LT3580EMS8E-TRPBF(RevC) Ver la hoja de datos (PDF) - Linear Technology

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LT3580EMS8E-TRPBF
(Rev.:RevC)

Boost/Inverting DC/DC Converter with 2A Switch, Soft-Start, and Synchronization

Linear Technology 

LT3580

APPLICATIONS INFORMATION

Compensation—Theory

Like all other current mode switching regulators, the

LT3580 needs to be compensated for stable and efficient

operation. Two feedback loops are used in the LT3580—

a fast current loop which does not require compensation,

and a slower voltage loop which does. Standard bode plot

analysis can be used to understand and adjust the voltage

feedback loop.

As with any feedback loop, identifying the gain and phase

contribution of the various elements in the loop is critical.

Figure 4 shows the key equivalent elements of a boost

converter. Because of the fast current control loop, the

power stage of the IC, inductor and diode have been re-

placed by the equivalent transconductance amplifier gmp.

gmp acts as a current source where the output current is

proportional to the VC voltage.

–

gmp

+

VOUT

CPL

RESR RL

VC

RC

RO

1.215V

REFERENCE

gma

COUT

R1

CC

R2

3580 F04

CC: COMPENSATION CAPACITOR

COUT: OUTPUT CAPACITOR

CPL: PHASE LEAD CAPACITOR

gma: TRANSCONDUCTANCE AMPLIFIER INSIDE IC

gmp: POWER STAGE TRANSCONDUCTANCE AMPLIFIER

RC: COMPENSATION RESISTOR

RL: OUTPUT RESISTANCE DEFINED AS VOUT DIVIDED BY ILOAD(MAX)

RO: OUTPUT RESISTANCE OF gma

R1, R2: FEEDBACK RESISTOR DIVIDER NETWORK

RESR: OUTPUT CAPACITOR ESR

Figure 4. Boost Converter Equivalent Model

From Figure 4, the DC gain, poles and zeros can be cal-

culated as follows:

Output Pole: P1=

2

2 •
• RL • COUT

Error Amp Pole: P2 =

1

2 •
• RO • CC

Error Amp Zero: Z1=

1

2 •
• RC • CC

DC Gain:

A

=

VREF

VOUT2

•

VIN

• gma

• RO

•

gmp

• RL

•

1

4

ESR Zero: Z2 =

1

2 •
• RESR • COUT

RHP Zero:

Z3

=

2

•

VIN2 • RL

• VOUT2

•

L

High Frequency Pole: P3 > fS

3

Phase Lead Zero: Z4 =

1

2 •
• R1• CPL

Phase Lead Pole:

P4

=

2

•

•

1

CPL •

R1• R2

R1+ R2

The current mode zero (Z3) is a right-half plane zero which

can be an issue in feedback control design, but is manage-

able with proper external component selection.

Note that the maximum output currents of gmp and gma are

finite. The limits for gmp are in the Electrical Characteristics

section (switch current limit), and gma is nominally limited

to about ±12μA.

3580fc

12

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