MP1423 データシートの表示（PDF） - Monolithic Power Systems

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MP1423

3A, 23V, 385KHz Step-Down Converter

Monolithic Power Systems 

TM

MP1423 – 3A, 23V, 385KHz STEP-DOWN CONVERTER

Compensation Components

MP1423 employs current mode control for easy

compensation and fast transient response. The

system stability and transient response are

controlled through the COMP pin. COMP pin is

the output of the internal transconductance

error amplifier. A series capacitor-resistor

combination sets a pole-zero combination to

control the characteristics of the control system.

The DC gain of the voltage feedback loop is

given by:

A VDC

= RLOAD

× GCS

× A VEA

×

VFB

VOUT

Where AVEA is the error amplifier voltage gain,

400V/V; GCS is the current sense

transconductance, 3.8A/V; RLOAD is the load

resistor value.

The system has two poles of importance. One

is due to the compensation capacitor (C3) and

the output resistor of error amplifier, and the

other is due to the output capacitor and the load

resistor. These poles are located at:

fP1

=

GEA

2π × C3 × A VEA

fP2

=

1

2π × C2 × RLOAD

Where GEA is the error amplifier

transconductance, 800µA/V.

The system has one zero of importance, due to

the compensation capacitor (C3) and the

compensation resistor (R3). This zero is located

at:

fZ1

=

1

2π × C3 × R3

The system may have another zero of

importance, if the output capacitor has a large

capacitance and/or a high ESR value. The zero,

due to the ESR and capacitance of the output

capacitor, is located at:

fESR

=

1

2π × C2 × RESR

In this case, a third pole set by the

compensation capacitor (C6) and the

compensation resistor (R3) is used to

compensate the effect of the ESR zero on the

loop gain. This pole is located at:

fP3

=

1

2π × C6 × R3

The goal of compensation design is to shape

the converter transfer function to get a desired

loop gain. The system crossover frequency

where the feedback loop has the unity gain is

important.

Lower crossover frequencies result in slower

line and load transient responses, while higher

crossover frequencies could cause system

unstable. A good rule of thumb is to set the

crossover frequency to approximately one-tenth

of the switching frequency. Switching frequency

for the MP1423 is 385KHz, so the desired

crossover frequency is around 38KHz.

Table 3 lists the typical values of compensation

components for some standard output voltages

with various output capacitors and inductors.

The values of the compensation components

have been optimized for fast transient

responses and good stability at given

conditions.

Table 3—Compensation Values for Typical

Output Voltage/Capacitor Combinations

VOUT

2.5V

3.3V

5V

12V

2.5V

3.3V

5V

12V

C2

22µF

Ceramic

22µF

Ceramic

22µF

Ceramic

22µF

Ceramic

560µF Al.

30mΩ ESR

560µF Al

30mΩ ESR

470µF Al.

30mΩ ESR

220µF Al.

30mΩ ESR

R3

3.9kΩ

4.7kΩ

7.5kΩ

16.9kΩ

91kΩ

120kΩ

100kΩ

169kΩ

C3

5.6nF

4.7nF

4.7nF

1.5nF

1nF

1nF

1nF

1nF

C6

None

None

None

None

150pF

120pF

120pF

39pF

MP1423 Rev. 1.1

www.MonolithicPower.com

7

1/6/2006

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© 2006 MPS. All Rights Reserved.

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