MP1580 – 2A, 380KHz STEP-DOWN CONVERTER

MP1580 Rev. 3.0

www.MonolithicPower.com

7

12/5/2005

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© 2005 MPS. All Rights Reserved.

TM

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:

OUT

FB

VEA

CS

LOAD

VDC

V

V

A

G

R

A

×

×

×

=

current sense gain, (roughly the output current

divided by the voltage at COMP), 1.95 A/V and

The system has 2 poles of importance, one is

due to the compensation capacitor (C3), and

the other is due to the output capacitor (C2).

These are:

VEA

EA

1

P

A

3

C

2

G

f

×

×

π

=

amplifier transconductance (770µA/V).

and

LOAD

2

P

R

2

C

2

1

f

×

×

π

=

The system has one zero of importance, due to

the compensation capacitor (C3) and the

compensation resistor (R3). The zero is:

3

R

3

C

2

1

Z

×

×

π

=

If a large value capacitor (C2) with relatively

high

equivalent-series-resistance

(ESR)

is

used, the zero due to the capacitance and ESR

of the output capacitor can be compensated by

a third pole set by R3 and C6. The pole is:

3

R

6

C

2

1

P

×

×

π

=

The system crossover frequency (the frequency

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

important. A good rule of thumb is to set the

crossover frequency to approximately 1/10 of

the switching frequency.

In this case, the switching frequency is 380KHz,

Lower crossover frequencies result in slower

response and worse transient load recovery.

Higher crossover frequencies can result in

instability.

Choosing the Compensation Components

The values of the compensation components

given in Table 4 yield a stable control loop for

the output voltage and capacitor given.

Table 4—Compensation Values for Typical

Output Voltage/Capacitor Combinations

C2

R3

C3

C6

2.5V

22µF Ceramic

7.5kΩ

2.2nF

None

3.3V

22µF Ceramic

10kΩ

2nF

None

5V

22µF Ceramic

15kΩ

1.2nF

None

12V

22µF Ceramic

33kΩ

1nF

None

2.5V

560µF/6.3V

(30mΩ ESR)

200kΩ

1nF

100pF

3.3V

560µF/6.3V

(30mΩ ESR)

200kΩ

1nF

82pF

5V

470µF/10V

(30mΩ ESR)

250kΩ

1nF

56pF

12V

220µF/25V

(30mΩ ESR)

250kΩ

1nF

27pF

To optimize the compensation components for

conditions not listed in Table 4, use the

following procedure:

Choose the compensation resistor to set the

desired crossover frequency.

Determine the

value by the following equation:

FB

OUT

CS

EA

C

V

V

G

G

f

2

C

2

3

R

×

×

×

×

π

=

Putting in the known constants and setting the

crossover frequency to the desired 40KHz:

OUT

8

V

2

C

10

37

.

1

3

R

×

×

×

≈

Choose the compensation capacitor to set the

zero below ¼ of the crossover frequency.

Determine the value by the following equation:

3

R

V

2

C

22

.

0

3

C

OUT

×

×

>