BQ24040 Datasheet, PDF (26/31 Pages) Texas Instruments – 800mA Single-Input Single Cell Li-Ion Battery Charger With Auto Start

bq24040
bq24041
SLUS941A – SEPTEMBER 2009 – REVISED SEPTEMBER 2009.................................................................................................................................... www.ti.com
Leakage Current Effects on Battery Capacity
To determine how fast a leakage current on the battery will discharge the battery is an easy calculation. The time
from full to discharge can be calculated by dividing the Amp-Hour Capacity of the battery by the leakage current.
For a 0.75AHr battery and a 10μA leakage current (750mAHr/0.010mA = 75000 Hours), it would take 75k hours
or 8.8 years to discharge. In reality the self discharge of the cell would be much faster so the 10μA leakage
would be considered negliable.
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 bq2405x, 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 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); this thermal pad is
also the main ground connection for the device. Connect the thermal pad to the PCB ground connection. It is
best to use muntiply 10mil vias in the power pad of the IC and in close proxcimity to conduct the heat to the
bottom ground plane. The bottom ground place should avoid traces that “cut off” the thermal path. The thiner
the PCB the less temperature rise. The EVM PCB has a thickness of 0.031 inchs and uses 2 oz. (2.8mil
thick) copper on top and bottom, and is a good example of optimal thermal performance.
SPACER
REVISION HISTORY
Changes from Original (August 2009) to Revision A ..................................................................................................... Page
• Changed the status of the devices From: Product Preview To: Prodcution Data ................................................................ 1
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