Datasheet

_______________Detailed Description
The MAX745 is a switch-mode, Li+ battery charger that
can achieve 90% efficiency. The charge voltage and
current are set independently by external resistor-
dividers at SETI and VADJ, and at pin connections at
CELL0 and CELL1. VADJ is connected to a resistor-
divider to set the charging voltage. The output voltage-
adjustment range is ±5%, eliminating the need for 0.1%
resistors while still achieving 0.75% set accuracy using
1% resistors.
The MAX745 consists of a current-mode, pulse-width-
modulated (PWM) controller and two transconductance
error amplifiers: one for regulating current (GMI) and
the other for regulating voltage (GMV) (Figure 2). The
error amplifiers are controlled through the SETI and
VADJ pins. Whether the MAX745 is controlling voltage
or current at any time depends on the battery state. If
the battery is discharged, the MAX745 output reaches
the current-regulation limit before the voltage limit,
causing the system to regulate current. As the battery
charges, the voltage rises to the point where the volt-
age limit is reached and the charger switches to regu-
lating voltage. The STATUS pin indicates whether the
charger is regulating current or voltage.
Voltage Control
To set the voltage limit on the battery, connect a resis-
tor- divider to VADJ from REF. A 0V to V
REF
change at
VADJ sets a ±5% change in the battery limit voltage
around 4.2V. Since the 0 to 4.2V range on VADJ results
in only a 10% change on the voltage limit, the resistor-
divider’s accuracy does not need to be as high as the
output voltage accuracy. Using 1% resistors for the
voltage dividers typically results in no more than 0.1%
degradation in output voltage accuracy. VADJ is inter-
nally buffered so that high-value resistors can be used
to set the output voltage. When the voltage at VADJ is
MAX745
Switch-Mode Lithium-Ion
Battery Charger
_______________________________________________________________________________________ 5
______________________________________________________________Pin Description
IBAT
Current-Sense Amplifier’s Analog Current-Source Output. See the Monitoring Charge Current section for a
detailed description.
2 DCIN Charger Input Voltage. Bypass DCIN with a 0.1µF capacitor.
3 VL Chip Power Supply. Output of the 5.4V linear regulator from DCIN. Bypass VL with a 4.7µF capacitor.
1
4 CCV Voltage-Regulation-Loop Compensation Point
5 CCI Current-Regulation-Loop Compensation Point
8 VADJ
Voltage-Adjustment Pin. VADJ is tied to a 1% tolerance external resistor-divider to adjust the voltage set
point by 10%, eliminating the need for precision 0.1% resistors. The input voltage range is 0V to V
REF
.
7 REF 4.2V Reference Voltage Output. Bypass REF with a 0.1µF or greater capacitor.
6
THM/
SHDN
Thermistor Sense-Voltage Input. THM/SHDN also performs the shutdown function. If pulled low,
the charger turns off.
13 STATUS
An open-drain MOSFET sinks current when in current-regulation mode, and is high impedance when in volt-
age-regulation mode. Connect STATUS to VL through a 1kΩ to 100kΩ pullup resistor. STATUS can also drive
an LED for visual indication of regulation mode (see MAX745 EV kit). Leave STATUS floating if not used.
11, 12
CELL1,
CELL0
Logic Inputs to Select Cell Count. See Table 1 for cell-count programming.
10 GND Analog Ground
9 SETI SETI is externally tied to the resistor-divider between REF and GND to set the charging current.
14 BATT Battery-Voltage-Sense Input and Current-Sense Negative Input
15 CS Current-Sense Positive Input
16 PGND Power Ground
17 DLO Low-Side Power MOSFET Driver Output
18 DHI High-Side Power MOSFET Driver Output
19 LX Power Connection for the High-Side Power MOSFET Source
20 BST Power Input for the High-Side Power MOSFET Driver
NAME FUNCTIONPIN