LP3945 Datasheet, PDF (11/15 Pages) National Semiconductor (TI) – Battery Charge Management System

Application Notes (Continued)
CHARGE CURRENT SELECTION
The LP3945 and LP3946 are designed to provide a charge
current ranging from 500 mA to 950 mA, in 50 mA resolution,
to support batteries with different capacity ratings. No exter-
nal resistor is required to set the charge current in the
LP3945 and LP3946. This entirely eliminates design time,
external component board space and stability issue.
The LP3945 uses the I²C interface to program the charge
current while the LP3946 has a pre-programmed charge
current.
BATTERY VOLTAGE SELECTION
The battery voltage regulation is set to 4.1V during the
manufacturing. The 4.2V option can be selected on LP3945
via the I²C interface or set at the factory for LP3946.
The Ni-MH charge mode is only available in LP3945.
END OF CHARGE (EOC) CURRENT SELECTION
The EOC thresholds can be programmed to 0.1C, 0.15C and
0.2C in the LP3945. The default value is 0.1C, which pro-
vides the highest energy storage, but at the expense of
longer charging time. On the other hand, 0.2C takes the
least amount of charging time, but yields the least energy
storage. The LP3946 has 0.1C as pre-programmed EOC
threshold. 0.15C and 0.2C options are available upon re-
quest.
No EOC function is available during Ni-MH charge cycle.
User must provide a reliable method for charge termination.
CHARGE CURRENT SENSE DIFFERENTIAL AMPLIFIER
The charge current is monitored across the internal 120 mΩ
current sense resistor. The differential amplifier provides the
analog representation of the charge current. Charge current
can be calculated using the following equation:
Where voltage at Diff Amp output (V_DIFF) is in volt, and
charge current (I_CHG) is in amps.
Monitoring the Diff Amp output during constant voltage cycle
can provide an accurate indication of the battery charge
status and the time remaining to EOC. This feature is par-
ticularly useful during Ni-MH charge cycle. The current
sense circuit is operational in the LDO mode as well. It can
be used to monitor the system current consumption during
testing.
20066514
FIGURE 4. Charge Current Monitoring Circuit (Diff-Amp)
LED CHARGE STATUS INDICATORS
The LP3945 and LP3946 are equipped with two open drain
outputs to drive a green LED and a red LED. These two
LEDs work together in combinations to indicate charge sta-
tus or fault conditions. Table 2 shows all the conditions.
TABLE 2. LED Indicator Summary
Charger Status
RED
LED
GREEN
LED
Charger Off
OFF
OFF
Charging Li Ion Battery*
ON
OFF
Maintenance Mode
OFF
ON
Charging Li Ion Battery after
Passing Maintenance Mode
OFF
ON
Charging Ni-MH in Constant
ON
OFF
Voltage Mode
EN Pin = LOW
OFF
ON
LDO Mode
OFF
OFF
5.6 Hr Safety Timer Flag
ON
ON
*Charging Li Ion battery for the first time after V_CHG-IN insertion.
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