MP1567 データシートの表示（PDF） - Monolithic Power Systems

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MP1567

1.2A Synchronous Rectified Step-Down Converter

Monolithic Power Systems 

TM

MP1567 – 1.2A SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER

75KHz or lower to insure stable operation.

Lower crossover frequencies result in slower

response and worse transient load recovery.

Higher crossover frequencies degrade the

phase and/or gain margins and can result in

instability.

Choosing the Compensation Components

The values of the compensation components

given in Table 1 yield a stable control loop for

the output voltage and capacitor given.

Table 1—Compensation Values for Typical

Output Voltage/Capacitor Combinations

VOUT

1.8V

2.5V

3.3V

1.8V

2.5V

3.3V

1.8V

2.5V

3.3V

C2

4.7µF Ceramic

4.7µF Ceramic

4.7µF Ceramic

10µF Ceramic

10µF Ceramic

10µF Ceramic

47µF Tantalum

(300mΩ)

47µF Tantalum

(300mΩ)

47µF Tantalum

(300mΩ)

R3

3.3kΩ

5.1kΩ

6.8kΩ

7.5kΩ

10kΩ

10kΩ

10kΩ

10kΩ

10kΩ

C3 C4

2.2nF

1.5nF

1.2nF

1nF

820pF

820pF

None

None

None

None

None

None

2.2nF 1.5nF

3.3nF 1.5nF

4.7nF 1.5nF

To optimize the compensation components for

conditions not listed in Table 1, use the

following procedure.

Choose the compensation resistor to set the

desired crossover frequency. Determine the

value by the following equation:

R3 = 2π × C2 × VOUT × fC

GEA × GCS × VFB

Putting in the known constants and setting the

crossover frequency to the desired 75KHz:

R3 ≈ 4.36 × 108 × C2 × VOUT

In this case, the actual crossover frequency is

less than the desired 75KHz, and it is

calculated by:

fC

=

R3 × GEA × GCS × VFB

2π × C2 × VOUT

Choose the compensation capacitor to set the

zero to one fourth of the crossover frequency.

Determine the value by the following equation:

C3 =

4 × C2 × VOUT

R32 × GEA × GCS × VFB

Determine if the second compensation

capacitor, C4, is required. It is required if the

ESR zero of the output capacitor occurs at less

than four times the crossover frequency, or:

8π × C2 × RESR × fC ≥ 1

Where RESR is the equivalent series resistance

of the output capacitor.

If this is the case, then add the second

compensation capacitor. Determine the value

by the equation:

C4 = C2 × RESR(MAX)

R3

Where RESR(MAX) is the maximum ESR of the

output capacitor.

For Example:

Given:

VOUT = 1.8V

C2 = 10µF Ceramic (ESR = 10mΩ max.)

Calculate:

R3 ≈ 4.36 × 10 8 (10µF) × (1.8V) = 7.85kΩ

(Use the nearest standard value of 7.5kΩ.)

C3 = 1.9 × 10 −14 = 1.05nF

10µF × 1.8V

(Use 1nF since it is a standard value.)

8π × C2 × RESR × fC = 0.19

which is less than 1, therefore the second

compensation capacitor (C4) is not required.

MP1567 Rev. 2.3

www.MonolithicPower.com

8

1/3/2006

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