EV2108DQ/DK-00A Ver la hoja de datos (PDF) - Monolithic Power Systems

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EV2108DQ/DK-00A

2A, 6V, 740KHz Synchronous Buck Converter

Monolithic Power Systems 

MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER

Calculate the required inductance value by the

equation:

( ) L = VOUT × VIN − VOUT

VIN × fSW × ∆I

Where ∆I is the peak-to-peak inductor ripple

current. It is recommended to choose ∆I to be

30%~40% of the maximum load current.

Compensation

The system stability is controlled through the

COMP pin. COMP 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 loop gain is:

A VDC

=

⎜⎜⎝⎛

VFB

VOUT

⎟⎟⎠⎞ × A VEA

× GCS

× RLOAD

Where VFB is the feedback voltage, 0.9V, AVEA

is the transconductance error amplifier voltage

gain, 400 V/V and GCS is the current sense

transconductance, (roughly the output current

divided by the voltage at COMP), 4.5A/V.

RLOAD is the load resistance:

R LOAD

=

VOUT

IOUT

Where IOUT is the output load current.

The system has 2 poles of importance, one is

due to the compensation capacitor (C3), and

the other is due to the load resistance and the

output capacitor (C2), where:

fP1

=

2π ×

GEA

A VEA

× C3

P1 is the first pole, and GEA is the error amplifier

transconductance (450µA/V) and

fP2

=

1

2π × RLOAD

× C2

The system has one zero of importance, due to

the compensation capacitor (C3) and the

compensation resistor (R3). The zero is:

f Z1

=

1

2π × R3 × C3

If large value capacitors with relatively high

equivalent-series-resistance (ESR) are used,

the zero due to the capacitance and ESR of the

output capacitor can be compensated by a third

pole set by R3 and C4. The pole is:

fP3

=

1

2π × R3 × C4

The system crossover frequency (the frequency

where the loop gain drops to 1dB or 0dB) is

important. Set the crossover frequency below

one tenth of the switching frequency 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.

Table 1—Compensation Values for Typical

Output Voltage/Capacitor Combinations

VOUT

C2

1.8V 22µF Ceramic

2.5V 22µF Ceramic

3.3V 22µF Ceramic

1.8V

47µF Tantalum

(300mΩ)

2.5V

47µF Tantalum

(300mΩ)

3.3V

47µF Tantalum

(300mΩ)

R3

6.8kΩ

9.1kΩ

12kΩ

13kΩ

C3

3.3nF

2.2nF

1.8nF

2nF

C4

None

None

None

1nF

18kΩ 1.2nF 750pF

24kΩ 1nF 560pF

Choosing the Compensation Components

The values of the compensation components

listed in Table 1 yields a stable control loop for

the given output voltage and capacitor. To

optimize the compensation components for

conditions not listed in Table 1, use the

following procedure.

MP2108 Rev 1.1

www.MonolithicPower.com

9

10/2/2006

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