National Semiconductor LM3647, Configuration Manual

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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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