM?

    Jiawei

  • 0 in reply to Jiawei Hu1
    TI__Mastermind 22299 points

    Hi Jiawei, 

    If you lowered the frequency and did not change the inductor then the inductor pk-pk ripple current ΔIL will be very large and hit peak current limit.

    The inductor pk-pk ripple current is ΔIL = T_on * (VIN-VOUT)/L.

    Also T_on = (VOUT/VIN)/F_sw

    The peak current limit is set by current sense resistor and current sense threshold.

    The 60mV threshold on LM25149-Q1 and the 5mΩ on the EVM will set a peak inductor current limit of 12A.


    You need to re-size the inductor for 440kHz.

    For 15A you also need to re-size the current sense resistor R11 to change the peak current limit.

    I suggest using the quickstart calculator for the updates to the design.

    Hope this helps,

    -Orlando