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BQ24450: BQ24450 External Transistor

Vladimir Naumenkov
Expert 2970 points
Part Number: BQ24450

Dear Sir, please answer

I tested circuit (please, see attach, "picture 2"). I set 3A initial current for charging 1-cell lead acid battery (2.3V).  This circuit works.  VT1 transistor  and VD1 diode must be placed on the heatsink. 

I would like to remove the VD1 diode. The 2.3 V battery is constantly connected to the device, and the 5.5 V power supply is periodically turned off.

If to remove VD1 diode in my circuit, then collector-base junction of the VT1 will be shifted in the forward direction, when power supply 5.5 V is turn off (as the battery remains connected to the device). Therefore, I can’t remove VD1 diode in my circuit. 

If I use external Quasi-Darlington pair or NPN  Emitter-Follower (please, see page 14 of the BQ24450 datasheet), then, if to turn off 5.5 V supply, then emitter-base junction of the Q2  (or Qext) will be closed. 

QUESTIONS:

1) Can I remove VD1, if I use this external Quasi-Darlington pair ? Since this pair has a large current gain, is it necessary to provide measure to prevent oscillation ?

2) Since the input voltage of 5.5 V is enough for the NPN Emitter-Follower circuit to work, can it be better for me to use this NPN Emitter-Follower circuit  (without VD1 diode) ?

3) What max current I can set through Rp resistor (NPN Emitter-Follower circuit )?  Is it possible to set 50 mA current?

TIA

Sincerely,

Vladimir Naumenkov

www.agat.by

Picture 2.docx

  • 0
    TI__Guru 67211 points

    Hello

    1) Can I remove VD1, if I use this external Quasi-Darlington pair ? Since this pair has a large current gain, is it necessary to provide measure to prevent oscillation ?---Bill J - VD1 diode will prevent leakage current battery to output.  You would need to evaluate the design without the diode to verify that it meets your requirements.

    2) Since the input voltage of 5.5 V is enough for the NPN Emitter-Follower circuit to work, can it be better for me to use this NPN Emitter-Follower circuit  (without VD1 diode) ? -- Bill J - As you noted several options are possible for the external pass transistor and show on page 13 & 14 in data sheet. As in question 1 you will need to evaluate the design to verify it meets your requirements.

    3) What max current I can set through Rp resistor (NPN Emitter-Follower circuit )?  Is it possible to set 50 mA current? -- Bill J - The data sheet indicates this is about 40mA.

  • 0 in reply to Bill Johns
    Expert 2970 points

    Dear Bill, thank you

    If possible, let's clarify something. You noted:  "VD1 diode will prevent leakage current battery to output."  (If  use  PNP transistor)

    If  use NPN transistor, then emitter-base junction acts as a VD1 diode ? And in my opinion, I can remove the VD1 diode. 

    TIA

    Sincerely,

    Vladimir Naumenkov

    www.agat.by