BQ24040 Datasheet, PDF (25/31 Pages) Texas Instruments – 800mA Single-Input Single Cell Li-Ion Battery Charger With Auto Start

bq24040
bq24041
www.ti.com.................................................................................................................................... SLUS941A – SEPTEMBER 2009 – REVISED SEPTEMBER 2009
CHG and PG
LED Status: connect a 1.5k resistor in series with a LED between the OUT pin and the CHG pin.
Connect a 1.5k resistor in series with a LED between the OUT pin and the and PG pin.
Processor Monitoring: Connect a pull-up resistor between the processor’s power rail and the CHG pin.
Connect a pull-up resistor between the processor’s power rail and the PG pin.
SELECTING IN AND OUT PIN CAPACITORS
In most applications, all that is needed is a high-frequency decoupling capacitor (ceramic) on the power pin, input
and output pins. Using the values shown on the application diagram, is recommended. After evaluation of these
voltage signals with real system operational conditions, one can determine if capacitance values can be adjusted
toward the minimum recommended values (DC load application) or higher values for fast high amplitude pulsed
load applications. Note if designed for high input voltage sources (bad adaptors or wrong adaptors), the capacitor
needs to be rated appropriately. Ceramic capacitors are tested to 2x their rated values so a 16V capacitor may
be adequate for a 30V transient (verify tested rating with capacitor manufacturer).
THERMAL PACKAGE
The bq2405x 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). The power pad should
be directly connected to the VSS pin. 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:
θ_JA = (T_J – T) / P
(0)
Where:
T_J = chip junction temperature
T = ambient temperature
P = device power dissipation
Factors that can influence the measurement and calculation of θ_JA include:
1. Whether or not the device is board mounted
2. Trace size, composition, thickness, and geometry
3. Orientation of the device (horizontal or vertical)
4. Volume of the ambient air surrounding the device under test and airflow
5. Whether other surfaces are in close proximity to the device being tested
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. Typically after fast charge begins the pack voltage
increases to 3.4V within the first 2 minutes. The thermal time constant of the assembly typically takes a few
minutes to heat up so when doing maximum power dissipation calculations, 3.4V is a good minimum voltage to
use. This is verified, with the system and a fully discharged battery, by plotting temperature on the bottom of the
PCB under the IC (pad should have multiple vias), the charge current and the battery voltage as a function of
time. The fast charge current will start to taper off if the part goes into thermal regulation.
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) + [V_(OUT) – V_(BAT)] × I_(BAT)
(0)
The thermal loop feature reduces the charge current to limit excessive IC junction temperature. It is
recommended that the design not run in thermal regulation for typical operating conditions (nominal input voltage
and nominal ambient temperatures) and use the feature for non typical situations such as hot environments or
higher than normal input source voltage. With that said, the IC will still perform as described, if the thermal loop
is always active.
Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s): bq24040 bq24041
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