LM3647 Datasheet, PDF (6/17 Pages) National Semiconductor (TI) – Universal Battery Charger for Li-Ion, Ni-MH and Ni-Cd Batteries

LED2 is an active-low output used to indicate charge or dis-
charge. It also sends out digitally what the LM3647 has read
at the mode selection pins and charge timeout.
LED3 is an active-low output used to indicate charge start/
stop and error.
VREF is the voltage reference analog input. The LM3647
uses this pin as a reference when measuring the other ana-
log inputs.
CEXT is a timing pin used by the LM3647, it must be con-
nected to a low loss capacitor.
CEL is an analog input that measures the battery voltage via
a resistor divider network.
CS is an analog input that is connected to a differential am-
plifier that measures the voltage over a small current sensing
resistor.
TEMP is an analog input that is connected to the temperature
sensing NTC-resistor (if used). If no temperature sensor is
used, the input must be biased to approximate 1.5-2V.
DISCHG is a digital output that controls a power-FET that
discharges the batteries before charging them. If this function
is not used then leave this pin unconnected.
SYSOK is an open drain output that resets the LM3647 in the
rare case of an internal illegal operating condition. This pin is
connected to the RESET pin to increase reliable operation of
the device in hostile operating environments (e.g., noisy en-
vironments).
BUZZER is a digital output that controls a small FET and
turns the buzzer on and off. The buzzer must have it’s own
oscillator drive circuitry.
PWM is a digital output that controls the charge voltage or
turns the external current source on and off (depending on
mode-selection).
4.3 Configurations
4.3.1 Maximum Battery Voltage
The maximum battery voltage corresponds to the number of
battery cells. The resistor network in the figure below scales
the battery voltage to a level suitable for the LM3647. For Ni-
Cd/Ni-MH batteries the tolerance of the network is not criti-
cal, and only defines the maximum battery voltage (which is
used as a backup termination method). For Li-Ion batteries
the network must be more accurate, and resistors with low
tolerances must be used (1% or better).
Ni-Cd/Ni-MH:
Each battery cell is at nominal voltage 1.2V, but the critical
voltage is rather the maximum voltage per cell specified at
1.85V. By multiplying the number of cells with the maximum
cell voltage, the Maximum Battery Voltage is achieved.
When the maximum battery voltage has been determined,
the voltage divider network can be dimensioned using the fol-
lowing formula:
MaximumBatteryVoltage × ₍---R----₆---R-₊---⁷--R----₇----₎= CEL = 3.017V
Resistor network selection Quick Guide:
No. of Cells
2
3
4
5
6
7
8
9
10
Normal
2.4V
3.6V
4.8V
6V
7.2V
8.4V
9.6V
10.8V
12V
Ni-Cd/Ni-MH
Max
3.7V
5.55V
7.4V
9.25V
11.1V
12.95V
14.8V
16.65V
18.5V
R6
R7
16k 11k
62k 30k
15k 5.6k
39k 10k
22k 3.9k
Example: A standard 9V Ni-Cd block battery is composed of
6 small Ni-Cd cells and therefore have a nominal voltage of
7.2V. See table above for resistor values.
Li-Ion:
The voltage divider network for Li-Ion must be selected with
great care for maximum utilization of the batteries. Li-Ion bat-
tery cells have a nominal voltage of 3.6V or 3.7V and the
maximum voltage per cell is specified at 4.1V or 4.2V respec-
tively. By multiplying the number of battery cells with the max-
imum cell voltage, it is possible to determine the Maximum
Voltage of the Battery Pack. When the maximum battery volt-
age has been determined, the voltage divider network has to
be dimensioned using the following formula:
MaximumBatteryVoltage × ₍---R----₆---R-₊---⁷--R----₇----₎= CEL = 2.675V
(2.740V if SEL3 is set to Vcc)
The LM3647 supports two different user selectable battery
input voltages on the cell pins. These are 2.675V (SEL3 tied
to GND) and 2.740V (SEL3 tied to Vcc). This selection pin
can be used to configure the charger to handle both 3.6V and
3.7V Li-Ion-cells, without changing resistor values. SEL3 can
also be used if there is problem in finding the right values in
the resistor network.
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