can be reduced by applying a correction factor to the

feedback resistor equation.

First, calculate the correction factor, k, by plugging the

desired output voltage into the following formula:

k represents the shift in the operating point at the feed-

back node (OUT).

Select the lower feedback resistor, R3, to be

≤35.7kΩ to

ensure stability and solve for R2:

Inductor Selection

A 4.7µH inductor with a saturation current of at least

800mA is recommended for the MAX1557 full-load

(600mA) application. For the MAX1556/MAX1556A appli-

cation with 1.2A full load, use a 3.3µH inductor with at

least 1.34A saturation current. For lower full-load cur-

rents the inductor current rating can be reduced. For

maximum efficiency, the inductor’s resistance (DCR)

should be as low as possible. Please note that the core

material differs among different manufacturers and

inductor types and has an impact on the efficiency. See

Table 2 for recommended inductors and manufacturers.

Capacitor Selection

Ceramic input and output capacitors are recommend-

ed for most applications. For best stability over a wide

temperature range, use capacitors with an X5R or bet-

ter dielectric due to their small size, low ESR, and low

temperature coefficients.

Output Capacitor

put voltage ripple small and to ensure regulation loop

ing frequency. A 22µF ceramic output capacitor is rec-

ommended for most applications. If a larger output

capacitor is used, then paralleling smaller capacitors is

suggested to keep the effective impedance of the

capacitor low at the switching frequency.

Input Capacitor

Due to the pulsating nature of the input current in a buck

converter, a low-ESR input capacitor at INP is required

for input voltage filtering and to minimize interference

with other circuits. The impedance of the input capacitor

cy. A minimum value of 10µF is recommended at INP for

most applications. The input capacitor can be increased

for better input filtering.

IN Input Filter

In all MAX1557 applications, connect INP directly to IN

and bypass INP as described in the

Input Capacitor

section. No additional bypass capacitor is required at

IN. For applications using the MAX1556 and

MAX1556A, an RC filter between INP and IN keeps

power-supply noise from entering the IC. Connect a

100

Ω resistor between INP and IN, and connect a

0.47µF capacitor from IN to GND.

Soft-Start Capacitor

operation of the MAX1556/MAX1556A/MAX1557. The

Start section. Soft-start times for various soft-start

capacitors are shown in the

Typical Operating

Characteristics.

PCB Layout and Routing

Due to fast-switching waveforms and high-current

paths, careful PCB layout is required. An evaluation kit

(MAX1556EVKIT) is available to speed design.

When laying out a board, minimize trace lengths

between the IC, the inductor, the input capacitor, and

the output capacitor. Keep these traces short, direct,

and wide. Keep noisy traces, such as the LX node

trace, away from OUT. The input bypass capacitors

should be placed as close as possible to the IC.

Connect GND to the exposed paddle and star PGND

and GND together at the output capacitor. The ground

connections of the input and output capacitors should

be as close together as possible.

075

3

32

.

Vk

V

R

RR

OUTPUT

−

⎛

⎝⎜

⎞

⎠⎟

=

+

()

kx

V x

VV

V

OUTPUT

.

.

.

=

−

⎛

⎝

⎜

⎞

⎠

⎟

−

106

10

075

36

2

Step-Down Converters

10

______________________________________________________________________________________

OUTPUT

OUT

R2

R3

SS

REFERENCE

1.25V

ERROR

AMPLIFIER

Figure 6. Adjustable Output Voltage