BQ25892: Not start without battery.

Alexandr Brychev

Prodigy 80 points

Part Number: BQ25892

I have charger on BQ25892.

With battery all work is good. But when i connect VBUS without battery U_SYS is only 1.8V. This is normal?

over 7 years ago

0 Jeff F over 7 years ago

TI__Guru**** 165886 points

HI Alexandr,

What is your system load? What is the voltage measured at the charger's VBUS or PMID pins? Is R79 tuned to 10kohm so that V(TS) ~= 1/2 V(REGN)? Can you send your I2C register settings when no battery is attached?

0 Alexandr Brychev over 7 years ago in reply to Jeff F

Prodigy 80 points

Hi Jeff, i tryed with load ~300ma and no load. Both times Vsys is 1.8V. Vbus and PMID ~5V. R79 NTC 10k. I can't write i2c register, because my device can't start from 1.8v. If Vsys 1.8v and i connect battery, charger start working and Vsys is normal.

0 Jeff F over 7 years ago in reply to Alexandr Brychev

TI__Guru**** 165886 points

When you say "charger start working", did you see a square-ish oscilloscope waveform on the SW pin? Did SYS always stay >3.5V even if battery voltage was <3.5V? Using an oscilloscope, can you measure current going into VBUS or coming out of the inductor and going to SYS and BAT?

0 Alexandr Brychev over 7 years ago in reply to Jeff F

Prodigy 80 points

Square impulses present without battery and with battery, but with different duty cycle. Vsys is 3.5V when battery 2.7V. I connect alcaline battery 1.5V, with it Vsys already 1.8V.

0 Jeff F over 7 years ago in reply to Alexandr Brychev

TI__Guru**** 165886 points

Alexandr,

Most alkaline batteries are not rechargeable and should not be connected to the charger.

Just to confirm, when no battery is connected between BAT and GND pins, VSYS = 1.8V but you see a square wave at the SW pin? Can you measure VBUS input current when no battery is connected?

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BQ25892: Not start without battery.