MP2354DS-LF-Z データシートの表示（PDF） - Monolithic Power Systems

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MP2354DS-LF-Z

2A, 23V, 380KHz Step-Down Converter

Monolithic Power Systems 

TM

MP2354 – 2A, 23V, 380KHz STEP-DOWN CONVERTER

APPLICATION INFORMATION

INPUT

4.75V to 23V

C5

10nF

3

2

OPEN

AUTOMATIC

STARTUP

OPEN

IF NOT USED

VIN

8

RUN

BST

4

LX

MP2354

1

6

SYNC

FB

GND

COMP

5

C6

OPEN

7C3

3.3nF

D1

B230A

OUTPUT

2.5V / 2A

MP2354_TAC_F02

Figure 2—Typical Application Circuit

Sync Pin Operation

The SYNC pin driving waveform should be a

square wave with a rise time less than 20ns.

Minimum High voltage level is 2.7V. Low level

is less than 0.8V. The frequency of the external

sync signal needs to be greater than 445KHz.

A rising edge on the SYNC pin forces a reset of

the oscillator. The upper transistor M1 is

switched off immediately if it is not already off.

250ns later M1 turns on connecting LX to VIN.

Setting the Output Voltage

The output voltage is set using a resistive

voltage divider from the output to FB (see

Figure 2). The voltage divider divides the output

voltage down by the ratio:

Inductor

The inductor is required to supply constant

current to the output load while being driven by

the switched input voltage. A larger value

inductor will result in less ripple current that will

result in lower output ripple voltage. However,

the larger value inductor will have a larger

physical size, higher series resistance, and/or

lower saturation current. A good rule for

determining the inductance to use is to allow

the peak-to-peak ripple current in the inductor

to be approximately 30% of the maximum

switch current limit. Also, make sure that the

peak inductor current is below the maximum

switch current limit. The inductance value can

be calculated by:

VFB

=

VOUT × R2

R1+ R2

Where VFB is the feedback voltage and VOUT is

the output voltage.

Thus the output voltage is:

L

=

VOUT

fS × ∆IL

× ⎜⎜⎝⎛1−

VOUT

VIN

⎟⎟⎠⎞

Where VIN is the input voltage, fS is the 380KHz

switching frequency, and ∆IL is the peak-to-

peak inductor ripple current.

VOUT

=

1.23 × (R1+ R2)

R2

R2 can be as high as 100kΩ, but a typical value

is 10kΩ. Using that value, R1 is determined by:

R1 = 8.18 × (VOUT − 1.23)(kΩ)

For example, for a 3.3V output voltage, R2 is

10kΩ, and R1 is 17kΩ.

Choose an inductor that will not saturate under

the maximum inductor peak current. The peak

inductor current can be calculated by:

ILP

= ILOAD

+

VOUT

2 × fS × L

× ⎜⎜⎝⎛1−

VOUT

VIN

⎟⎟⎠⎞

Where ILOAD is the load current.

MP2354 Rev. 1.4

www.MonolithicPower.com

6

1/6/2006

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© 2006 MPS. All Rights Reserved.

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