MP4566―36V, 1MHz, 0.6A, STEP-DOWN CONVERTER WITH 35μA QUIESCENT CURRENT

MP4566 Rev. 1.01

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11

9/22/2011

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APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider from output voltage

to FB pin is used to set the output voltage. The

feedback resistor R1 also sets the feedback loop

bandwidth

with

the

internal

compensation

capacitor. Choose R1 to be around 150~200kΩ.

R2 is then given by:

out

R1

R2

V

1

1.0

=

−

Table 1 lists the recommended T-type resistors

value for common output voltages.

Table 1—Resistor Selection for Common

Output Voltages

R1 (kΩ)

R2 (kΩ)

1.8

150(1%)

187(1%)

2.5

150(1%)

100(1%)

3.3

187(1%)

80.6(1%)

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 will result in less ripple current that will

result in lower output ripple voltage with the same

output capacitors. However, the larger value

inductor will have a larger physical size, higher

series resistance, and/or lower saturation current

as well as the slow load transient dynamic. 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. The inductance

value can be calculated by:

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f

×−

=

×Δ ×

current.

Choose an inductor that will not saturate under

the maximum inductor peak current. The peak

inductor current can be calculated by:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Selecting the 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 diode.

Choose a diode whose maximum reverse voltage

rating is greater than the maximum input voltage,

and whose current rating is greater than the

maximum load current. Table 2 lists example

Schottky diodes and manufacturers.

Table 2—Output Schottky Diodes

Manufacturer

Part Number

Voltage

Rating

(V)

Current

Rating

(A)

Package

Diodes Inc.

B240-13-F

40V

2A

SMA

Diodes Inc.

B340-13-F

40V

3A

SMA

Central semi

CMSH2-40M

40V

2A

SMA

Central semi

CMSH3-40MA

40V

3A

SMA

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required to

supply the AC current to the step-down converter

while maintaining the DC input voltage. Use low

ESR capacitors for the best performance.

Ceramic capacitors with X5R or X7R dielectrics

are highly recommended because of their low

ESR and small temperature coefficients. For

most applications, a 10µF capacitor is sufficient.

For higher output voltage, 22uF may be needed

for more stable system.

Since the input capacitor absorbs the input

switching current it requires an adequate ripple

current rating. The RMS current in the input

capacitor can be estimated by:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

The worse case condition occurs at VIN =

2VOUT, where:

2

I

I

LOAD

1