MP28367 – 1A, 21V, 1.4MHz STEP-DOWN CONVERTER IN A TSOT23-6

MP28367 Rev. 0.9

www.MonolithicPower.com

7

12/13/2007

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© 2007 MPS. All Rights Reserved.

TM

APPLICATION INFORMATION

Setting Output Voltage

The external resistor divider is used to set the

output voltage (see the schematic on front

page). The feedback resistor R1 also sets the

feedback loop bandwidth with the internal

compensation capacitor (see Figure 1). R2 can

be determined by:

1

V

81

.

0

V

1

R

2

R

=

Table 1—Resistor Selection for Common

Output Voltages

R1 (kΩ)

R2 (kΩ)

1.8

80.6 (1%)

64.9 (1%)

2.5

49.9 (1%)

23.7 (1%)

3.3

49.9 (1%)

16.2 (1%)

5

49.9 (1%)

9.53 (1%)

Selecting the Inductor

A 1µH to 10µH inductor with a DC current rating

of at least 25% percent higher than the

maximum load current is recommended for

most applications. For highest efficiency, the

inductor’s DC resistance should be less than

200mΩ. For most designs, the required

inductance value can be derived from the

following equation.

OSC

L

IN

OUT

IN

OUT

f

I

V

)

V

V

(

V

L

×

Δ

×

−

×

=

Choose the inductor ripple current to be 30% of

the maximum load current. The maximum

inductor peak current is calculated from:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Under light load conditions below 100mA, a

larger inductance is recommended for improved

efficiency. See Table 2 for suggested inductors.

Also note that the maximum recommended load

current is 1A if the duty cycle exceeds 35%.

Selecting the Input Capacitor

The input capacitor reduces the surge current

drawn from the input and the switching noise

from the device. The input capacitor impedance

at the switching frequency should be less than

the input source impedance to prevent high

frequency

switching

current

from

passing

through the input. Ceramic capacitors with X5R

or X7R dielectrics are highly recommended

because of their low ESR and small temperature

coefficients. For most applications, a 4.7µF

capacitor is sufficient.

Selecting the Output Capacitor

The output capacitor keeps the output voltage

ripple small and ensures loop stability. The

output capacitor impedance should be low at the

switching frequency. Ceramic capacitors with

X5R or X7R dielectrics are recommended for

their

low

ESR

characteristics.

For

most

applications, a 22µF ceramic capacitor will be

sufficient.

PC Board Layout

The high current paths (GND, IN and SW)

should be placed very close to the device with

short, direct and wide traces. The input capacitor

needs to be as close as possible to the IN and

GND pins. The external feedback resistors

should be placed next to the FB pin. Keep the

switch node traces short and away from the

feedback network.

External Bootstrap Diode

It is recommended that an external bootstrap

diode be added when a 5V rail is available in the

system. This helps improve the efficiency of the

regulator. The bootstrap diode can be a low cost

one such as IN4148 or BAT54.

MP28367

SW

BS

10nF

5V (External)

Figure 2—External Bootstrap Diode

This diode is also recommended for high duty

cycle operation (when

IN

OUT

V

V

>65%) applications.