BQ25751: buck-boost switching frequency confirmation and circuit function confirmation

Hello Sun,

sun qiyun said:

My circuit is designed 30V power input, 26.4V float voltage for 24V lead acid battery charging, max charging current is 1A. Switching frequency is 300KHz.

When I  adjust the input voltage as 28V, the frequency is 150KHz.

Can you provide a scope capture when this behavior occurs?

sun qiyun said:

About the UV/OV function, from the datasheet, I find if the input voltage is over voltage or under voltage, the chip will close the ACFET by the pin ACDRV. 

From the functional block diagram, I find if the ACFET is short, the OV/UV function is also active, and the chip will close the buck-boost controller, right?

Yes, This is correct.

sun qiyun said:

For the inductor selection, datasheet recommends inductor is 2.2~15uH, DCR is 1.75~60mohm. Is it necessary to select inductor per this? Or I can select inductor according to buck circuit caculation?

Yes, You will need to select the inductor based on these parameters.

Best Regards,

Christian

Hi， Moyer

Christian Moyer said:

Can you provide a scope capture when this behavior occurs?

Please see the attached test picture.

C1 is SW1 waveform, C2 is input power, C3 is charging current (in my design, the max charging current is 1A).

When the input power is 28V, the SW1 frequency is about 150KHz.

Christian Moyer said:

sun qiyun said:

About the UV/OV function, from the datasheet, I find if the input voltage is over voltage or under voltage, the chip will close the ACFET by the pin ACDRV. 

From the functional block diagram, I find if the ACFET is short, the OV/UV function is also active, and the chip will close the buck-boost controller, right?

Yes, This is correct.

From the block diagram, I only find the OVP signal Vsys_rev_ov, no UV signal, does it show when the ACFET is short, the controller is only be closed when the power is over voltage, but  not be closed in under voltage, right? And I have not find the value Vsys_rev_ov, could you please tell me if ACFET is short, how the controller is closed when OV/UV happens?

Thanks!

Hello Sun,

sun qiyun said:

When the input power is 28V, the SW1 frequency is about 150KHz.

What is the battery voltage when this behavior occurs? Do you see any difference at different battery voltages? Can you also measure SW2 when this behavior occurs?

sun qiyun said:

From the block diagram, I only find the OVP signal Vsys_rev_ov, no UV signal, does it show when the ACFET is short, the controller is only be closed when the power is over voltage, but  not be closed in under voltage, right? And I have not find the value Vsys_rev_ov, could you please tell me if ACFET is short, how the controller is closed when OV/UV happens?

Yes, If ACFET is shorted. The converter will shut off in both overvoltage conditions and under voltage conditions. Im not sure why this is not in block diagram, but I confirmed it in the lab.

Best Regards,

Christian