LT1185 LINER [Linear Technology], LT1185 Datasheet - Page 9

• Manufacturer Part Number: LT1185
• Description: Low Dropout Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Shutdown Techniques
The LT1185 can be shut down by open-circuiting the REF
pin. The current flowing into this pin must be less than
0.4 A to guarantee shutdown. Figure 3 details several
ways to create the “open” condition, with various logic
levels. For variations on these schemes, simply remember
that the voltage on the REF pin is 2.4V negative with
respect to the ground pin.
Output Overshoot
Very high input voltage slew rate during start-up may
cause the LT1185 output to overshoot. Up to 20% over-
shoot could occur with input voltage ramp-up rate exceed-
ing 1V/ s. This condition cannot occur with normal 50Hz
to 400Hz rectified AC inputs because parasitic resistance
and inductance will limit rate of rise even if the power
switch is closed at the peak of the AC line voltage. This
assumes that the switch is in the AC portion of the circuit.
U U
V
+
–
*R1 SHOULD BE CONNECTED DIRECTLY TO GROUND LEAD, NOT TO THE LOAD,
IN
SO THAT r
ERRORS CREATED BY r
INCREASES WITH INCREASING GROUND PIN CURRENT. R2 SHOULD BE CONNECTED
DIRECTLY TO LOAD FOR REMOTE SENSING
W
Figure 2. Proper Connection of Positive Sense Lead
V
a
IN
0 . THIS LIMITS THE OUTPUT VOLTAGE ERROR TO (I
REF
R
LIM
LT1185
a
ARE MULTIPLIED BY (1 + R2/R1). NOTE THAT V
U
GND
V
OUT
FB
LEAD RESISTANCES
r
a
PARASITIC
If instead, a switch is placed directly in the regulator input
so that a large filter capacitor is precharged, fast input slew
rates will occur on switch closure. The output of the
regulator will slew at a rate set by current limit and output
capacitor size; dVdt = I
= 2.2 F, the output will slew at 1.6V/ s and overshoot can
occur. This overshoot can be reduced to a few hundred
millivolts or less by increasing the output capacitor to
10 F and/or reducing current limit so that output slew rate
is held below 0.5V/ s.
A second possibility for creating output overshoot is
recovery from an output short. Again, the output slews at
a rate set by current limit and output capacitance. To avoid
overshoot, the ratio I
0.5 10
to C
supply bypass capacitors that total more than C
OUT
R1*
2.37k
R2
– r
I
GND
for this calculation. Many loads will have multiple
6
b
. Remember that load capacitance can be added
+
LOAD
GND
)(r
OUT
b
).
LT1185 • F02
LIM
V
OUT
+
–
LIM
/C
OUT
/C
OUT
. With I
should be less than
LIM
= 3.6A and C
LT1185
OUT
.
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OUT