Datasheet
17
LT1764A Series
1764afb
OUTPUT VOLTAGE (V)
012345678910
REVERSE OUTPUT CURRENT (mA)
1764 F12
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
LT1764A-2.5
LT1764A-3.3
LT1764A-1.8
LT1764A
T
J
= 25°C
V
IN
= OV
CURRENT FLOWS INTO
OUTPUT PIN
V
OUT
= V
ADJ
(LT1764A)
V
OUT
= V
FB
(LT1764A-1.5
LT1764A-1.8, LT1764A-2.5,
LT1764A-3.3)
LT1764A-1.5
In situations where the ADJ pin is connected to a resistor
divider that would pull the ADJ pin above its 7V clamp
voltage if the output is pulled high, the ADJ pin input
current must be limited to less than 5mA. For example, a
resistor divider is used to provide a regulated 1.5V output
from the 1.21V reference when the output is forced to 20V.
The top resistor of the resistor divider must be chosen to
limit the current into the ADJ pin to less than 5mA when the
ADJ pin is at 7V. The 13V difference between OUT and ADJ
pins divided by the 5mA maximum current into the ADJ pin
yields a minimum top resistor value of 2.6k.
In circuits where a backup battery is required, several
different input/output conditions can occur. The output
voltage may be held up while the input is either pulled to
ground, pulled to some intermediate voltage, or is left
open circuit. Current flow back into the output will follow
the curve shown in Figure 5.
When the IN pin of the LT1764A-X is forced below the OUT
pin or the OUT pin is pulled above the IN pin, input current
Figure 5. Reverse Output Current
will typically drop to less than 2µA. This can happen if the
input of the device is connected to a discharged (low
voltage) battery and the output is held up by either a
backup battery or a second regulator circuit. The state of
the SHDN pin will have no effect on the reverse output
current when the output is pulled above the input.
APPLICATIONS INFORMATION
WUU
U
TYPICAL APPLICATIO S
U
–
+
A1
LT1006
–
+
C1B
1/2 LT1018
–
+
C1A
1/2 LT1018
LT1004
1.2V
1764 TA03
1µF
1N4148
1N4148
10k
10k
750Ω
750Ω
2.4k
22µF
1N4002
TO
ALL “V
+
”
POINTS
200k
34k*
12.1k*
0.1µF
V
+
V
+
V
+
V
+
10k
V
+
0.033µF
+
10000µF
+
22µF
V
OUT
3.3V
3A
+
1N4002 1N4002
1k
10V AC
AT 115V
IN
1N4148
L1
500µH
90V AC
TO 140V AC
10V AC
AT 115V
IN
LT1764A-3.3
GND
IN
SHDN
OUT
FB
L1: COILTRONICS CTX500-2-52
L2: STANCOR P-8560
*1% FILM RESISTOR
NTE5437
L2
NTE5437
“SYNC”
SCR Preregulator Provides Efficiency Over Line Variations
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