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BQ2057W: External pass-transistor selection and application consult

Jayce Luo
Prodigy 590 points
Part Number: BQ2057W
Other Parts Discussed in Thread: BQ2057

Hi TI expert,

External pass-transistor of BQ2057W can be Transistor or MOSFET, right? What's the difference of these two kinds solution? What's the key spec need to pay attention in selection?

It's common to see parallel MOSFETs in switching applications. However, is it a good idea to use parallel transistors or MOSFETs in position of external pass-transistor in LDO? Is there any problems here?

B R,

Jayce L

  • 0
    TI__Mastermind 23410 points

    Hi Jayce,

    Yes, the external transistor can be a transistor or a MOSFET. These can be  seen in Figure 1 with a transistor and Figure 4 with a MOSFET. The main difference would be the impact is the power dissipation and how it relates to the junction temperature in terms of C/W. The information in selecting can be found within the datasheet. 

    In regards to your question for it it's a good idea, the BQ2057 is designed to work with the external pass-transistor. If it is referring to other transistors or MOSFETS, yes. Charge time may be increase. 

  • 0 in reply to Anthony P Pham
    TI__Prodigy 590 points

    Hi Anthony,

    Any current sharing issue where use a BQ2057 co-work a pair of parallel transistors? Parallel MOSFET is OK I believe, since MOSFET is driven by the voltage or small amount of charge. However, when using transistors, things changed. That's exactly what I concerns about.

    B R,

    Jayce L

  • 0 in reply to Jayce Luo
    TI__Mastermind 23410 points

    Hi Jayce,

    Sorry, I'm having a bit of trouble understading your question. Can you explain what you mean by "things changed". Are these transistors being placed at the output? 

  • 0 in reply to Anthony P Pham
    TI__Prodigy 590 points

    Hi  Anthony,

    Forget that, another urgent issue here to discuss. What's the function of R5 in the MOSFET configuration circuit in datasheet of BQ2057W, Fig 4. Thanks!

    B R,

    Jayce L

  • 0 in reply to Jayce Luo
    TI__Mastermind 23410 points

    Hi Jayce,

    The R5 resistor is used with the CMD67-22SRUC dual-chip LED to ensure the max voltage on the STAT pin does not exceed the LEDs tested current rating mentioned in its datasheet.