LT1431
7
1431fe
applicaTions inForMaTion
Typical applicaTions
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 necessar-
ily 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 capacitance
between COLL and REF (phase lead). More complicated
feedback loops may require shaping of the frequency
response of the LT1431 with dominant pole or 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 effective-
ness of the compensation schemes, but also decreasing
their necessity.
2.5V Reference
3-Pin Package
2.5V Reference
8-Pin Package 5V Reference
Increasing 5V Reference
Programmable Reference with Adjustable
Current Limit
V
IN
L
2.5V
LT1431 TA02
CATHODE
REF
LT1431Z
ANODE
V
IN
L
2.5V
LT1431 TA03
COLL
REF
LT1431
GND-F
V
+
GND-S
V
IN
L
5V
LT1431 TA04
COLL
REF
LT1431
GND-F
V
+
GND-S
R
TOP
R
MID
V
IN
L
5V + ∆
LT1431 TA05
R
∆ = R • (0.5mA) ±25% PROCESS TOLERANCE
∆ ≤ 500mV
COLL
REF
LT1431
GND-F
V
+
GND-S
R
TOP
R
MID
V
IN
L
LT1431 TA06
R
LIM
V
OUT
=
( )
1 +
R1
R2
V
REF
R1
R2
I
LIMIT
=
0.7
R
LIM
+ 3.6
AT 25°C
COLL
REF
LT1431
GND-F
V
+
GND-S
