BQ27500_08 Datasheet, PDF (24/39 Pages) Texas Instruments – System-Side Impedance Track™ Fuel Gauge

bq27500
bq27501
System-Side Impedance Track™ Fuel Gauge
SLUS785D – SEPTEMBER 2007 – REVISED APRIL 2008
www.ti.com
5.3.4 Power Path Control With the BAT_GD Pin
The bq27500/1 must operate in conjunction with other electronics in a system appliance, such as chargers
and other ICs and subcircuits that draw appreciable power. After a battery is inserted into the system, this
electronics must be disabled, so that an accurate OCV can be read. The OCV is used for helping
determine which battery profile to use, as it constitutes part of the battery impedance measurement.
When a battery is inserted into a system, the Impedance Track™ algorithm requires that no charging of
the battery takes place and that any discharge is limited to less than C/20—these conditions are sufficient
for the fuel gauge to take an accurate OCV reading. To disable these functions, the BAT_GD pin is merely
set high (floating output pulled high). Once an OCV reading has be made, the BAT_GD pin is pulled low,
thereby enabling battery charging and regular discharge of the battery. The Operation Configuration
[BATG_POL] bit can be used to set the polarity of the battery good signal, should the default configuration
need to be changed.
The flowchart of Figure 5-1 details how the BAT_GD pin functions in the context of battery insertion and
removal, as well as NORMAL vs SLEEP modes.
In PFC 1, the BAT_GD pin is also used to disable battery charging when the bq27500/1 reads battery
temperatures outside the range defined by [Charge Inhibit Temp Low, Charge Inhibit Temp High]. The
BAT_GD line is returned to low once temperature falls within the range [Charge Inhibit Temp Low +
Temp Hys, Charge Inhibit Temp High – Temp Hys].
5.3.5 Battery Detection Using the BI/TOUT Pin
During power-up or hibernate activities, or any other activity where the bq27500/1 must determine whether
a battery is connected or not, the fuel gauge applies a test for battery presence. First, the BI/TOUT pin is
put into high-Z status. The weak 1.8-MΩ pullup resistor keeps the pin high while no battery is present.
When a battery is inserted (or is already inserted) into the system device, the BI/TOUT pin is pulled low.
This state is detected by the fuel gauge, which polls this pin every second when the gauge has power. A
battery disconnected status is assumed when the bq27500/1 reads a thermistor voltage that is near 2.5 V.
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FUNCTIONAL DESCRIPTION
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