6

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 0.01

µF

and 18

µFcausesreducedphasemarginwithsomeringing

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

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

LT1431 • TA02

CATHODE

REF

LT1431Z

ANODE

2.5V

LT1431 • TA03

COLL

REF

LT1431

GND-F

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

TYPICAL APPLICATIO S

5V

LT1431 • TA04

COLL

REF

LT1431

GND-F

GND-S

5V +

∆

LT1431 • TA05

R

∆ = R × (0.5mA) ±25% PROCESS TOLERANCE

∆ ≤ 500mV

COLL

REF

LT1431

GND-F

GND-S

LT1431 • TA06

()

1 + R1

R2

R1

R2

0.7

AT 25°C

COLL

REF

LT1431

GND-F

GND-S