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A Topping charge, or Top-Up Charge, is performed on
Nickel-based cells to ensure a full charge.
TABLE 9. Li-Ion/Ni-MH/Ni-Cd Current Range When
Feedback is Enabled
Range
J10
J9
J13
390 mA
1–2
3–4
3–4
470 mA
1–2
5–6
5–6
560 mA
1–2
7–8
7–8
750 mA
1–2
9–10
9–10
830 mA
2–3
3–4
3–4
1000 mA
2–3
5–6
5–6
1200 mA
2–3
7–8
7–8
1600 mA
2–3
9–10
9–10
A Topping charge, or Top-Up Charge, is performed on
Nickel-based cells to ensure a full charge. See the following
table:
TABLE 10. Topping Charge and Maximum Charge
Scaled Topping Charge
J18
2.4C
3–4
1.2C
5–6
0.7C
7–8
0.5C
9–10
A thermistor at the charging cell can be used with the
LM3647 to provide an enhanced charging solution. A ther-
mistor to measure the temperature of the battery pack is
optional. See the following table:
TABLE 11. Optional Temperature Sensor
Temperature
Sense
J8
J3
Not Used
1–2
x
Used
N.C.
Note: Connect NTC thermistor
Example 1: Charging a Nickel-Cadmium Battery Pack
Battery Pack: 4.8V pack: 4 cells
@
1.2V, 650 mAh
Charger Requirement: Low-cost, Discharge Before Charge
The minimum input voltage required at full load is:
minimum V
supply
= N ( V
pk
) + V
reg
→
minimum V
supply
=
4(1.85V) + 1.25V = 8.65V
Charging at a rate faster than 1.0C is not normally recom-
mended. Thus, a charge current of 650 mA will charge the
battery in approximately one hour. The value of the regulator
resistor is obtained via:
R
reg
= (1.25V - 0.7V) / 650 mA = 0.85
Ω ≈
1.0
Ω
The power dissipated by the regulator resistor is approxi-
mately 0.42 watt. Thus, a 1.0
Ω
resistor with a power rating
greater than 0.5 watt (i.e., 2 watts - 5 watts) should be
connected to J17.
A discharge rate of 0.2C is typical for most discharge before
charge applications. The value of the discharge resistor is
obtained via:
C = 650 mAh
→
0.2C = 130 mAh
In order to discharge within 30 minutes, a current of 260 mA
will be required — resulting in a discharge resistor value:
R
dischg
= 4(0.9v) / 200 mA = 13.8
Ω ≈
14
Ω
The power dissipated by the discharge resistor is approxi-
mately 0.9 watts.
Note: V
reg
is the minimum input to output voltage drop across LM317
Charger Settings for 4.8V Nickel-Cadmium Pack
Jumper
Connection
Comment
Jumper
Connection
Comment
J1
Battery Pack
4 Ni-Cd cells
J10
X
Only for Fast PWM
J2
N.C.
Disch. before charge
J11
13-14
Ni 4-cells Range
J3
N.C.
No NTC used
J12
13-14
Ni 4-cells Range
J4
Power Supply
minimum V
supply
= 8.65v
J13
X
Only for Fast PWM
J5
GND
Ni-Cd Battery
J14
X
Only for Fast PWM
J6
GND
Ext. Current Control
J15
R
dischg
14
Ω
/ 5W Resistor
J7
Slow
Slow for Ext. Control
J16
N.C.
No J16 on Ref. Board
J8
1-2
Shorted
J17
R
reg
1.0
Ω
/ 5W Resistor
J9
X
Only for Fast PWM
J18
9-10
0.5C Topping
AN-1
165
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