BQ24072 Datasheet, PDF (30/38 Pages) Texas Instruments – 1.2A USB-FRIENDLY Li-Ion BATTERY CHARGER AND POWER-PATH MANAGEMENT IC

bq24072, bq24073
bq24074, bq24075
SLUS810A – SEPTEMBER 2008 – REVISED DECEMBER 2008...................................................................................................................................... www.ti.com
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)
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 Adaptors
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 bq2407x
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.
Sleep Mode
When the input is between V_UVLO and V_IN(DT), the device enters sleep mode. After entering sleep mode for
>20mS the internal FET connection between the IN and OUT pin is disabled and pulling the input to ground will
not discharge the battery, other than the leakage on the BAT pin. If one has a full 1000mAHr battery and the
leakage is 10µA, then it would take 1000mAHr/10µA = 100000 hours (11.4 years) to discharge the battery. The
battery’s self discharge is typically 5 times higher than this.
Layout Tips
• To obtain optimal performance, the decoupling capacitor from IN to GND (thermal pad) and the output filter
capacitors from OUT to GND (thermal pad) should be placed as close as possible to the bq2407x, with short
trace runs to both IN, OUT and GND (thermal pad).
• All low-current GND connections should be kept separate from the high-current charge or discharge paths
from the battery. Use a single-point ground technique incorporating both the small signal ground path and the
power ground path.
• The high current charge paths into IN pin and from the OUT pin must be sized appropriately for the maximum
charge current in order to avoid voltage drops in these traces
• The bq2407x 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); this thermal pad is
also the main ground connection for the device. Connect the thermal pad to the PCB ground connection. Full
PCB design guidelines for this package are provided in the application note entitled: QFN/SON PCB
Attachment Application Note (SLUA271).
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