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bq25504: Max acceptable current from Vbat to Vstor, Ron of internal transistor

Martin Schmidt93
Intellectual 320 points
Other Parts Discussed in Thread: BQ25504

I'm using the bq25504 in an application, where a LiFePO4 battery is connected to Vbat and a load connected to Vstor.

I'm now looking at a scenario where the booster of the bq25504 is passive and current flows from the charged battery into Vbat, through the internal transistor and out of Vstor into my load.

The load consists of a uC that draws a few uA most of the time, plus a motor that draws up to a few hundred mA occasionally. The rechargeable battery is capable of delivering several A in current, it can deliver a lot more than the load will ever ask for. The limiting factor for the current is not the battery, it is the load and the bq25504 itself.

I would like to know what current my load is allowed to draw out of Vstor and what voltage drop I have to expect between Vbat and Vstor (Ron = ?).

I'd appreciate any help on this.

  • 0
    TI__Mastermind 20570 points

    You can find the RDSON value of the FET between VBAT and VSTOR in the electrical char table. It should be less than 2 ohms.  There are not interal current limit circuit from battery to VSTOR, just high Rdson.

  • 0 in reply to Jing Zou
    Intellectual 320 points

    Thank you , Jing Zou. This answers my question.
    I will connect my motor driver directly to VBAT, as 2 ohms will result in too much of a voltage drop.

  • 0 in reply to Martin Schmidt93
    Expert 1790 points

    My application is similar, driving a pager motor which draws about 100 mA and whose resistance is 13 ohms.  I am using the BAT_OK signal to switch a MOSFET and dump the entire charge on the storage into the motor, as often as possible (whenever voltage on storage rises to VBAT_OK_HYST voltage say 2.8 V and stops dumping when voltage on the storage falls to VBAT_OK_PROG  say 2.4V)

    I'm not sure the max RDSon of the internal transistor is the entire answer.  At 3V and say 1 ohm typical (which the data sheet does not say), that is 3A.  Couldn't you burn up the internal transistor?  I see that on the BQ25504 EVM board schematic, it says "VSTOR 5.25V, 200mA peak" near the jumper J5 in the upper right corner, suggesting that you should limit the current draw?  The transient starting (locked rotor) current of motors can be higher than the running current.  The current is probably not more than a few hundred mA, but the internal transistor might not be rated for that current (and the datasheet doesn't say?)


    Also, for my motor, 1 ohm drop on the internal transistor is negligible compared to the 13 ohm resistance of the motor.  I'm actually considering putting a current limiting resistor in series with my motor, to limit the current to say 200mA.


    I have tried it both ways, attaching load to VSTOR and to VBAT.  It actually seems to work better attached to VSTOR.  In both cases, when the motor is switched on, it drains the storage so quickly that the voltage on VSTOR falls to 1.7V which is below the VSTOR_CHGEN threshold of  1.8V and the chip enters cold start, even though it is programmed with VBAT_OK to 2.4V, so that the chip should be turning off the internal transistor from VBAT to VSTOR at 2.4V.  I would prefer that it not enter cold start.

    In the datasheet section 9.1.34, this is discussed (large fast system load transients and what you can do about it.) 

    I haven't yet tried

    1. a larger CSTOR capacitance

    2. a current limiting resistor to the load.

    3.  reprogramming  VBAT_OK_HYST to say 4V and VBAT_OK_PROG to 3V.

    I am using a 0.33mF polymer tantalum capacitor for storage (on VBAT, plus C3 the 0.1mF capacitor built into the EVM board.) The resistance is low.  It is barely enough charge to turn my motor about one revolution.  The motor is a 4mm diameter pager motor rated at 1.5 volt drawing about 100mA typical.