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LM5175: LM5175 How the current limit works in this situation?

Andrey Yakimov
Prodigy 190 points
Part Number: LM5175

Hi Team,

I'm trying to analyze

www.ti.com/.../PMP20660

132W 4-Switch Synchronous Buck-Boost CC-CV Supercapacitor Charging Circuit Reference Design

Can you explain for what reason they use DIODE Dx3 LL4148-13 and transistor Qx1 2N7002BK,215 between VOSNS-   and  ISNS-  ?

How the current limit works in this situation?

 Thanks!

  • 0
    TI__Guru 72565 points
    We will get back to you tomorrow. Thanks
  • 0 in reply to Youhao Xi
    TI__Guru 72565 points
    Sorry for the delay. I talked with the engineer who created this reference design. The transistor is used to disconnect the average current sense from the circuit during startup.

    During startup when the LM5175 is running through soft start (SS), the boost high side FET is off (HDRV2 low). However, the charging current into the large output capacitor as well as the out load current may exceed the dc current limit threshold. The transistor simply blank out the current sense signal and allows the SS to complete.

    Once SS is complete, HDRV2 will be high, or switching, through the diode (Dx3), the transistor is turned on, connecting the current sense signal to the IC, and operating the circuit with average current limit.

    Hope this clarifies,

    Best Regards,
    Youhao Xi, System and Applications Engineer, APP-BMC-BCS, TI