National Instruments LM3647 - Manual (Page 2)

Charge Current
The charge settings for LM3647 current control are shown
below. If the external LM317 is used to control the charge
current then the jumpers J9, J10 and J13 have no relevance,
when using LM317 regulation mode, this jumper must be
placed in either position. If external (LM317) regulation is
used then set jumper J7 to position
″
slow
″
; for LM3647
regulation set J7 to
″
fast
″
.
LM3647 Current Regulation
The I jumper J10 is used to select between different current
sense resistors. The values available are 0.047
Ω
and
0.100
Ω
.
The charge current is set with jumpers J9 and J13. The
figure shows two possible currents that depend on how
jumper J10 is set. The higher current is selected when J10 is
set to 0.047
Ω and the lower current is selected when J10 is
set to 0.100
Ω
.
1.0 FUNCTIONAL DESCRIPTION
1.1 General
The LM3647 has voltage and current sensing inputs that are
used to control a PWM-output. The voltage input is
connected to the battery via a resistor divider network, and
the current input is connected to an operational amplifier that
amplifies the voltage across a current sense resistor located
at the positive battery terminal. The PWM-output can be
configured as a high speed PWM, or as a low speed
(ON/OFF) output for an external current regulator. The latter
is for low cost Ni-Cd/Ni-MH charger applications, eliminating
the need for any operational amplifiers or current feedback
circuitry.
The high speed PWM is filtered to a DC-level and fed into an
operational amplifier that controls a power-NPN transistor.
The LM3467 requires charge current feedback to control the
charge current.
1.2 Modes of Operation
1.2.1 Charging Ni-Cd/Ni-MH Batteries
The charger detects that a battery is connected when the
CEL-pin
> 1.0V. The charger can also detect a battery that
has been deeply discharged and does not have any voltage
across the battery terminals. This is accomplished by
applying a small ’pre-charge’ current once every minute for
up to 15 seconds. The deeply discharged battery will accept
this charge and the battery potential will eventually rise
above the 1.0V limit to initiate normal charging. When the
charger has detected a battery (CEL-pin
> 1.0V), it checks
to see if the temperature is within range to start charging. If
it is, then it applies a small current of 0.2C for approximately
5 minutes. If the battery voltage exceeds the maximum
battery voltage (CEL-pin
> 3.017V), the LM3647 stops
charging and stays in error mode until the battery is
removed. If the battery voltage has not risen above the bad
battery threshold (CEL-pin
< 1.2V), then the battery is
considered to be defective and the charger goes into error
mode. If the battery passes all tests, then after the five
minutes have passed, the charger starts the next phase:
Fast Charge.
During Fast Charge, the charger applies a constant current
to the battery and monitors both battery voltage and
temperature. The charger is looking for a drop in the battery
voltage that normally occurs at the end of the Fast Charge
cycle. The size of the voltage drop differs depending on
battery type (Ni-Cd/Ni-MH). For Ni-Cd it’s approximately 50
mV/cell and for Ni-MH it’s approximately 17 mV/cell. If the
temperature rise is larger than 50 mV/minute (
∼
1˚C/minute)
when charging Ni-MH batteries, the battery has reached the
end of the Fast Charge cycle.
During charging the temperature-input is constantly
measured to ensure that the battery’s temperature is within
proper range. If the temperature is out of range the charger
aborts the charge and goes into error mode.
During the next charge phase (Topping Charge) the LM3647
applies a small current of 0.2C for a time set by the
time-selection RC-network (see section below). This phase
may be followed by a Maintenance Charge phase,
depending on selection-pins.
1.2.2 Charging Li-Ion Batteries
The charger detects that a battery is connected when the
CEL-pin
> 1.0V. The charger can also detect a battery that
has been deeply discharged and does not have any voltage
across the battery terminals. This is accomplished by
applying a small ’pre-charge’ current once every minute for
up to 15 seconds. The deeply discharged battery will accept
this charge and the battery potential will eventually rise
above the 1.0V limit to initiate normal charging. When the
charger has detected a battery (CEL-pin
> 1.0V), it checks
to see if the temperature is within range to start charging. If
it is, then it applies a small current of 0.2C for approximately
1 minute. If the battery voltage is close to fully charged, the
charger will not reach the charging voltage within 1 minute,
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