LT1117/LT1117-2.85
LT1117-3.3/LT1117-5
APPLICATIO HI TS
The 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)
V
IN
IN
ADJ
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
V
IN
IN
V
IN
+
IN
ADJ
I
ADJ
50µA
R2
V
OUT
= V
REF
1 + — + I
ADJ
R2
R1
Figure 2. Basic Adjustable Regulator
6
U U
LT1117
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 3-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
×
R2
+
R1
,R
P
=
Parasitic Line Resistance
R1
LT1117
OUT
ADJ
R1
CONNECT
R1 TO CASE
R2
R
L
R
P
PARASITIC
LINE RESISTANCE
OUT
R1
+
V
OUT
C
OUT
150µF
+
C
ADJ
10µF
R2
LT1117 • TA02
Figure 1
LT1117
LT1117 • TA04
OUT
V
REF
R1
V
OUT
CONNECT
R2 TO LOAD
( )
Figure 3. Connections for Best Load Regulation
R2
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.
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