LTC3785EUF-1(RevA) View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC3785EUF-1
(Rev.:RevA)

10V, High Efficiency, Buck-Boost Controller with Power Good

Linear Technology 

LTC3785-1

APPLICATIONS INFORMATION

CLOSING THE FEEDBACK LOOP

The LTC3785-1 incorporates voltage mode control. The

control to output gain is given by:

GBuck = 1.6 • VIN, Buck Mode

GBOOST

=

1.6

• VOUT2

VIN

,

Boost

Mode

The output filter exhibits a double-pole response and is

given by:

fFILTER _ POLE = 2 • π •

1

L • COUT

where COUT is the output filter capacitor.

The output filter zero is given by:

fFILTER

_

ZERO =

2

•

π

•

1

RESR

•

COUT

where RESR is the capacitor equivalent series resistance.

A troublesome feature in boost mode is the right half plane

zero (RHP), and is given by:

fRHPZ

=

2

•

π

VIN2

•IOUT • L

•

VOUT

The loop gain is typically rolled off before the RHP zero

frequency.

A simple type I compensation network (Figure 5) can be

incorporated to stabilize the loop but at a cost of reduced

bandwidth and slower transient response. To ensure proper

phase margin, the loop must cross over almost a decade

before the L-C double pole.

The unity gain frequency of the error amplifier with the

type 1 compensation is given by:

fUG

=

2

•

π

•

1

R1•

CP1

Most applications demand an improved transient response

to allow a smaller output filter capacitor. To achieve a higher

bandwidth, type III compensation is required as shown

in Figure 6. Two zeros are required to compensate for the

double pole response.

fPOLE1≈

1

2 • π • 32e3 • CP1 • R1

(a

very

low

frequency)

fZERO1=

2

•

π

1

• RZ

• CP1

fZERO2

=

2

•

π

•

1

R1•

C

Z1

fPOLE2 ≈

2

•

π

1

• RZ

• CP2

ERROR

AMP

1.225V

FB

VOUT

R1

VC CP1

R2

37851 F05

Figure 5. Error Amplifier with Type I Compensation

ERROR

AMP

1.225V

FB

VOUT

R1

CZ1

VC

RZ

CP1

R2

CP2

37851 F06

Figure 6. Error Amplifier with Type III Compensation

37851fa

14

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