30BQ015 データシートの表示（PDF） - Monolithic Power Systems

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30BQ015

2A, 380 KHz Step-Down Converter

Monolithic Power Systems 

TM

MP1580 – 2A, 380KHz STEP-DOWN CONVERTER

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

= RLOAD

× GCS

× A VEA

×

VFB

VOUT

Where AVEA is the transconductance error

amplifier voltage gain, 400 V/V, GCS is the

current sense gain, (roughly the output current

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

RLOAD is the load resistance (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 output capacitor (C2).

These are:

fP1

=

GEA

2π × C3 × A VEA

Where P1 is the first pole and GEA is the error

amplifier transconductance (770µA/V).

and

fP2

=

1

2π × C2 × RLOAD

The system has one zero of importance, due to

the compensation capacitor (C3) and the

compensation resistor (R3). The zero is:

fZ1

=

2π ×

1

C3 × R3

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:

fP3

=

1

2π × C6 × R3

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,

so use a crossover frequency, fC, of 40KHz.

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

VOUT

2.5V

3.3V

5V

12V

2.5V

3.3V

5V

12V

C2

22µF Ceramic

22µF Ceramic

22µF Ceramic

22µF Ceramic

560µF/6.3V

(30mΩ ESR)

560µF/6.3V

(30mΩ ESR)

470µF/10V

(30mΩ ESR)

220µF/25V

(30mΩ ESR)

R3

7.5kΩ

10kΩ

15kΩ

33kΩ

C3

2.2nF

2nF

1.2nF

1nF

C6

None

None

None

None

200kΩ 1nF 100pF

200kΩ 1nF 82pF

250kΩ 1nF 56pF

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:

R3 = 2π × C2 × fC × VOUT

GEA × GCS VFB

Putting in the known constants and setting the

crossover frequency to the desired 40KHz:

R3 ≈ 1.37 × 108 × C2 × VOUT

Choose the compensation capacitor to set the

zero below ¼ of the crossover frequency.

Determine the value by the following equation:

C3 > 0.22 × C2 × VOUT

R3

MP1580 Rev. 3.0

www.MonolithicPower.com

7

12/5/2005

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© 2005 MPS. All Rights Reserved.

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