Rainbow Electronics MAX8922L - Manual (Page 10)

MAX8922L
Thermal Considerations
The MAX8922L is in a thermally enhanced TDFN pack-
age with an exposed pad. Connect the exposed pad of
the package to a large copper ground plane to provide a
thermal contact between the device and the circuit board.
The exposed pad transfers heat away from the device,
allowing the IC to charge the battery with maximum cur-
rent, while minimizing the increase in die temperature.
Input Sources
The MAX8922L operates from well-regulated DC
sources. The charger input voltage range is 4.45V to
7V. The device survives input voltages up to 30V with-
out damage to the IC. If the input voltage is greater
than 7.5V (typ), the IC stops charging. An appropriate
power supply must provide at least 4.2V plus the volt-
age drop across the internal-pass transistor when
sourcing the desired peak charging current.
V IN(MIN) > 4.2V + I FAST-CHARGE(MAX) x R ON
where R ON is the input-to-BAT resistance. Failure to
meet this requirement results in an incomplete charge
or increased charge time.
Recommended PCB Layout and Routing
Place all bypass capacitors for IN and BAT as close as
possible to the IC. Connect the battery to BAT as close
as possible to the IC to provide accurate battery volt-
age sensing. Provide a large copper ground plane to
allow the exposed pad to sink heat away from the
device. Make all high-current traces short and wide to
minimize voltage drops. A sample layout is available in
the MAX8922L Evaluation Kit to speed designs.
Typical Application Circuits
AC Adapter Charge
Figure 4 shows the MAX8922L as a Li+ battery charger
with an AC adapter. The MAX8922L detects the pres-
ence of an input supply resulting in POK pulled low.
Once POK is pulled low, the MAX8922L begins charg-
ing the battery when EN/SET is low or unconnected.
The system can program the charge current by EN/SET
pulses. By monitoring CHG, the system can detect the
top-off threshold and terminate the charge through
EN/SET. The MAX8922L also provides an overvoltage-
protected 4.94V LDO output to a low-voltage-rated USB
system input.
30V Li+ Linear Battery Charger
with GSM Test Mode in 3mm x 2mm TDFN
10 ______________________________________________________________________________________
AC
ADAPTER
IN BAT
EN/SET
GND
LDO
MIN
SETI
EP
POK
CHG
SYSTEM
VBUS
GPIO
GPIO
GPIO
R1
1.87k Ω
C1
1 µF
C2
2.2
µF
C3
1
µF
VI/O
4.2V
Li+
R2
3k Ω
TEST
MAX8922L
Figure 4. AC Adapter Charger Application