LT1431
APPLICATIO S I FOR ATIO
Frequency Compensation
As a shunt regulator, the LT1431 is stable for all capacitive
loads on the COLL pin. Capacitive loading between 0.01µF
and 18µF causes reduced phase margin with some ringing
under transient conditions. Output capacitors should not
be used arbitrarily because output noise is not necessarily
reduced.
Excess capacitance on the REF pin can introduce enough
phase shift to induce oscillation when configured as a
reference >2.5V. This can be compensated with capaci-
tance between COLL and REF (phase lead). More compli-
cated feedback loops may require shaping of the fre-
quency response of the LT1431 with dominant pole or
TYPICAL APPLICATIO S
2.5V Reference
3-Pin Package
R
L
V
IN
CATHODE
REF
LT1431Z
ANODE
LT1431 • TA02
2.5V
V
IN
V
+
GND-S
Increasing 5V Reference
R
L
V
IN
R
V
+
R
TOP
LT1431
R
MID
GND-S
GND-F
LT1431 • TA05
5V +
∆
COLL
REF
∆
= R
×
(0.5mA) ±25% PROCESS TOLERANCE
∆ ≤
500mV
6
U
pole-zero compensation. This can be accomplished with a
capacitor or series resistor and capacitor between COLL
and COMP.
The compensation schemes mentioned above use voltage
feedback to stabilize the circuits. There must be voltage
gain at the COLL pin for them to be effective, so the COLL
pin must see a reasonable AC impedance. Capacitive
loading of the COLL pin reduces the AC impedance,
voltage gain, and frequency response, thereby decreasing
the effectiveness of the compensation schemes, but also
decreasing their necessity.
2.5V Reference
8-Pin Package
R
L
2.5V
COLL
REF
LT1431
GND-F
LT1431 • TA03
W
U
U U
5V Reference
R
L
V
IN
V
+
5V
COLL
REF
LT1431
R
MID
GND-F
LT1431 • TA04
R
TOP
GND-S
Programmable Reference with Adjustable
Current Limit
R
L
V
IN
V
+
LT1431
GND-S
R
LIM
GND-F
I
LIMIT
=
0.7
AT 25°C
R
LIM
+ 3.6
LT1431 • TA06
V
OUT
= 1 + R1 V
REF
R2
COLL
REF
R2
R1
( )
LINER [ LINEAR TECHNOLOGY ]
