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Layout and Bill of Materials
6.3
Thermal Performance
This section shows a thermal image of the bq24257 running at 6-V input and 1-A system load, a 3.8-V
battery is used and charging at rate 1000 mA. There is no air flow and the ambient temperature is 25°C.
The peak temperature of the IC (60.4°C) is well below the maximum recommended operating condition
listed in the data sheet.
Figure 9. Thermal Image
7
Layout and Bill of Materials
7.1
Printed-Circuit Board Layout Guideline
1. Place the BOOT, PMID, IN, BAT, and LDO capacitors as close as possible to the IC for optimal
performance.
2. Connect the inductor as close as possible to the SW pin, and the CSIN cap as close as possible to the
inductor minimizing noise in the path.
3. Place a 1-
μ
F PMID capacitor as close as possible to the PMID and PGND pins, making the high-
frequency current loop area as small as possible.
4. The local bypass capacitor from SYS/CSIN to GND must be connected between the SYS/CSIN pin
and PGND of the IC. This minimizes the current path loop area from the SW pin through the LC filter
and back to the PGND pin.
5. Place all decoupling capacitors close to their respective IC pins and as close as possible to PGND (do
not place components such that routing interrupts power-stage currents). All small control signals must
be routed away from the high-current paths.
6. To reduce noise coupling, use a ground plane, if possible, to isolate the noisy traces from spreading its
noise all over the board. Put vias inside the PGND pads for the IC.
7. The high-current charge paths into IN, Micro-USB, BAT, SYS/CSIN, and from the SW pins must be
sized appropriately for the maximum charge current to avoid voltage drops in these traces.
8. For high-current applications, the balls for the power paths must be connected to as much copper in
the board as possible. This allows better thermal performance because the board conducts heat away
from the IC.
13
SLUUA08A – March 2013 – Revised April 2013
bq2425xEVM-150, Single-Cell Li-Ion Switch-Mode Charger
Copyright © 2013, Texas Instruments Incorporated