LT1764AMPQ Linear Technology, LT1764AMPQ Datasheet - Page 17

• Manufacturer Part Number: LT1764AMPQ
• Description: IC,VOLT REGULATOR,ADJUSTABLE,+1.21 TO +20V,BIPOLAR,SIP,5PIN,PLASTIC
• Manufacturer: Linear Technology

APPLICATIONS
TYPICAL APPLICATIO S
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
TO 140V AC
U
90V AC
ALL “V
POINTS
INFORMATION
TO
+
U
”
L1: COILTRONICS CTX500-2-52
L2: STANCOR P-8560
*1% FILM RESISTOR
“SYNC”
L2
+
1N4002
1N4002
U
10V AC
AT 115V
10V AC
AT 115V
SCR Preregulator Provides Efficiency Over Line Variations
22µF
W
IN
IN
NTE5437
NTE5437
2.4k
750Ω
1N4002
1N4148
U
1k
0.033µF
+
1/2 LT1018
–
C1A
500µH
V
L1
+
750Ω
+
1/2 LT1018
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.
10000µF
C1B
1N4148
+
–
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
V
V
0
+
+
34k*
12.1k*
200k
0
10k
1N4148
LT1764A-1.5
LT1764A-1.8
LT1764A
1
IN
SHDN
LT1764A-3.3
0.1µF
2
GND
Figure 5. Reverse Output Current
1µF
OUTPUT VOLTAGE (V)
OUT
3
FB
LT1006
V
+
A1
4
+
–
+
5
6
22µF
10k
7
LT1764A Series
LT1764A-3.3
LT1764A-2.5
V
3.3V
3A
OUT
1764 TA03
8
10k
LT1004
1.2V
9
1764 F12
V
+
10
T
V
CURRENT FLOWS INTO
OUTPUT PIN
V
V
LT1764A-1.8, LT1764A-2.5,
LT1764A-3.3)
J
IN
OUT
OUT
= 25°C
= OV
= V
= V
ADJ
FB
(LT1764A-1.5
(LT1764A)
17
1764afb