National Semiconductor LM3647, User Manual

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The actual timeouts (with RCIN @ 2.5 MHz) is:
Phase Timeout
Fast Charge
∼
330 Minutes
Topping Charge
∼
80 Minutes
EXAMPLE 2:
The actual timeouts (with RCIN @ 2.5 MHz) is:
Phase Timeout
Fast Charge Constant Current − 130 Minutes
Topping Charge Constant Voltage − 190 Minutes
1.2.2.4 Setting The 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 the voltage drop over it is not too small, since the
signal will be more susceptible to noise and offsets in the
amplification-stage. The resistance should not be too large
either (especially in high-current applications), because this
will only generate more heat from the component. A suitable
value is one where 50 mV dropped across the resistor when
maximum current flows through it. The differential signal is
then amplified, inverted and centered around the 2.5V
reference by the operational amplifier and fed to the CS pin
on the LM3647. The gain must be dimensioned by setting
the appropriate ratio between R1 (R2) and R3 (R4). The
figure below is dimensioned for a maximum current of about
1.1A. This was dimensioned using the following formula:
1.2.2.5 Setting Maximum Battery Voltage
The resistor network (see the figure below) scales the
battery voltage to a suitable level for the LM3647. For
Ni-Cd/Ni-MH batteries the network is less critical, but limits
the maximum battery voltage, it is only used as a backup
termination method. For Li-Ion batteries the network must be
more accurate, requiring precision resistors with low
tolerances.
•
For Ni-Cd/Ni-MH:
The dimensioning is accomplished in the following manner:
First calculate the maximum battery voltage for the specific
battery pack. See example below.
BatteryVoltage/Cell = 1.2V NumberOfCells = 5
→
Battery-
PackVoltage = 1.2x5 = 6V
MaximumBatteryVoltage/Cell = 1.85V
→
MaximumBattery-
Voltage = 1.85x5 = 9.25V
When the Maximum Battery Voltage has been determined,
the voltage divider network has to be dimensioned using the
following formula:
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