LM3647 Datasheet, PDF (7/17 Pages) National Semiconductor (TI) – Universal Battery Charger for Li-Ion, Ni-MH and Ni-Cd Batteries

4.3.3 Charge Current
The charge-current is selected by setting the current sensing
resistor and the gain of the differential amplification stage.
The current sensing resistor (R5) should be dimensioned
such that a voltage drop over it is not too small, because the
signal will be more susceptible to noise and offsets in the am-
plification-stage. The resistance should not be too large ei-
ther (especially in high-current applications), because this
will only generate more heat from the component. A suitable
value is one that develops about 50mV across the resistor
when maximum current flows through it.
R1
Resistor network selection Quick Guide:
No. of Cells
1
2
3
4
Normal
3.6V
7.2V
10.8V
14.4V
Li-Ion (3.6V cell)
Max
R6
3.675V
16k
7.35V
62k
11.025V
27k
14.7V
22k
R7
30k
30k
7.5k
3.9k
No. of Cells
1
2
3
4
Normal
3.7V
7.4V
11.1V
14.8V
Li-Ion (3.7V cell)
Max
R6
3.74V
16k
7.48V
62k
11.22V
27k
14.96V
22k
R7
30k
30k
7.5k
3.9k
4.3.2 Charge Timeout
The LM3647 uses the charge timeout value as a backup ter-
mination method if the normal termination methods fail. The
charge timeout also controls the length of some of the phas-
es, (e.g., the Topping Charge phase). The timeout is select-
able between a charge rate of 3.2C down to 0.4C. The table
below shows R-C values and their resulting timeout.
R Value
100 kΩ
100 kΩ
100 kΩ
100 kΩ
100 kΩ
100 kΩ
100 kΩ
100 kΩ
C Value
internal
10 nF
15 nF
22 nF
33 nF
47 nF
68 nF
100 nF
Appropriate Charge Rates
3.2C
2.4C
1.4C
1.2C
0.9C
0.7C
0.5C
0.4C
R2
The current-sensing signal is amplified, inverted and cen-
tered on the 2.5V reference by the operational amplifier and
fed into the CS pin on the LM3647. The amplification stage
must be dimensioned by setting the appropriate ratio be-
tween R1 (R3) and R2 (R4). The figure below is dimensioned
for a maximum current of about 1.1A. This was dimensioned
using the following formula:
MaxCurrent = -⁽--R----²---⁾----^⁄--⁽---R----¹---⁾-
R5
R1 = R3 R2 = R4
R2 = 5.1kΩ R1 = 100kΩ R5 = 0.047Ω
MaxCurrent ≈ 1.09 Ampere
4.3.4 Dimensioning the reset circuitry
The reset-circuitry is designed to hold the RESET-pin until
the power supply to the LM3647 has stabilized. The RC-net-
work (R21 and C4) should be dimensioned in the following
way:
(R21 × C4) > 5xPowerSupplyRiseTime
The diode D2 discharges the capacitance C4 when power
loss occurs. The resistor R20 is used to protect the SYSOK-
pin, and its value is not critical (typical value is 2kΩ). The con-
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