Rainbow Electronics MAX1508 - Manual (Page 8)

MAX1508
Linear Li+ Battery Charger with Integrated Pass FET,
Thermal Regulation, and
ACOK
in 3mm x 3mm TDFN
8 _______________________________________________________________________________________
EN V IN V BATT I BATT
CHG STATE
X V BATT V IN 0 Hi-Z Shutdown
Low 4.25V ≤ V
IN
≤ 7V < 2.5V 10% of I FAST Low Prequalification
Low 4.25V ≤ V
IN
≤ 7V ≥ 2.5V I FAST * Low Fast Charge
Low 4.25V ≤ V
IN
≤ 7V 4.2V 10% of I
FAST
Hi-Z Full Charge
Low >7V X 0 Hi-Z Overvoltage
High X X 0 Hi-Z Disabled
X = Don’t care.
*I
FAST
is reduced as necessary to maintain the die temperature at +100°C.
Table 1. CHG States
Capacitor Selection
Connect a ceramic capacitor from BATT to GND for
proper stability. Use a 1µF X5R ceramic capacitor for
most applications.
Connect a 1µF ceramic capacitor from IN to GND. Use
a larger input bypass capacitor for high input voltages
or high charging currents to reduce supply noise.
Connect a 0.47µF ceramic capacitor from VL to GND.
Thermal Considerations
The MAX1508 is in a thermally enhanced thin DFN pack-
age with exposed paddle. Connect the exposed paddle
of the MAX1508 to a large copper ground plane to pro-
vide a thermal contact between the device and the circuit
board. The exposed paddle transfers heat away from the
device, allowing the MAX1508 to charge the battery with
maximum current, while minimizing the increase in die
temperature.
DC Input Sources
The MAX1508 operates from well-regulated DC
sources. The full-charging input voltage range is 4.25V
to 7V. The device can stand up to 13V on the input
without damage to the IC. If V
IN is greater than 7V, then
the MAX1508 stops charging.
An appropriate power supply must provide at least
4.25V when sourcing the desired peak charging cur-
rent. It also must stay below 6.5V when unloaded.
Application Circuits
Stand-Alone Li+ Charger
The MAX1508 provides a complete Li+ charging solu-
tion. The Typical Operating Circuit on the front page
shows the MAX1508 as a stand-alone Li+ battery
charger. The 2.8k
Ω resistor connected to ISET sets a
charging current of 520mA. The LED indicates when
either fast-charge or precharge qualification has
begun. When the battery is full, the LED turns off.
Microprocessor-Interfaced Charger
Figure 3 shows the MAX1508 as a µP-cooperated Li+
battery charger. The MAX1508 starts charging the bat-
tery when EN is low. The µP can drive EN high to dis-
able the charger. The MAX1508’s ACOK output
indicates the presence of a valid AC adapter to the µP.
CHG can be used to detect the charge status of a bat-
tery. By monitoring V
ISET
, the system can measure the
charge current.
USB-Powered Li+ Charger
The universal serial bus (USB) provides a high-speed
serial communication port as well as power for the
remote device. The MAX1508 can be configured to
charge its battery at the highest current possible from
the host port. Figure 4 shows the MAX1508 as a USB
battery charger. To make the circuit compatible with
either 100mA or 500mA USB ports, the circuit initializes
at 95mA charging current. The microprocessor then
interrogates the host to determine its current capability.
If the host port is capable, the charging current is
increased to 435mA. The 435mA current was chosen to
avoid exceeding the 500mA USB specification.
Layout and Bypassing
Connect a 1µF ceramic input capacitor as close to the
device as possible. Provide a large copper GND plane
to allow the exposed paddle to sink heat away from the
device. Connect the battery to BATT as close to the
device as possible to provide accurate battery voltage
sensing. Make all high-current traces short and wide to
minimize voltage drops. For an example layout, refer to
the MAX1507/MAX1508 evaluation kit layout.
Chip Information
TRANSISTOR COUNT: 1812
PROCESS: BiCMOS