MP1580 – 2A, 380KHz STEP-DOWN CONVERTER

MP1580 Rev. 3.0

www.MonolithicPower.com

5

12/5/2005

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TM

APPLICATION INFORMATION

COMPONENT SELECTION

Sync Pin Operation

The SYNC pin driving waveform should be a

square wave with a rise time less than 20ns.

The Minimum High voltage level is 2.7V and the

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 3). 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

222

.

1

+

×

=

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

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

)

222

.

1

V

(

18

.

8

1

R

×

≅

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 results in less ripple current that in turn

results in lower output ripple voltage.

However, the larger value inductor has a larger

physical size, higher series resistance and/or

lower saturation current. Choose an inductor

that does not saturate under the worst-case

load conditions.

A good rule for determining the inductance is to

allow the peak-to-peak ripple current in the

inductor to be approximately 30% of the

maximum load current. Also, make sure that the

peak inductor current (the load current plus half

the peak-to-peak inductor ripple current) is

below the 2.4A minimum current limit.

The inductance value can be calculated by the

equation:

I

f

V

)

V

V

(

V

L

IN

OUT

IN

OUT

∆

×

×

−

×

=

frequency and ∆I is the peak-to-peak inductor

ripple current. Table 1 lists a number of suitable

inductors from various manufacturers.

Table 1—Inductor Selection Guide

Package

Dimensions

(mm)

Vendor/

Model

Core

Type

Core

Material

W

L

H

Sumida

CR75

Open

Ferrite

7.0

7.8

5.5

CDH74

Open

Ferrite

7.3

8.0

5.2

CDRH5D28 Shielded Ferrite

5.5

5.7

5.5

CDRH5D28 Shielded Ferrite

5.5

5.7

5.5

CDRH6D28 Shielded Ferrite

6.7

6.7

3.0

CDRH104R Shielded Ferrite

10.1 10.0 3.0

Toko

D53LC

Type A

Shielded Ferrite

5.0

5.0

3.0

D75C

Shielded Ferrite

7.6

7.6

5.1

D104C

Shielded Ferrite

10.0 10.0 4.3

D10FL

Open

Ferrite

9.7

11.5 4.0

Coilcraft

DO3308

Open

Ferrite

9.4

13.0 3.0

DO3316

Open

Ferrite

9.4

13.0 5.1