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 BQ2409x, 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 BQ2409x 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 multiple 10mil vias in the power
pad of the IC and in close proximity to conduct the heat to the bottom ground
plane. The bottom ground place should avoid traces that “cut off” the thermal
path. The thinner the PCB the less temperature rise. The EVM PCB has a thickness
of 0.031 inches and uses 2 oz. (2.8 mil thick) copper on top and bottom, and is
a good example of optimal thermal performance.