EV2354DS-00A Ver la hoja de datos (PDF) - Monolithic Power Systems

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EV2354DS-00A

2A, 23V, 380KHz Step-Down Converter

Monolithic Power Systems 

TM

MP2354 – 2A, 23V, 380KHz STEP-DOWN CONVERTER

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 MP2354 is 380KHz, so the

desired crossover frequency is around 38KHz.

Table 4 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 4—Compensation Values for Typical

Output Voltage/Capacitor Combinations

VOUT

2.5V

3.3V

5V

12V

2.5V

3.3V

5V

12V

L1

10µH

min.

15µH

min.

15µH

min.

22µH

min.

10µH

min.

15µH

min.

15µH

min.

22µH

min.

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 C3

5.6kΩ 4.7nF

7.5kΩ 3.3nF

11kΩ 2.2nF

27kΩ 1nF

140kΩ 1nF

187kΩ 1nF

237kΩ 1nF

267kΩ 1nF

C6

None

None

None

None

120pF

82pF

56pF

22pF

To optimize the compensation components for

conditions not listed in Table 4, the following

procedure can be used.

1) Choose the compensation resistor (R3) to

set the desired crossover frequency. Determine

the R3 value by the following equation:

R3 = 2π × C2 × fC × VOUT

GEA × GCS VFB

2) Choose the compensation capacitor (C3) to

achieve the desired phase margin. For

applications with typical inductor values, setting

the compensation zero, fZ1, to less than one

forth of the crossover frequency provides

sufficient phase margin. Determine the C3

value by the following equation:

C3 >

4

2π × R3 × fC

Where R3 is the compensation resistor value.

3) Determine if the second compensation

capacitor (C6) is required. It is required if the

ESR zero of the output capacitor is located at

less than half of the 380KHz switching

frequency, or the following relationship is valid:

1

< fS

2π × C2 × RESR 2

If this is the case, then add the second

compensation capacitor (C6) to set the pole fP3

at the location of the ESR zero.

Determine the C6 value by the equation:

C6 = C2 × RESR

R3

MP2354 Rev. 1.4

www.MonolithicPower.com

9

1/6/2006

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