MAX1556_1 Datasheet, PDF(10/12 Page) Maxim Integrated Products – 16μA IQ, 1.2A PWM Step-Down DC-DC Converters

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MAX1556_1 Datasheet, PDF (10/12 Pages) Maxim Integrated Products – 16μA IQ, 1.2A PWM Step-Down DC-DC Converters

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16ÂµA IQ, 1.2A PWM DC-DC

Step-Down Converters

Inductor Selection

A 4.7ÂµH inductor with a saturation current of at least

800mA is recommended for the MAX1557 full-load

(600mA) application. For the MAX1556 application with

1.2A full load, use a 3.3ÂµH inductor with at least 1.34A

saturation current. For lower full-load currents the

inductor current rating can be reduced. For maximum

efficiency, the inductorâs resistance (DCR) should be as

low as possible. Please note that the core material dif-

fers 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

The output capacitor COUT is required to keep the out-

put voltage ripple small and to ensure regulation loop

stability. COUT must have low impedance at the switch-

ing frequency. A 22ÂµF 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

CINP should be kept very low at the switching frequen-

cy. A minimum value of 10ÂµF 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 sec-

tion. No additional bypass capacitor is required at IN.

For applications using the MAX1556, an RC filter

between INP and IN keeps power-supply noise from

entering the IC. Connect a 100â¦ resistor between INP

and IN, and connect a 0.47ÂµF capacitor from IN to GND.

Soft-Start Capacitor

The soft-start capacitor, CSS, is required for proper

operation of the MAX1556/MAX1557. The recommend-

ed value of CSS is discussed in the Soft-Start section.

Soft-start times for various soft-start capacitors are

shown in the Typical Operating Characteristics.

ERROR

AMPLIFIER

REFERENCE

1.25V

OUTPUT

OUT

Figure 6. Adjustable Output Voltage

PC Board Layout and Routing

Due to fast-switching waveforms and high-current

paths, careful PC board layout is required. An evalua-

tion 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 to the IC as possible.

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.

Chip Information

TRANSISTOR COUNT: 7567

PROCESS: BiCMOS

10 ______________________________________________________________________________________

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