The switching node rise and fall
times should be minimized for minimum switching loss. Proper layout of the
components to minimize high frequency current path loop (see Figure 11-1) is important to prevent electrical and magnetic field radiation and high
frequency resonant problems. Follow this specific order carefully to achieve the
proper layout.
- Place input capacitor as close as possible to PMID pin and GND
pin connections and use shortest copper trace connection or GND plane. Add 1-nF
small size (such as 0402 or 0201) decoupling cap for high frequency noise filter
and EMI improvement.
- Place inductor input pin to SW pin as close as possible.
Minimize the copper area of this trace to lower electrical and magnetic field
radiation but make the trace wide enough to carry the charging current. Do not
use multiple layers in parallel for this connection. Minimize parasitic
capacitance from this area to any other trace or plane.
- Put output capacitor near to the inductor and the device.
Ground connections need to be tied to the IC ground with a short copper trace
connection or GND plane.
- Route analog ground separately from power ground. Connect
analog ground and connect power ground separately. Connect analog ground and
power ground together using thermal pad as the single ground connection point.
Or using a 0-Ω resistor to tie analog ground to power ground.
- Use single ground connection to tie charger power ground to
charger analog ground. Just beneath the device. Use ground copper pour but avoid
power pins to reduce inductive and capacitive noise coupling.
- Place decoupling capacitors next to the IC pins and make trace
connection as short as possible.
- It is critical that the exposed thermal pad on the backside of
the device package be soldered to the PCB ground. Ensure that there are
sufficient thermal vias directly under the IC, connecting to the ground plane on
the other layers.
- Ensure that the number and sizes of vias allow enough copper
for a given current path.
See the BQ25618 BMS024 Evaluation Module EVM User's
Guide and BQ25619 BMS025 Evaluation Module
EVM User's Guide for the recommended component placement
with trace and via locations. For the VQFN information, refer to Quad Flatpack No-Lead
Logic Packages Application Report and QFN and SON PCB
Attachment Application Report.