BQ24075T Datasheet, PDF (24/29 Pages) Texas Instruments – 1.5A USB-Friendly Li-Ion Battery Charger and Power-Path Management IC

bq24075T
bq24079T
SLUS937 – DECEMBER 2009
www.ti.com
SELECTING IN, OUT AND BAT CAPACITORS
In most applications, all that is needed is a high-frequency decoupling capacitor (ceramic) on the power pin,
input, output and battery 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 high input voltage sources (bad adapters or wrong
adapters), 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 bq2407xT 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 V_SS. Full PCB design guidelines for this package are provided in the application note
entitled: QFN/SON PCB Attachment Application Note. 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: = (T_J – T) / P
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
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
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 Equation 5 when a battery pack is being charged :
P = [V_IN – V_OUT] × I_OUT + [V_OUT – V_BAT] × I_BAT
(5)
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.
Half-Wave Adapters
Some 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 adapters under those conditions, the bq2407xT
family keeps the charger on for at least 20 msec (typical) after the input power puts the part in sleep mode. This
feature enables use of external adapters using 50 Hz networks. The input must not drop below the UVLO voltage
for the charger to work properly. Thus, the battery voltage should be above the UVLO to help prevent the input
from dropping out. Additional input capacitance may be needed.
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