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

MP2354 Rev. 1.4

www.MonolithicPower.com

6

1/6/2006

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© 2006 MPS. All Rights Reserved.

TM

APPLICATION INFORMATION

MP2354_TAC_F02

MP2354

LX

VIN

BST

FB

SYNC

COMP

GND

RUN

OUTPUT

2.5V / 2A

INPUT

4.75V to 23V

OPEN

IF NOT USED

OPEN

AUTOMATIC

STARTUP

C6

OPEN

C3

3.3nF

C5

10nF

D1

B230A

32

4

6

7

5

1

8

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.

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:

2

R

1

R

2

R

V

V

OUT

FB

+

×

=

the output voltage.

Thus the output voltage is:

(

)

2

R

2

R

1

R

23

.

1

+

×

=

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

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

()( )

Ω

−

×

=

k

23

.

1

V

18

.

8

1

R

OUT

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

10kΩ, and R1 is 17kΩ.

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:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

IN

OUT

L

S

OUT

V

V

1

∆I

f

V

L

peak inductor ripple current.

Choose an inductor that will not saturate under

the maximum inductor peak current. The peak

inductor current can be calculated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

+

=

IN

OUT

S

OUT

LOAD

LP

V

V

1

L

f

2

V

I

I