BQ25306: BQ25306RTER

Hello, 

Thank you for reaching out via E2E. 

Switching R11 to 1.33MΩ in your design is the correct resistance value to set full charge voltage to approx. 8.4V. R1 = 1..33MΩ and R2 = 200kΩ will set VBATREG to 8.41 using the equation from table 7-1 Pin Functions in the BQ25306 datasheet. 

FirazAhmed Peerjade said:

The voltage is not changing even after replacing the FB resistor (R11 with 1.33MΩ).

You mention the voltage is not changing, but please understand the BQ25306 device charges battery in constant current mode then constant voltage mode. This is shown in Figure 9-1 of the datasheet. Once FB resistor is changed if battery voltage is still at 3.8V the device will be in precharge phase of charge cycle. 

Do you have two series batteries connected in your testing after changing the FB resistor? 

Best Regards,

Garrett

Yes, two batteries connected in series, and my battery voltage is around 6.9V. 

Hello, 

Thank you for your reply. 

Given the battery voltage is around 6.9V what charge current do you observe? Is it the 2A you expect to see based on resistor value at ICHG pin connection? 

Best Regards,

Garrett

Yes, the current was set to 2Amp

Hello,

Thank you for the confirmation that charge current is still set to 2A. 

What is the actual charge current you measure during your test? If you do not have the ability to monitor charge current what is the input current draw at VBUS by the BQ25306 IC? 

Additionally, I see in your schematic that input rail is 5V. Have you increased VIN above 8.4V now that you are trying to charge 2 series battery cells rather than 1? 

Best Regards,

Garrett 

I switched the input voltage from 5V to 12V and didn't measure the output current. I set the current as per the documentation and assumed it would supply 2Amp, but although I followed the schematic as a reference, I am still not getting an 8.4V output—please tell me what might be wrong.

Hello, 

Thank you for the reply and confirming you switched your input supply to 12V. 

FirazAhmed Peerjade said:

but although I followed the schematic as a reference, I am still not getting an 8.4V output—please tell me what might be wrong.

Please understand the BQ25306 will not automatically regulate output to 8.4V all the time. I recommend you refer to the Battery Charging Profile section of the datasheet (section 9.3.2.1). Given battery voltage is approx. 6.9V the device will be in 'fast charge' phase where buck converter operates in constant current mode at 2A rather than in voltage regulation mode. 

The 2 series battery needs to be approaching full charge for voltage to be 8.4V. 

I requested you to check whether expected 2A charge current was observed in your test to verify whether BQ25306 was behaving normally. If you cannot verify input current or charge current during your test please allow extended time for charging to check whether battery voltage will increase to 8.4V. 

Best Regards,

Garrett

How do I set it to 7.4 V? I'm using two 3.7 V 18650 cells connected in series, which yields a total voltage of 7.4 V, so I want to set the output voltage to 7.4 V.

Yes, im using 2 18650 cells in series. 

Hello, 

FirazAhmed Peerjade said:

Yes, im using 2 18650 cells in series. 

Are they Li-ion cells? The typical full charge voltage will differ based on battery cell chemistry. For example, LiFePo4 is generally charged to a lower voltage compared to Li-ion. 

If you are using Li-ion I recommend you double check the specification for full charge voltage in your battery datasheet. 

Best Regards,

Garrett 

I don’t have much time to extend this discussion. I just wanted to know why, even though I designed my circuit as per the reference design, I’m still not getting 8.4V. How can we fix this? I’m using this IC in one of my products, and the delivery dates are near. If there’s anything you can do to help me achieve the desired output, please let me know. Thank you for your valuable time.