MP2459 – 0.5A, 55V, 480kHz, STEP-DOWN CONVERTER IN A TSOT23-6

MP2459 Rev. 1.0

www.MonolithicPower.com

10

3/4/2013

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© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting Output Voltage

The external resistor divider sets the output

voltage (see the Typical Application schematic).

Table 1 lists resistors for common output

voltages. The feedback resistor (R1) also sets

the feedback loop bandwidth with the internal

compensation capacitor (see Figure 1). R2 is:

1

V

81

.

0

V

1

R

2

R

=

Table 1: Resistor Selection for Common

Output Voltages

R1 (k )

R2 (k )

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

Use an inductor with a DC current rating at

least 25% percent higher than the maximum

load current for most applications. For best

efficiency, the inductor’s DC resistance should

be less than 200m . 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:

2

I

I

I

L

LOAD

)

MAX

(

L

∆

+

=

Under light-load conditions (below 100mA), use

a larger inductance to improve efficiency.

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. Use ceramic capacitors with

X5R or X7R dielectrics for their low ESRs and

small

temperature

coefficients.

For

most

applications, a 4.7µF capacitor will 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.

Use

ceramic

capacitors with X5R or X7R dielectrics for their

low ESR characteristics. For most applications,

a 22µF ceramic capacitor will sufficient.

PCB Layout Guide

PCB layout is very important to stability. Please

follow these guidelines and use Figure 2 as

reference.

1)

Keep the path of switching current short

and minimize the loop area formed by the

input capacitor, high-side MOSFET, and

Schottky diode.

2)

Keep the connection from the power

ground→Schottky diode→SW pin 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 to

large copper areas to cool the chip for

improved thermal performance and long-

term reliability. For single layer PCBs, avoid

soldering the exposed pad

.