MP1591 – 2A, 32V, 330KHz STEP-DOWN CONVERTER

MP1591 Rev. 2.3

www.MonolithicPower.com

7

9/27/2006

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© 2006 MPS. All Rights Reserved.

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

REF

VDC

R

G

A

V

V

A

×

×

×

=

current sense gain (roughly the output current

divided by the voltage at COMP), 3.5 A/V.

The system has 2 poles of importance; one is

due to the compensation capacitor (C4) and the

other is due to the output capacitor (C5). These

are:

)

4

C

A

2

(

G

f

VEA

MEA

1

P

×

×

π

=

amplifier transconductance (770µS) and

)

5

C

R

2

(

1

f

LOAD

2

P

×

×

π

=

The system has one zero of importance due to

the compensation capacitor (C4) and the

compensation resistor (R3) which is

)

4

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 C3

)

3

C

3

R

2

(

1

P

×

×

π

=

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 one

tenth of the switching frequency. In this case,

the switching frequency is 330KHz, so use a

crossover

frequency

of

33KHz.

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 given capacitor.

Table 4—Compensation Values for Typical

Output Voltage/Capacitor Combinations

C5

R3

C3

C4

2.5V

22µF Ceramic

3.9kΩ

None

4.7nF

3.3V

22µF Ceramic

5.1kΩ

None

3.9nF

5V

22µF Ceramic

7.5kΩ

None

2.7nF

12V

22µF Ceramic

18kΩ

None

1.2nF

2.5V

47µF SP-Cap

8.2kΩ

None

2.2nF

3.3V

47µF SP-Cap

10kΩ

None

2.2nF

5V

47µF SP-Cap

16kΩ

None

1.5nF

12V

47µF SP-Cap

36kΩ

None

1nF

2.5V

560µF/6.3V, AL

30mΩ ESR

100kΩ

150pF

1nF

3.3V

560µF/6.3V, AL

30mΩ ESR

120kΩ

120pF

1nF

5V

470µF/10V, AL

30mΩ ESR

150kΩ

82pF

1nF

12V

220µF/25V, AL

30mΩ ESR

180kΩ

33pF

1nF

Note: “AL” = Electrolytic