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LM5122: Burn issue

KJ Liu
Intellectual 530 points
Part Number: LM5122

Hi Sir,

My customer is using LM5122 as boost structure and one of board is burned. There is several question need to be checked

spec : input 42~60V , Output 60V, 3A 

1. Could LM5122 be use as non sync boost structure?  From the result , the mos (SW to gnd ) is burned and diode is fine. 

2. Boost Diode DBST : Is 1N4148 is ok for this and is possible the reason to induce LM5122 burn ?

3.If input cap and output cap is very small (around 1u) , is possible the reason to induce LM5122 burn ?

  • 0
    TI__Guru* 78070 points
    Hi KJ,

    Thank you and your customer for considering the Lm5122 in the design. You description is not clear what exactly is burned.

    To answer your questions:
    (1) Yes you can use it as a non sync boost. You just need to leave the HO, HB, and SW grounded , and not feeding HB from VCC. For 60V output, you should select at least 80V MOSFET to handle the possible votlage stress.

    (2) I don't think 1N4148 can support 3A load. Its rating is 300mA. You should select a diode 100V and >3A rated.

    (4) If the output capacitor is very small, you will has higher output ripples and that may cause over voltage on the low side FET. Pleas choose the FET according to the LM5122 datasheet design guidelines.

    Thanks,
    Youhao Xi, Applications Engineering
  • 0 in reply to Youhao Xi
    Intellectual 530 points
    Hi Youhao,

    Thanks for your reply,the burn issue isn't happen all the time (1 of 7)

    For


    (1). the different between your suggestion and customer is my customer leave HO floatimg, SW feed to system (inductor, drian of mos and diode , same as sync boost ). what is HB? do you have recommend schematic to share ?
    (2). For 1N4148 , it's be used in serial with the boost cap for charge pump function. Does it still need 3A capability?
    (3). same as previous, since the load is battery ,customer doesn't use bigger cap as input and output cap , is it make sense ?
    (4). Mos, We choose 100V Nmos, the reason doesn't use 2 Nmos as sync boost is system's input and outout all battery and customer afraid that diode of mos will reverse the energy to input, is it possible ? if it possible, do you have any suggestion ?