Datasheet

14
LT1505
1505fc
Nickel-Cadmium and Nickel-Metal-Hydride Charging
The circuit in the 4A Lithium Battery Charger (Figure 1) can
be modified to charge NiCd or NiMH batteries. For
example, 2-level charging is needed; 2A when Q1 is on,
and 200mA when Q1 is off (Figure 8).
APPLICATIONS INFORMATION
WUU
U
Figure 6. High Input Voltage Shudown
Figure 7. Synchronizing with External Clock
R2
5.49k
R1
49.3k
PROG
C
PROG
1µF
Q1
LT1505
1505 F07
5V TO 20V
VN2222
PULSE WIDTH > 200ns
5k
SYNC
1505 F06
LT1505
Figure 8. 2-Level Charging
SHDN
1505 F05
3M
V
IN
24V ≤ V
CC
< 27V
OPEN DRAIN
3.3µF
LT1505
amplifier CA1. They are not needed if V
CC
is at least 2.5V
higher than V
BAT
. The PFET M3 is optional and can be
replaced with a diode if V
IN
is at least 3V higher than V
BAT
.
The gate control pin INFET turns on M3 when V
IN
gets up
above the undervoltage lockout level set by R5 and R6 and
is clamped internally to 8V below V
CC
. In sleep mode when
V
IN
is removed, INFET will clamp M3 V
SG
to 0.2V.
Shutdown
When adapter power is removed, V
CC
will drift down and
be held by the body diode of the topside NFET switch. As
soon as V
CC
goes down to 0.2V above V
BAT
, the LT1505 will
go into sleep mode drawing only 10µA from the battery.
There are two ways to stop switching: pulling the SHDN
pin low or pulling the V
C
pin low. Pulling the SHDN pin low
will also turn off V
GBIAS
and CA1 input currents. Pulling the
V
C
pin low will only stop switching and V
GBIAS
stays high.
Make sure there is a pull-up resistor on the SHDN pin even
if the SHDN pin is not used, otherwise internal pull-down
current will keep the SHDN pin low and switching off when
power turns on.
Each TGATE and BGATE pin has a 50k pull-down resistor
to keep the external power FETs off when shut down or
power is off.
Note that maximum operating V
CC
is 24V. For short
transients the LT1505 can be operated as high as 27V. For
V
CC
higher than 24V it is preferred to use the V
C
pin to shut
down. If the SHDN pin has to be used to shut down at V
CC
higher than 24V, the Figure 6 pull-up circuit must be used
to slow down the V
GBIAS
ramp-up rate when the SHDN pin
is released. Otherwise, high surge current charging the
bypass capacitor might damage the LT1505. For V
CC
less
than 24V, only a 100k resistor and no capacitor is needed
at SHDN pin to V
IN
for pull-up.
Synchronization
The LT1505 can be synchronized to a frequency range
from 240kHz to 280kHz. With a 200ns one-shot timer on
chip, the LT1505 provides flexibility on the synchronizing
pulse width. Sync pulse threshold is about 1.2V (Figure 7).
For 2A full current, the current sense resistor (R
S1
) should
be increased to 0.05Ω so that enough signal (10mV) will
be across R
S1
at 0.2A trickle charge to keep charging
current accurate.
For a 2-level charger, R1 and R2 are found from:
R1
2.465 4000
I
R2
2.465 4000
II
LOW HI LOW
=
()()
=
()()
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