BQ24180 Datasheet, PDF (34/39 Pages) Texas Instruments – Fully Integrated Switch-Mode One-Cell Li-Ion Charger with Full USB Compliance and Accessory Power Connection

bq24180
SLUSA02 A – FEBRUARY 2010 – REVISED FEBRUARY 2010
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This is a standard value. If it is not a standard value, then choose the next close value and calculate the real
charge current. Calculate the power dissipation on the sense resistor:
P_(RSNS) = I_(CHARGE) ² × R_(SNS)
P_(RSNS) = 125² × 0.068
P_(RSNS) = 0.106 W
Select 0805 0.25-W 68-mΩ 2% sense resistor, i.e. Sosomu RL122OT-R068-G or RL0816T-R068-F 68-mΩ,
0.125W, 0603, 1%.
PCB LAYOUT CONSIDERATION
It is important to pay special attention to the PCB layout. The following provides some guidelines:
• To obtain optimal performance, the power input capacitors, connected from input to PGND, should be placed
as close as possible to the bq24180. The output inductor should be placed close to the IC and the output
capacitor connected between the inductor and PGND of the IC. The intent is to minimize the current path loop
area from the SW pin through the LC filter and back to the PGND pin. To prevent high frequency oscillation
problems, proper layout to minimize high frequency current path loop is critical (see Figure 39). The sense
resistor should be adjacent to the junction of the inductor and output capacitor. Route the sense leads
connected across the RSNS(R1) back to the IC, close to each other (minimize loop area) or on top of each
other on adjacent layers (do not route the sense leads through a high-current path, see Figure 40).
• Place all decoupling capacitor close to their respective IC pin and as close as to PGND (do not place
components such that routing interrupts power stage currents). All small control signals should be routed
away from the high current paths.
• The PCB should have a ground plane (return) connected directly to the return of all components through vias
(two vias per capacitor for power-stage capacitors, two vias for the IC PGND, one via per capacitor for
small-signal components). A star ground design approach is typically used to keep circuit block currents
isolated (high-power/low-power small-signal) which reduces noise-coupling and ground-bounce issues. A
single ground plane for this design gives good results. With this small layout and a single ground plane, there
is no ground-bounce issue, and having the components segregated minimizes coupling between signals.
• The high-current charge paths into VBUS, PMID and from the SW pins must be sized appropriately for the
maximum charge current in order to avoid voltage drops in these traces. The PGND pins should be
connected to the ground plane to return current through the internal low-side FET.
• Place 4.7mF input capacitor as close to PMID pin and PGND pin as possible to make high frequency current
loop area as small as possible. Place 1mF input capacitor as close to VBUS pin and PGND pin as possible to
make high frequency current loop area as small as possible (see Figure 41).
VBUS
L1
R1
SW
V _BAT
VIN
C1
PMID
C2
High
Frequency
Current
Path
PGND
BAT
C3
Figure 39. High Frequency Current Path
34
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