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LT1117CST Datasheet Download

Part No.:
LT1117CST
Download:
Download Datasheet
Description:
[800mA Low Dropout Positive Regulators Adjustable and Fixed 2.85V, 3.3V, 5V]
File Size:
283 K
Page:
12 Pages
Logo:
Manufacturer:
LINER [ LINEAR TECHNOLOGY ]
PCB Prototype
 LT1117CST Datasheet Page:2LT1117CST Datasheet Page:3LT1117CST Datasheet Page:4LT1117CST Datasheet Page:5LT1117CST Datasheet Page:7LT1117CST Datasheet Page:8LT1117CST Datasheet Page:9LT1117CST Datasheet Page:10 
LT1117/LT1117-2.85
LT1117-3.3/LT1117-5
APPLICATI
HI TS
to set the overall output voltage. Normally this current is
chosen to be the specified minimum load current of 10mA.
Because I
ADJ
is very small and constant when compared
to the current through R1, it represents a small error and
can usually be ignored. For fixed voltage devices R1 and
R2 are included in the device.
Load Regulation
Because the LT1117 is a three-terminal device, it is not
possible to provide true remote load sensing. Load regu-
lation will be limited by the resistance of the wire connect-
ing the regulator to the load. The data sheet specification
for load regulation is measured at the output pin of the
device. Negative side sensing is a true Kelvin connection,
with the bottom of the output divider returned to the
negative side of the load. Although it may not be immedi-
ately obvious, best load regulation is obtained when the
top of the resistor divider (R1) is returned directly to the
output pin of the device, not to the load. This is illustrated
in Figure 3. Connected as shown, R
P
is not multiplied by
the divider ratio. If R1 were connected to the load, the
effective resistance between the regulator and the load
would be:
R
P
×
Figure 1.
adjust pin can be driven, on a transient basis,
±25V
with
respect to the output without any device degradation.
Diodes between input and output are not usually needed.
The internal diode between the output and input pins of the
device can withstand microsecond surge currents of 10A
to 20A. Normal power supply cycling can not generate
currents of this magnitude. Only with extremely large
output capacitors, such as 1000µF and larger, and with the
input pin instantaneously shorted to ground can damage
occur. A crowbar circuit at the input of the LT1117 in
combination with a large output capacitor could generate
currents large enough to cause damage. In this case a
diode from output to input is recommended, as shown in
Figure 1.
D1
1N4002
(OPTIONAL)
LT1117
V
IN
IN
ADJ
OUT
R1
+
C
ADJ
10µF
Output Voltage
The LT1117 develops a 1.25V reference voltage between
the output and the adjust terminal (see Figure 2). By
placing a resistor between these two terminals, a constant
current is caused to flow through R1 and down through R2
LT1117
V
IN
IN
ADJ
I
ADJ
50µA
OUT
V
REF
R1
V
OUT
V
IN
IN
R2
V
OUT
= V
REF
1 + — + I
ADJ
R2
R1
( )
Figure 2. Basic Adjustable Regulator
6
U
UO
+
V
OUT
C
OUT
150µF
R2
LT1117 • TA02
R2
+
R1
,
R
P
=
Parasitic Line Resistance
R1
LT1117
OUT
ADJ
R1
CONNECT
R1 TO CASE
R2
R
L
R
P
PARASITIC
LINE RESISTANCE
CONNECT
R2 TO LOAD
LT1117 • TA04
R2
Figure 3. Connections for Best Load Regulation
LT1117 • TA03
For fixed voltage devices the top of R1 is internally Kelvin
connected, and the ground pin can be used for negative
side sensing.