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# BQ25155: VBUS to BAT minimum headroom

Part Number: BQ25155

Hello,

Our customer has a question about minimum headroom voltage from VBUS to BAT.

As I checked in the datasheet and found out it seems to be defined by VDPPM and Ron(PMID to BAT). Since PMID voltage is regulated 4.5V default condition for example, understanding the voltage drop from PMID to BAT is necessary.

If above understanding is correct, could you let me know how we can estimate PMID to BAT voltage drop at 245mA charge current case ?

If above understanding is incorrect, could you let me know, how we can estimate VBUS (IN) to BAT minimum headroom voltage ?

Best regards,

• Hi,

With regards to minimum headroom voltage, this is determined by the PMID voltage above BAT as the voltage across the BATFET determines the ability to charge. To understand the question correctly, you want to minimize the voltage between PMID to BAT by regulating PMID to the minimum voltage needed to charge accurately?

PMID will typically have to be 200 mV (VDPPM) above BAT to ensure charging is not reduced. Voltage drop at 245mA will depend on Rdson of the BATFET during charging which can vary somewhat based on temperature. The best way to determine the voltage drop is by using the ADC channels to measure the voltages at the two pins. The charge current ADC channel can also provide information to ensure that charge current is not being reduced.

Best Regards.

• Thank you very much for your feedback.

I am not sure the reason he asked, but I understood that the minimum head room voltage IN to BAT V(IN to BAT) = input current x Ron(IN-PMID) (520m-ohm max) + VDPPM (200mV) in battery tracking operation.

best regards,

• Hi Toshiro,

Typically the 200 mV will be greater than the Charge Current x Ron, but these values do not need to be added.

Best Regards.

• Hi Juan,

Does it mean the minimun headroom voltage is priciesely estimated by input current x Ron(IN-PMID) + VDPPM but roughly estimated by VDPPM, because VDPPM >> Input current x Ron(IN-PMID) at typical condition ?

If my understanding is correct, i will close this thread.

Best regards,

• Hi Toshiro,

I misread your previous question, I thought you meant minimum headroom voltage at PMID. Yes Input minimum headroom would be described by VDPPM + (IIN x Ron). Ron can be estimated at typical conditions for most use but there can be some variations with temperature.

Best Regards,

Juan Ospina

• Thank you very much.

We can close this Q&A.