LTC3775IMSE-TRPBF 查看數據表（PDF） - Linear Technology

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LTC3775IMSE-TRPBF

High Frequency Synchronous Step-Down Voltage Mode DC/DC Controller

Linear Technology 

LTC3775

APPLICATIONS INFORMATION

Figure 1 shows a Type 3 amplifier. The transfer function of

this amplifier is given by the following equation:

( ) VCOMP =

– 1+ sR2C1
1+ s(RA + R3)C3

( )( )( ) VOUT sRA C1+ C2 1+ s(C1||C2)R2 1+ sC3R3

The RC network across the error amplifier and the feed-

forward components R3 and C3 introduce two pole-zero

pairs to obtain a phase boost at the system unity-gain

frequency, fC. In theory, the zeros and poles are placed

symmetrically around fC, and the spread between the zeros

and the poles is adjusted to give the desired phase boost

at fC. However, in practice, if the crossover frequency

is much higher than the LC double-pole frequency, this

method of frequency compensation normally generates

a phase dip within the unity bandwidth and creates some

concern regarding conditional stability.

If conditional stability is a concern, move the error ampli-

fier’s zero to a lower frequency to avoid excessive phase

dip. The following equations can be used to compute the

feedback compensation component values:

fSW = switching frequency

fLC = 2π

1

LCOUT

fESR

=

1

2π RESR

COUT

choose:

fC =

crossover frequency

=

fSW

10

fZ1(ERR)

=

fLC

=

1

2πR2C1

( ) fZ2(RES) =

fC

5

=

2π

RA

1

+ R3

C3

fP1(ERR)

=

fESR

=

1

2πR2(C1|| C2)

fP2(RES)

=

5fC

=

1

2πR3C3

Required error amplifier gain at frequency fC:

A V(CROSSOVER)

( )
40log



1+ 

fC

2



–

20log

 fLC 



1+ 

fC

2



 fESR 

– 20log

AMOD

20log R2

RA

•



1+



fLC

fC



1+

fP2(RES)

fC

+

fP2(RES) – fZ2(RES)

fZ2(RES)







1+



fC

fESR

+

fLC

fESR –



fLC 1+

fP2(RES)

fC





where AMOD is the modulator and line feedforward gain

and is equal to:

AMOD ≈

VIN(MAX) • DCMAX

VSAW

=

40V • 0.95

1.25V

≈ 30V/V

Once the value of resistor RA and the pole and zero loca-

tions have been decided, the values of C1, R2, C2, R3 and

C3 can be obtained from the above equations.

Compensating a switching power supply feedback loop is

a complex task. The applications shown in this data sheet

show typical values, optimized for the power components

shown. Though similar power components should suffice,

substantially changing even one major power component

may degrade performance significantly. Stability also may

depend on circuit board layout. To verify the calculated

component values, all new circuit designs should be

prototyped and tested for stability.

VOUT

C3

C2

C1

R2

RA R3 –

FB

RB

+

VREF

–1

GAIN

0

COMP

PHASE

+1

–1

BOOST

FREQ

–90

–180

–270

–380

3775 F01

Figure 1. Type 3 Amplifier Compensation

3775fa

11

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