MP2359 – 1.2A, 24V, 1.4MHz STEP-DOWN CONVERTER IN A TSOT23-6

MP2359 Rev. 1.1

www.MonolithicPower.com

6

6/23/2006

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© 2006 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). Table 1 shows a list of resistor selection

for common output voltages. 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. Refer to Table 2 for suggested surface

mount inductors. For most designs, the required

inductance value can be derived from the

following equation.

SW

L

IN

OUT

IN

OUT

f

I

V

)

V

V

(

V

L

×

∆

×

−

×

=

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

∆

+

=

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 supply 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 feedback 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 the input voltage is no

greater than 5V or the 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.

MP2359

SW

BS

10nF

5V (External) or

Figure 2—External Bootstrap Diode

This diode is also recommended for high duty cycle

operation (when

IN

OUT

V

V

>65%) applications.