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BQ25895EVM-664: Boost Mode Issue

Harshraj Atodaria
Prodigy 20 points
Part Number: BQ25895EVM-664
Other Parts Discussed in Thread: BQ25895

Hello TI Community,

I am working on charging and discharging of Single cell Li-ion battery using Charger bq25895 and for that I am using the evaluation board BQ25895EVM-664 to confirm before I start with my board.

I am using this evaluation board as a standalone board without I2C communication. For charging part it behaves correctly the way I expected it to behave. 

Issue arrives when I try to get the boost output to the load.For load I am using different power resistors.

To start with I have followed the necessary conditions to enable the boost mode (8.2.5) of the following datasheet link such as pull up the OTG pin, Vbus is disconnected and VBat above threshold value, as soon as this is satisfied I am getting 5V at PMID. That looks good so far!!

www.ti.com/.../bq25895.pdf

Further when I start connecting the load at PMID to draw current it behaves nicely for values greater then or equal to 20 ohms. As I go below 20 ohms to draw more power PMID voltage drops to 3.4V and current readings are also incosistent. This chip is capable of delivering 5V at 3.1A and it behaves differently when I try to draw more then 250mA current.

I tried changing the battery and also replaced it with a lab power source but din't change the result. Now I am kind of in a turmoil on how to resolve this issue.

Thank you for help in advance and will be eagerly waiting for the response.

Regards,

Harshraj  

  • 0
    TI__Guru*** 143950 points

    Harshraj,

    Does the EVM properly function in charge (buck) mode?  When in OTG mode, what is the voltage measured at BAT pin on EVM?  Can you provide an oscope shot of V(BAT), V(SYS), V(SW) and V(PMID) when the EVM is working properly in OTG mode and when it is not working properly?  You mention "current readings".  What do you mean by that?  Do you have series current meters installed? 

  • 0 in reply to Jeff F
    Prodigy 20 points

    Hello Jeff,

    Thank you for your quick response.

    In response to your question, yes for current meters.After your mail I did go through the voltage levels and note that battery voltage which in my case a power source was dropping low due to wiring connections involved. Improving those connections chip started to show some improvements although it did not fulfilled the expectation as boost output (PMID) was not consistent below 3.9 battery voltage.

    Further I removed the PMID load and added system loaded to measure how much current can I draw. Connecting it with low value power resistor chip failed and that ends my tests.

    If you could recommend a charger that can help draw 6 amps at 3.3 battery Voltage then it would be great or it would be nice to have boost built in.

    Thank you for your support and will be waiting for your response.

    Regards,

    Harshraj   

  • 0 in reply to Harshraj Atodaria
    TI__Guru*** 143950 points
    Harshraj,

    The bq25895 is the best choice for your application. Its IBAT(FET_OCP), the peak discharge current is 9A with 6A being the maximum continuous discharge current. Current meters , especially when set to auto range, add too much series resistance. I suggest removing them and try again. It can provide 5.0V/3.1A down to 3.0V. If there is too much series resistance and/or their is a fast transient from low current to 3.1A that causes V(BAT) to dip below V(OTG_BAT) = 2.9V then the boost converter may not function properly.