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LM5122: Change current sensor resistor and 48V out

ArTzy
Genius 13999 points
Part Number: LM5122

Hi Experts,

Good day. I would like to ask your assistance on this request from a valued client about boost regulator - LM5122.

They're going to change current limit (1A)@24V with LM5122EVM-1PH evaluation, and 48V with another LM5122.
They changed current sensor resistor (R2) value 12mR, 25mR, but Vout dropped to much.
Their reference is LM5122 Webench circuit. The difference between 5122 Eval board and Webench is R5/6(100R), Webench doesn't.

Please advise which parts should they change.

The other question for 48V application, Q2-3, C5-9 are changed to 75V version.
After they change R28 for 48V, they don't see 48V out.

Please advise this one as well.

Thank you for your expertise.

Regards,
Archie A.

  • 0
    TI__Intellectual 2775 points

    Hi Archie A,

    Thank you for your promoting our device. I'm a little confused, I'll check with you first.

    Is the current customer required to be modified by two EVMs? 

    1）24V/1A output?

    2) 48V output ?

    Could you sharing me what is input voltage and output current ?

    About the 24V/1A EVM what load there add and what is output voltage ?

    About 48V evm what value there change about feedback res? what happen they find after change ?

    BR Dehuan

  • 0 in reply to dehuan Qi
    TI__Genius 13999 points

    Hi Dehuan,

    Thank you for your response.

    I was able to get the feedback from client as follows:

    Are you required to modify two EVMs?
    1）24V/1A output?
    2) 48V output ?

    Now upconverting requirement is changed.

    1. LM5122EVM-1PH

    2. Input 24V è Output 55V @ 4.5A

    3. Application is for Charging Capacitors( 3V380F x 24)

    4. After FB resistors (R28=0, R27=47.5K, R26-1.05K), Vout is +55V.

    5. Vout is 55V, But it is hard to charge Capacitor Banks, L1 is still 10uH, R2(current sensor) is 0.004R.

    I didn’t check current flow to Capacitor bank.

    6. After I changed Vout with Webench, L1 = 33.76uH, R2 is 2.68mR.

    Can you give me advise to get this work?

    Thank you.

    Regards,
    Archie A.

  • 0 in reply to ArTzy
    TI__Intellectual 2775 points

    Hi Archie A.

    You need change output cap voltage range  more than 55V， now the EVM cap voltage cannot support.
    Q2/Q3 need change to more than 60V.  and R2 advise follow webench， and L1 if you are inconvenient, you can use the original first.
    then check current flow to cap bank. 
    BR Dehuan
  • 0 in reply to dehuan Qi
    TI__Genius 13999 points

    Hello Dehuan,

    Thank you for your guidance.

    Client followed the advise and Webench design to get 55V@1A constant current, but board doesn’t work this application.

    Can you advise us how to get constant current?

    Thank you.

    Regards,
    Archie A.

  • 0 in reply to ArTzy
    TI__Intellectual 2775 points

    Hi  Archie A.

    Does it output 55V if there is no load?

    Could you measure the output voltage/ input voltage / output current/ SW by oscilloscope？
    BR Dehuan