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LM5158-Q1: Question of Output Vload during non power supply

Part Number: LM5158-Q1
Other Parts Discussed in Thread: LM5158

Hello, TI Team

I want to design LM5158 boost converter, and we control BIAS input using FET on/off like below picture.

We need to prevent to supply 12V to Vload as necessary. So, I use 2 type enable signal from MCU, 1st of BIAS supply prevent and 2nd of UVLO control.

i) Case 1 : Boost Enable1 off && Boost Enable2 off && 3.3V supplied to PGOOD

Vload was measured 1.12V.

ii) Case 2 : Boost Enable1 off && Boost Enable2 off && 3.3V no supplied to PGOOD

Vload was measured 0.8V

I guessed 0.8V of case2 is caused by internal FET of SW pin.

but i don't know why 1.12V was measured in case1 depending on PGOOD 3.3V supply. Please let me know why.

  • Hi Haeun,

    thank you for using the E2E forum.

    When measuring this do you have any load on the output side - i assume not.

    For the case in question, I assume that the supply on PGOOD is providing through some leakage paths (esp. the ESP protection circuits) a current path to the SW which will show as the small voltage you could measure.

    To avoid or double check on that you can try to connect the pull up resistor to one of the switched signals or to the Vcc pin of the LM5158.

    Best regards,


  • Firstly, thank you for your reply.

    I don't understand which leackage path the PGOOD voltage is applied to SW pin. I cannot predict that point in the block diagram.

    Can you explain detail?

  • Hi Haeun,

    sorry for the miss understanding - i would like to understand the path the voltage coming to the output side.

    So can you let me know what the load of the system is when doing these measurements.

    I also cannot follow the explanation for case 2 - if all is off how can the output be 0.8V?

    Can you please confirm that this voltage is really static over long time.

    Best regards,


  • Thank you for your concerning.

    I think it causes from MCU setup. I am checking MCU port setting with SW team. And then i share the result to you. please wait.

  • Hello Stefan,

    I didn't connect load as you mentioned. 

    I resolved the problem. I found other 3.3V pull up voltage was supplied in PGOOD line through MCU internal pull up line. (Even if i disconnected 3.3V in HW ciruit.)

    I guessed that MCU pull up voltage was supplied in LM5158, so it could measure 0.8V in Vload output line. PGOOD protection circuit can make output path. Is it right?

  • HI Haeun,

    thank you for the update - glad that you found the issue.

    Can you just rephrase your last question (or add a drawing) - I did not understand that.

    Best regards,


  • Hello Stefan,

    I cannot see another path from PGOOD to BIAS or SW in block diagram.

    Only PGOOD 3.3V was supplied without supplying BIAS like case 1 or case 2. However I can measure Vload above 0.8V by PGOOD voltage.

    So, I guess that PGOOD pin seems to be connected to BIAS or SW by some internal circuit of LM5158. (ex. IC protectoin circuit..)

    I want to know if what i think is right.

  • Hi Haeun,

    Thanks for the clarification - let me check with design.

    Will come back in 2-3 days.

    Best regards,


  • Hi Haeun,

    sorry for the delay in answering this question:

    There is a ESD diodes between PGOOD and BIAS. If PGOOD goes above BIAS, this diode will be forward biased defining a voltage at Bias with a diode drop.

    Best regards, 


  • Hello Stefan,

    Thank you for your reply. It made me fully understand. 

    Thank you for your kindness.