BQ2406X Datasheet, PDF (26/31 Pages) Texas Instruments – 1A SINGLE-CHIP Li-Ion/Li-Pol CHARGE MANAGEMENT IC WITH THERMAL REGULATION

bq2406x
SLUS689 – JUNE 2006
www.ti.com
APPLICATION INFORMATION (continued)
ISET/BAT
For stability reasons, it may be necessary to put a 0.47-µF capacitor between the ISET and
BAT pin..
STAT1/2 and PG Optional (LED STATUS – See below, Processor Monitored; or no status)
STAT1
Connect the cathode of a red LED to the open-collector STAT1 output, and connect the
anode of the red LED to the input supply through a 1.5 kΩ resistor that limits the current.
STAT2
Connect the cathode of a green LED to the open-collector STAT2 output, and connect the
anode of the green LED to the input supply through a 1.5 kΩ resistor that limits the current.
PG
Connect the cathode of an LED to the open-collector PG output, and connect the anode of
the LED to the input supply through a 1.5 kΩ resistor to limit the current.
THERMAL CONSIDERATIONS
The bq2406x family is packaged in a thermally enhanced MLP package. The package includes a thermal pad to
provide an effective thermal contact between the IC and the printed circuit board (PCB). Full PCB design
guidelines for this package are provided in the application note entitled: QFN/SON PCB Attachment Application
Note (SLUA271).
The most common measure of package thermal performance is thermal impedance (θ_JA ) measured (or
modeled) from the chip junction to the air surrounding the package surface (ambient). The mathematical
expression for θ_JA is:
q(JA)
+
T_J
*
P
T_A
(6)
Where:
T_J = chip junction temperature
T_A = ambient temperature
P = device power dissipation
Factors that can greatly influence the measurement and calculation of θ_JA include:
• Whether or not the device is board mounted
• Trace size, composition, thickness, and geometry
• Orientation of the device (horizontal or vertical)
• Volume of the ambient air surrounding the device under test and airflow
• Whether other surfaces are in close proximity to the device being tested
The device power dissipation, P, is a function of the charge rate and the voltage drop across the internal
PowerFET. It can be calculated from the following equation when a battery pack is being charged :
P = [V(IN) – V(OUT)] × I(OUT)
Due to the charge profile of Li-Ion batteries the maximum power dissipation is typically seen at the beginning of
the charge cycle when the battery voltage is at its lowest. See the charging profile, Figure 1 .
If the board thermal design is not adequate the programmed fast charge rate current may not be achieved under
maximum input voltage and minimum battery voltage, as the thermal loop can be active effectively reducing the
charge current to avoid excessive IC junction temperature.
USING ADAPTERS WITH LARGE OUTPUT VOLTAGE RIPPLE
Some low cost adapters implement a half rectifier topology, which causes the adapter output voltage to fall
below the battery voltage during part of the cycle. To enable operation with low cost adapters under those
conditions the bq2406x family keeps the charger on for at least 30 msec (typical) after the input power was not
detected. This feature enables use of external low cost adapters using 50 Hz networks.
The backgate control circuit prevents any reverse current flowing from the battery to the adapter terminal during
the charger off delay time.
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