Datasheet

6
LT1431
1413fc
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 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 effectiveness of the compensation schemes, but also
decreasing their necessity.
V
IN
R
L
2.5V
LT1431 • TA02
CATHODE
REF
LT1431Z
ANODE
V
IN
R
L
2.5V
LT1431 • TA03
COLL
REF
LT1431
GND-F
V
+
GND-S
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
R
L
5V
LT1431 • TA04
COLL
REF
LT1431
GND-F
V
+
GND-S
R
TOP
R
MID
V
IN
R
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
R
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
APPLICATIO S I FOR ATIO
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TYPICAL APPLICATIO S
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