鈥?/div>
4.2V
4.2V
Figure 5. Disconnecting Voltage Divider
Some battery manufacturers recommend termination of
constant-voltage float mode after charging current has
dropped below a specified level (typically 50mA to 100mA)
and
a further time-out period of 30 minutes to 90 minutes
has elapsed. This may extend the life of the battery, so
check with manufacturers for details. The circuit in Figure
7 will detect when charging current has dropped below
75mA. This logic signal is used to initiate a time-out
U
W
U
U
period, after which the LT1510 can be shut down by
pulling the V
C
pin low with an open collector or drain.
Some external means must be used to detect the need for
additional charging if needed, or the charger may be
turned on periodically to complete a short float-voltage
cycle.
Current trip level is determined by the battery voltage, R1
through R3, and the internal LT1510 sense resistor
(鈮?0.18鈩?pin-to-pin). D2 generates hysteresis in the trip
level to avoid multiple comparator transitions.
Nickel-Cadmium and Nickel-Metal-Hydride Charging
The circuit in Figure 6 uses the 8-pin LT1510 to charge
NiCd or NiMH batteries up to 12V with charging currents
of 0.5A when Q1 is on and 50mA when Q1 is off.
C1
D1
0.22碌F 1N5819
SW
V
CC
D3
1N5819
+
C
IN
*
10碌F
1碌F
300鈩?/div>
WALL
ADAPTER
R1
100k
R2
11k
Q1
VN2222
BOOST PROG
L1**
33碌H
D2
1N914
LT1510
GND
V
C
I
BAT
SENSE
* TOKIN OR MARCON CERAMIC
SURFACE MOUNT
** COILTRONICS CTX33-2
BAT
0.1碌F
1k
+
V
BAT
C
OUT
22碌F
TANT
+
2V TO
20V
ON: I
BAT
= 0.5A
OFF: I
BAT
= 0.05A
1510 F05.5
Figure 6. Charging NiMH or NiCd Batteries
(Efficiency at 0.5A
鈮?/div>
90%)
For a 2-level charger, R1 and R2 are found from:
I
BAT
(
2000
)(
2
.
465
)
=
R
PROG
I
LOW
(
2
.
465
)(
2000
)
R1
=
(
2
.
465
)(
2000
)
R2
=
I
HI
鈭?/div>
I
LOW
All battery chargers with fast-charge rates require some
means to detect full charge state in the battery to terminate
the high charging current. NiCd batteries are typically
charged at high current until temperature rise or battery
11
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