MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER

MP2108 Rev 1.1

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8

10/2/2006

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rate as the voltage at SS, forcing a linear

output voltage ramp from 0V to the desired

regulation voltage during soft-start.

The soft-start period is determined by the

equation:

5

C

45

.

0

×

=

Where C5 (in nF) is the soft-start capacitor from

period. Determine the capacitor required for a

given soft-start period by the equation:

SS

t

22

.

2

5

C

×

=

Use values for C5 between 10nF and 22nF to

set the soft-start period between 4ms and

10ms.

Setting the Output Voltage (see Figure 2)

Set the output voltage by selecting the resistive

voltage divider ratio. The voltage divider drops

the output voltage to the 0.9V feedback voltage.

Use 10kΩ for the low-side resistor of the

voltage divider. Determine the high-side resistor

by the equation:

1

R

1

V

9

.

0

V

2

R

OUT

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

=

Where R2 is the high-side resistor, R1 is the

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, so a capacitor is required to

supply the AC current to the step-down

converter while maintaining the DC input

voltage. A low ESR capacitor is required to

keep the noise at the IC to a minimum. Ceramic

capacitors are preferred, but tantalum or low

ESR electrolytic capacitors are also an option.

The capacitor can be electrolytic, tantalum or

ceramic. Because it absorbs the input switching

current, it must have an adequate ripple current

rating. Use a capacitor with RMS current rating

greater than 1/2 of the DC load current.

For stable operation, place the input capacitor

as close to the IC as possible. A smaller high

quality 0.1µF ceramic capacitor may be placed

closer to the IC with the larger capacitor placed

further away.

If using this technique, it is recommended that

the larger capacitor be a tantalum or electrolytic

type. All ceramic capacitors should be placed

close to the IC. For most applications, a 10µF

ceramic capacitor will work.

Selecting the Output Capacitor

The output capacitor (C2) is required to

maintain the DC output voltage. Low ESR

capacitors are preferred to keep the output

voltage ripple to a minimum. The characteristics

of the output capacitor also affect the stability of

the

regulation

control

system.

Ceramic,

tantalum, or low ESR electrolytic capacitors are

recommended.

The output voltage ripple is:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

+

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

2

C

f

8

1

R

V

V

1

L

f

V

V

SW

ESR

IN

OUT

SW

OUT

RIPPLE

the output capacitors.

Choose an output capacitor to satisfy the output

ripple requirements of the design. A 22µF

ceramic

capacitor

is

suitable

for

most

applications.

Selecting the 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 is likely to have a

larger

physical

size

and

higher

series

resistance. Choose an inductor that does not

saturate under the worst-case load conditions.

A good rule for determining the inductance is to

allow

peak-to-peak

ripple

current

to

be

approximately 30% to 40% of the maximum

load current. Make sure that the peak inductor

current (the load current plus half the peak-to-

peak inductor ripple current) is below 2.5A to

prevent loss of regulation due to the current

limit.