MP2357

– 0.5A, 24V, 1.4MHz STEP-DOWN CONVERTER IN A TSOT23-6

MP2357 Rev. 1.0

www.MonolithicPower.com

7

6/13/2013

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2013 MPS. All Rights Reserved.

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

OUT





Table 1

—Resistor Selection for Common

Output Voltages

R1 (k

Ω)

R2 (k

Ω)

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

200m

Ω. 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













Where

ΔI

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 0.5A 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.

PCB layout is very important to achieve stable

operation. Please follow these guidelines and

take Figure2 for references.

1)

Keep the path of switching current short

and minimize the loop area formed by Input

cap,

high-side

MOSFET

and

schottky

diode.

2)

Keep the connection of schottky diode

between SW pin and input power ground

as short and wide as possible.

3)

Ensure all feedback connections are short

and direct. Place the feedback resistors

and compensation components as close to

the chip as possible.

4)

Route SW away from sensitive analog

areas such as FB.

5)

Connect IN, SW, and especially GND

respectively to a large copper area to cool

the chip to improve thermal performance

and long-term reliability. For single layer,

do not solder exposed pad of the IC.