MP9141ES-LF-Z データシートの表示（PDF） - Monolithic Power Systems

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MP9141ES-LF-Z

1.8A Step-Down DC to DC Converter

Monolithic Power Systems 

MP9141 – 1.8A STEP-DOWN DC TO DC CONVERTER

MPS CONFIDENTIAL AND PROPRIETARY INFORMATION– INTERNAL USE ONLY

The system has one zero of importance, due to

the compensation capacitor (C5) and the

compensation resistor (R1). This is:

Choose the compensation resistor to set the

desired crossover frequency. Determine the

value by the following equation:

f Z1

=

1

2π × R1× C5

If a large value capacitor (C7) 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 R1 and C4:

fP3

=

1

2π × R1× C4

The system crossover frequency (the frequency

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

important. A good standard is to set the

crossover frequency to approximately one tenth

of the switching frequency. In this case, the

switching frequency is 380KHz. Therefore, 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

C7

R1

C5 C4

2.5V 22µF Ceramic 7.5kΩ 2.2nF None

3.3V 22µF Ceramic 10kΩ 1.5nF None

5V 22µF Ceramic 10kΩ 2.2nF None

12V 22µF Ceramic 10kΩ 5.6nF None

2.5V

560µF/6.3V

(30mΩ ESR)

10kΩ 30nF None

3.3V

560µF/6.3V

(30mΩ ESR)

10kΩ 39nF None

5V

470µF/10V

(30mΩ ESR)

10kΩ 47nF None

12V

220µF/25V

(30mΩ ESR)

10kΩ 56nF None

To optimize the compensation components for

conditions not listed in Table 4, use the

following procedure:

R1

=

2π

×

C7

×

VOUT

×

(GEA

×

fC

GCS

×

VFB

)

Putting in the known constants and setting the

crossover frequency to the desired 40KHz:

R1 ≈ 1.37 × 108 × C7 × VOUT

The value of R1 is limited to 10kΩ to prevent

output overshoot at startup. Therefore, if the value

calculated for R1 is greater than 10kΩ, use 10kΩ.

In this case, the actual crossover frequency is

less than the desired 40KHz, and is calculated by:

fC

=

R1× GEA × GCS × VFB

2π × C7 × VOUT

or:

fC

≈

2.92 × 10 −4 × R1

C7 × VOUT

Choose the compensation capacitor to set the

zero to one fourth of the crossover frequency.

Determine the value by the following equation:

C5 = 0.22 × C7 × VOUT

R1

Determine if the second compensation

capacitor, C4 is required. It is required if the

ESR zero of the output capacitor occurs at less

than four times the crossover frequency.

8π × C7 × RESR × fC ≥ 1

or:

7.34 × 10 −5 × R1× RESR ≥ 1

VOUT

Where RESR is the equivalent series resistance

of the output capacitor. If this is the case, add

the second compensation capacitor. Determine

the value by the equation:

C4 = C7 × RESR(MAX)

R1

Where RESR(MAX) is the maximum ESR of the

output capacitor.

MP9141 Rev. 1.11

www.MonolithicPower.com

8

4/18/2011

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2011 MPS. All Rights Reserved.

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