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BQ25505: Battery is not charging

Part Number: BQ25505
Other Parts Discussed in Thread: BQ25570EVM-206


We are using BQ25505 boost charger to recharge a secondary battery.

Test setup description as below:

A discharged battery of 2.75V is connected across VBAT_SEC pin..

VIN_DC is supplied from a bench top power supply unit with 3.3V and 0.5A.

VSTOR output is directly connected to the VOC_SAMP pin on the board.

VREF_SAMP is connected to a low leakage 10nF capacitor.

With proper biasing setup through resistor drivers, we still don't see a charging current to the battery. With this context we observe the following:

1. VREF_SAMP is 16sec periodic signal with approx. 300msec pulse width and amplitude is around 2.10V.

2. VOC_SAMP & VBAT_SEC signal waveforms are similar - both are a pulsating train with 16sec repeatability and peak is 4.20V and it drops to 3.20V within 300msec. 

3. We don't have the BQ25505 EVM. However when replicating the setup with BQ25570EVM-206, battery charging peak current is 40mA.

Could you please help us in understanding this behavior of BQ25505 device - why battery is not getting charged?

Is VREF_SAMP signal require to be static, not pulsating? Currently, a 10nF cap is connected, do you see it to be increased?

  • Hi Rajesh,

    VREF_SAMP should not discharge much between sampling intervals.  Placing a scope probe or DMM with 10Mohm resistance to ground will discharge it.  Or using a 10nF cap with low insulation resistance (i.e. low voltage rating).  Regarding not charging, is the boost converter switching, i.e. is there a sporadic squarish waveform at LBOOST similar to the datasheet picture below?  If so, is the load on VSTOR more than the input source can provide?