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BQ2057C: Reverse current flows on PNP Transistor , when input power remove

Shinichi Inoue
Guru 21676 points
Part Number: BQ2057C
Other Parts Discussed in Thread: BQ2057, BQ24090, BQ24001, BQ24040,

Dear Specialists,

My customer is considering BQ2057 and has inquiries.

I would be grateful if you could advise.

---

I am evaluating the circuit of datasheet P.7 (Figure 1.) with power IC and MCU. 

Power IC is for MCU(+3.3V),  it is connected to Li+ and MCU is to control.

I observed the phenomenon listed below.

Could you please advise. 

Supply 5V to DC +,
Charge a lithium ion battery (3.6V nominal voltage) by using BQ2057CTS.

Next, when the 5V supply is removed, current flows from the lithium-ion battery in the opposite direction of Q1.
(Current flows from the collector of Q1 toward the emitter of Q1. At this time, VCE is about 1.0V.)

As a result, 2.5V to 3.0V voltage is applied to VCC of BQ2057CTS.

(1)Is there no problem if reverse current flows through transistor Q1?

(2) When 5V is not supplied to DC+, the VCC of BQ2057CTS has a voltage of 2.5V to 3.0V.

→ Is this VCC necessary for operation for BQ2057CTS?

---

I appreciate your great help.

Best regards,

Shinichi

  • 0
    TI__Guru* 75676 points

    Hello Shinichi

    The BQ2057C is a very old device released about 2001.  We have a number of other devices that charge single cell Li-Ion from a 5V supply with higher efficiency and very few external parts.  See BQ24040, BQ24090 small high efficiency solutions.  Also see the BQ24001 this is older device much smaller solution than BQ2057C.

    My recommendation is to move away from this device.

  • 0 in reply to Bill Johns
    Guru 21676 points

    Hi Bill,

    Thank you for your reply.

    I confirmed the customer.

    They have already bought 250pcs, they must use.

    Considering this situation, could you please answer the question.

    I appreciate your great help and patience.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    TI__Guru* 75676 points

    Hi Shinichi

    During an off or no input power state the CC pin to Q1 base is high impedance.  This could possibly increase the leakage current thru the device.  A couple of things to look at, lower leakage current transistor or high value resistor B to E.  Also diode on input to circuit to block reverse current.

    The BQ24040 or BQ24090 leakage current in this condition will be no greater than 1uA in this condition with no external parts added.

  • 0 in reply to Bill Johns
    Guru 21676 points

    Hi Bill,

    Thank you for your reply.

    I understand leakage current may cause the transistor fault on.

    I'll share two workarounds you mentioned with the customer.

    If they have an additional question, I cousult you again.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi