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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:

Resistor network selection Quick Guide:

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:

(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.

MaximumBatteryVoltage

R7

R6

R7

+

()

-------------------------

×

CEL

=

3.017V

=

No. of Cells

Ni-Cd/Ni-MH

Normal

Max

R6

R7

2

2.4V

3.7V

3

3.6V

5.55V

4

4.8V

7.4V

16k

11k

5

6V

9.25V

62k

30k

6

7.2V

11.1V

15k

5.6k

7

8.4V

12.95V

8

9.6V

14.8V

39k

10k

9

10.8V

16.65V

10

12V

18.5V

22k

3.9k

MaximumBatteryVoltage

R7

R6

R7

+

()

-------------------------

×

CEL

=

2.675V

=