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Li-ion BQ2057C charger, help!

DJ15246
Prodigy 130 points
Other Parts Discussed in Thread: BQ2057C, LM3658, BQ2057W

 

Hi All

I have made a Li-ion charger BQ2057C 4.2V 1A.
http://focus.ti.com/lit/ds/symlink/bq2057.pdf


I have made my design just like Figure 4 on page 10.

The only components i have changed are RSNS =0.1R, so i can get 1A output and MOSFET is IRF7416
http://www.irf.com/product-info/datasheets/data/irf7416.pdf

Other then that nothing has changed.

Ok my problems

Vin can be from 5V to a max of 18V.

But Vout should be no more then 4.2.

So when Vin is 5V, Vout it about 4.6V, and a Vin increase, Vout stablizes to about 5V. 

Tthis seems strange as I first i though it could be the IC's, so i changed them. It made no differnce. But as the Vout is not linear to Vin, i know the Mosfet is working.

And another  issue is, if i connect a  battery even it is full, the Vout goes to 4.2V. But if the battery is low and it's charging, the voltage will be about 3.45V.

BUT my problem comes, when Vin starts to increase.... The output current starts increasing above 1A , i have seen it going to about 1.7A. At this point, i switched it off to avoid any damage. The sense resistor is set for 1A, but why does current incease.

I have used the Lm3658 previosuly, i was expecting this IC to work in similar way, where by Vout is always 4.2V 1A, but as Vin increase between 4.3V to 7V, current input decrease, as it get the power from the higher voltage.

 

Should the BQ2057C be behaving in such way.

I also think it could be due to the MOSFET and Vgs threshold, could someone advice me on this.

 

My intention was to have charge with Vin from 5-16V 1A, charging a bettary 4.2V 1A.

 



Regards

DJ

  • 0
    Prodigy 30 points

    Hi DJ,

     

    A little late to respond to this, but I just used the same circuit figure 4 from the datasheet with a BQ2057W, 9V input and 2-LiPol cells in series using the recommended MOSFET and I encountered exactly the same issue: charge current being much higher than I set it to.

     

    I fixed this by adding a 1K series resistor  between R2 and the CC pin, effectively making a voltage divider for the MOSFET gate-source voltage. I suspect indeed that this chip is not designed for ultra-low Vgs MOSFET, therefore I had to do this.

     

    Any TI employee that can confirm this ?

     

    regards,

     

    Pgriep

  • 0 in reply to pgriep
    TI__Mastermind 31120 points

    The IC may be rated for 18V, but it is not practical to use an input voltage much larger than the regulation voltage because of the heat.  Don't know if the heat is also causing an issue.

    A voltage divider on the gate may be necessary for two cell design if the gate of the FET 's gate is not rated for the source voltage.  Sound like the FET may have been damaged.

    This product has sold in the tens of millions and has had no feedback like this, so i am at a loss for exactly what is going on.