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BQ24250: Charging current too weak

Shahriar Rohman
Prodigy 40 points
Part Number: BQ24250

Hello,

I am working on a BQ24250 application to power a 1S 2200mAh LiPo battery. I'm having issues with the current delivered to the battery which seems to be too weak, both when controlled in I2C or in standalone mode (the charging current is 20mA in average while it is supposed to be more than 500mA). I tried to simplify the schematic as much as possible to identify the issue, but I am not able to see what is going wrong. Could you please help me ?

I am attaching herewith the schematic in standalone mode, which is mostly based on datasheet. For tests purpose, the charge is simply a LED. Vin is a connected to a voltage power supply set to 5V than can deliver up to 1A. Capacitors are all MLCCs.


I put a 1Ω resistor in serie with battery in order to measure current going through the battery, as shown below (green is input, yellow is voltage on resistor).

Thanks in advance.

  • 0
    TI__Guru**** 166576 points

    Hi Shahriar,

    On the schematic, LDO requires a capacitor of at least 1uF.  Also, if not used, we typically recommend leaving SCL and SDA floating.

    EN1=EN2=GND sets input current limit to 500mA regardless of ILIM pin resistor.  See Table 1 on page 21.  Any load current on SYS gets prioritized over charge current to BAT.  

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Guru**** 166576 points

    Also, the 1 ohm resistor is too high.  For example, 1A charge current gives a 1V drop so even if the battery pack is at 7.4V, the charger will think that charge is complete at 8.4V.  I recommend lowering to 0.1ohm or 0.01 ohm.

    Jeff

  • 0 in reply to Jeff F
    Prodigy 40 points

    Hello Jeff,

    Thanks for your fast answer. I forgot to add capacitor on LDO in my schematic, but it was added in circuit.

    I just tested it again with leaving SCL and SDA floating and replacing 1 ohm resistor by a 10 mOhm resistor but still have the same issue... I could not reach the 500mA limit.

    However, it makes me wonder does this limit correspond to a current peak ? Or is it the limit for an average current ? Because as we can see in my first oscilloscope screen in the initial post, the charging current seems to be a periodical current, so average current appears to be much lower than peak current. Is it the normal behaviour ?

    Thanks again for your time.

  • 0 in reply to Shahriar Rohman
    TI__Guru**** 166576 points

    Hi Shahriar,

    The input current limit is the average current.  With EN1=EN2=GND, input current limit = 500mA.  So input power = 5V*0.5V = 2.5W.  efficiency = POUT/PIN=VBAT*IBAT/(VIN*IIN) so IBAT=efficiency*(VIN*IIN)/VBAT.  At VBAT=4.0V, your max charge current after factoring in efficiency is 0.92*(5V*0.5)/4.0V = 575mA.  

    Can you retake the scope shot and show V(IN), I(IN), V(SW) and V(BAT)?

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 40 points

    I see...  So it is not supposed to be as low.

    Here are the scope shots you asked :

    In yellow : V(IN)

    In purple : Voltage over a 1 Omh resistor after input to measure I(IN) (a 10 mOhm was too little to see anything relevant)

    And here are V(SW) (yellow) and V(BAT) (green) :

    Thanks for your help !

  • 0 in reply to Shahriar Rohman
    TI__Guru**** 166576 points

    Hi Shahriar,

    The 1 ohm resistor is too large and will cause VINDPM to trip, which will only complicate the issue.  So please retake the plots without that resistor, showing only the voltages IN and SW. Please zoom in to show all of one of the bursts.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 40 points

    Hello Jeff,

    Here are the plots of one burst of V(IN) in yellow and V(SW) in green :

    Thanks for your help.

  • 0 in reply to Shahriar Rohman
    TI__Guru**** 166576 points

    Hi Shahriar,

    The waveforms look odd.  VIN should not have those spikes and SW should be a square wave that goes between the input voltage and GND at each pulse.  Is the IC and its external components grounded properly, including the IC power pad (assuming you are using QFN)?  If you zoom in one or two switching pulses at the beginning (while not going to ground) what do they look like?  What is the DC voltage at BOOT (it should be close to 10V)?

    Regards,

    Jeff