MP2108 – 2A, 6V, 720KHz SYNCHRONOUS BUCK CONVERTER

INITIAL RELEASE – SPECIFICATIONS SUBJECT TO CHANGE

MP2108 Rev. 0.93

www.MonolithicPower.com

7

2/28/2006

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© 2006 MPS. All Rights Reserved.

TM

Calculate the required inductance value by the

equation:

()

I

f

V

V

V

V

L

SW

IN

OUT

IN

OUT

∆

×

×

−

×

=

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:

LOAD

CS

VEA

OUT

FB

VDC

R

G

A

V

V

A

×

×

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

=

is the transconductance error amplifier voltage

transconductance, (roughly the output current

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

OUT

OUT

LOAD

I

V

R

=

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:

3

C

A

2

G

f

VEA

EA

1

P

×

×

π

=

transconductance (300µA/V) and

2

C

R

2

1

f

LOAD

2

P

×

×

π

=

The system has one zero of importance, due to

the compensation capacitor (C3) and the

compensation resistor (R3). The zero is:

3

C

3

R

2

1

Z

×

×

π

=

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:

4

C

3

R

2

1

P

×

×

π

=

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

C2

R3

C3

C4

1.8V 22µF Ceramic

6.8kΩ

3.3nF

None

2.5V 22µF Ceramic

9.1kΩ

2.2nF

None

3.3V 22µF Ceramic

12kΩ

1.8nF

None

1.8V

47µF Tantalum

(300mΩ)

13kΩ

2nF

1nF

2.5V

47µF Tantalum

(300mΩ)

18kΩ

1.2nF 750pF

3.3V

47µF Tantalum

(300mΩ)

24kΩ

1nF

560pF

Choosing the Compensation Components

The values of the compensation components

given in Table 1 yields a stable control loop for

the output voltage and capacitor given. To

optimize the compensation components for

conditions not listed in Table 1, use the

following procedure.