LT1117 Datasheet, PDF (6/12 Pages) Linear Technology – 800mA Low Dropout Positive Regulators Adjustable and Fixed 2.85V, 3.3V, 5V

LT1117/LT1117-2.85
LT1117-3.3/LT1117-5
APPLICATI HI TS
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
OUT
ADJ
+
R1
V_OUT
C_OUT
150µF
+ _CADJ
10µF
R2
LT1117 • TA02
Figure 1.
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
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 × ^R2R+_1R1,R_P = Parasitic Line Resistance
R_P
LT1117
PARASITIC
LINE RESISTANCE
V_IN
IN
OUT
ADJ
R1
R_L
CONNECT
R1 TO CASE
R2
LT1117
V_IN
IN
OUT
V_OUT
ADJ
V_REF
R1
I_ADJ
50µA
( ) ^{VOUT = VREF 1 + RR—21 + IADJ R2}
R2
LT1117 • TA03
Figure 2. Basic Adjustable Regulator
CONNECT
R2 TO LOAD
LT1117 • TA04
Figure 3. Connections for Best Load Regulation
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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