BQ24742 Datasheet, PDF (27/35 Pages) Texas Instruments – Li-Ion or Li-Polymer Battery Charger with Low Iq and Accurate Trickle Charge

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bq24741, bq24742
SLUS875B – MARCH 2009 – REVISED OCTOBER 2009
MOSFETs.
3. Place inductor input terminal to switching MOSFET’s output terminal 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.
4. The charging current sensing resistor should be placed right next to the inductor output. Route the sense
leads connected across the sensing resistor back to the IC in same layer, close to each other (minimize loop
area) and do not route the sense leads through a high-current path (see Figure 36 for Kelvin connection for
best current accuracy). Place decoupling capacitor on these traces next to the IC.
5. Place output capacitor next to the sensing resistor output and ground.
6. Output capacitor ground connections need to be tied to the same copper that connects to the input capacitor
ground before connecting to system ground.
7. Use single ground connection to tie charger power ground to charger analog ground. Just beneath the IC
use analog ground copper pour but avoid power pins to reduce inductive and capacitive noise coupling.
8. Route analog ground separately from power ground. Connect analog ground to AGND and connect power
ground to PGND separately. Connect analog ground and power ground together using power pad as the
single ground connection point. Or using a 0Ω resistor to tie analog ground to power ground (power pad
should tie to analog ground in this case).
9. Decoupling capacitors should be placed next to the IC pins and make trace connection as short as possible.
10. It is critical that the exposed power pad on the backside of the IC 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.
11. The via size and number should be enough for a given current path.
SW
L1
R1
VBAT
High
VIN
Frequency
Current
BAT
C1
Path
PGND
C2
C3
Figure 35. High Frequency Current Path
Charge Current Direction
RSNS
To Inductor
To Capacitor and battery
Current Sensing Direction
To CSP - CSN pin or ACP - ACN pin
Figure 36. Sensing Resistor PCB Layout
Refer to the EVM design (SLUU284) for the recommended component placement with trace and via locations.
For the QFN information, refer to SCBA017 and SLUA271.
Copyright © 2009, Texas Instruments Incorporated
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