MP9141 データシートの表示（PDF） - Monolithic Power Systems

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MP9141

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

Output Capacitor

The output capacitor is required to maintain the

DC output voltage. Low ESR capacitors are

preferred to keep the output voltage ripple low.

The characteristics of the output capacitor also

affect the stability of the regulation control

system. Ceramic, tantalum or low ESR

electrolytic capacitors are recommended. In the

case of ceramic capacitors, the impedance at

the switching frequency is dominated by the

capacitance and so the output voltage ripple is

mostly independent of the ESR. The output

voltage ripple is estimated to be:

VRIPPLE

≅ 1.4 × VIN

×

⎜⎜⎝⎛

fLC

f

⎟⎟⎠⎞ 2

Where VRIPPLE is the output ripple voltage, VIN is

the input voltage, fLC is the resonant frequency

of the LC filter and f is the switching frequency.

In the case of tantalum or low-ESR electrolytic

capacitors, the ESR dominates the impedance

at the switching frequency, and so the output

ripple is calculated as:

VRIPPLE ≅ ∆I × RESR

Where VRIPPLE is the output voltage ripple, ∆I is

the inductor ripple current and RESR is the

equivalent series resistance of the output

capacitors.

Output Rectifier Diode

The output rectifier diode supplies the current to

the inductor when the high-side switch is off. To

reduce losses due to the diode forward voltage

and recovery times, use a Schottky rectifier.

Table 2 provides the Schottky rectifier part

numbers based on the maximum input voltage

and current rating.

Table 2—Schottky Rectifier Selection Guide

Table 3 lists some rectifier manufacturers.

Table 3—Schottky Diode Manufacturers

Vendor

Web Site

Diodes, Inc.

www.diodes.com

Fairchild Semiconductor www.fairchildsemi.com

General Semiconductor www.gensemi.com

International Rectifier www.irf.com

On Semiconductor

www.onsemi.com

Pan Jit International

www.panjit.com.tw

Choose a rectifier whose maximum reverse

voltage rating is greater than the maximum

input voltage, and has a current rating greater

than 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:

A VDC

=

VFB

VOUT

× A VEA

× GCS × RLOAD

Where VFB is the feedback threshold voltage,

VOUT is the desired output regulation voltage,

AVEA is the transconductance error amplifier

voltage gain, GCS is the current sense gain

(roughly the output current divided by the

voltage at COMP) equal to 1.95 A/V and RLOAD

is the load resistance (VOUT / IOUT where IOUT is

the output load current)

The system has two poles of importance, one is

due to the compensation capacitor (C5), and

the other is due to the output capacitor (C7).

These are:

VIN (Max)

15V

20V

1.8A Load Current

Part Number

Vendor

30BQ015

4

B220

1

SK23

6

SR32

6

fP1

=

2π ×

GEA

A VEA

× C5

Where P1 is the first pole, and GEA is the error

amplifier transconductance (770µA/V), and:

fP2

=

1

2π × RLOAD

× C7

MP9141 Rev. 1.11

www.MonolithicPower.com

7

4/18/2011

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

© 2011 MPS. All Rights Reserved.

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