BQ27500-V120 Datasheet, PDF (24/36 Pages) Texas Instruments – System-Side Impedance Track™ Fuel Gauge

bq27500-V120
System-Side Impedance Track™ Fuel Gauge
SLUS880 – OCTOBER 2008
www.ti.com
5.3.4 Power Path Control with the BAT_GD Pin
The bq27500 must operate in conjunction with other electronics in a system appliance, such as chargers
or other ICs and application circuits that draw appreciable power. After a battery is inserted into the
system, there should be no charging or discharging current higher than C/20, 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 negated from the default setting. Once an OCV reading has be made, the BAT_GD pin is asserted,
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.
In PFC 1, the BAT_GD pin is also used to disable battery charging when the bq27500 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 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 reads a thermistor voltage that is near 2.5 V.
5.4 TEMPERATURE MEASUREMENT
The bq27500 measures battery temperature via its TS input, in order to supply battery temperature status
information to the fuel gauging algorithm and charger-control sections of the gauge. Alternatively, it can
also measure internal temperature via its on-chip temperature sensor, but only if the [TEMPS] bit of the
Operation Configuration register is cleared.
Regardless of which sensor is used for measurement, a system processor can request the current battery
temperature by calling the Temperature( ) function (see Section 4.1.1, Standard Data Commands, for
specific information).
The bq27500 external temperature sensing is optimized with the use of a high accuracy negative
temperature coefficient (NTC) thermistor with R₂₅ = 10.0K ± 1% and B25/85 = 3435K ± 1% (such as
Semitec NTC 103AT). The bq27500 can also be configured to use its internal temperature sensor. When
an external themistor is used, a 18.2k pull up resistor between BT/TOUT and TS pins is also required.
Additional circuit information for connecting this thermistor to the bq27500 is shown in Section 8,
Reference Schematic.
5.5 OVERTEMPERATURE INDICATION
5.5.1 Overtemperature: Charge
If during charging, Temperature( ) reaches the threshold of OT Chg for a period of OT Chg Time and
AverageCurrent( ) > Chg Current Threshold, then the [OTC] bit of Flags( ) is set. When Temperature( )
falls to OT Chg Recovery, the [OTC] of Flags( ) is reset.
If OT Chg Time = 0, then the feature is completely disabled.
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FUNCTIONAL DESCRIPTION
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