16µA I , 1.2A PWM DC-DC
Q
Step-Down Converters
can be reduced by applying a correction factor to the
feedback resistor equation.
OUTPUT
R2
First, calculate the correction factor, k, by plugging the
desired output voltage into the following formula:
ERROR
AMPLIFIER
OUT
⎛
⎞
V
− 0.75V
3.6V
−2
OUTPUT
k = 1.06 x 10 V x
⎜
⎟
R3
REFERENCE
1.25V
⎝
⎠
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:
⎛
⎞
0.75V −k
R3
R3+R2
=
⎜
⎟
V
⎝
⎠
OUTPUT
SS
Inductor Selection
Figure 6. Adjustable Output Voltage
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.
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
100Ω resistor between INP and IN, and connect a
0.47µF capacitor from IN to GND.
6/MAX157
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.
Soft-Start Capacitor
The soft-start capacitor, C , is required for proper
SS
operation of the MAX1556/MAX1556A/MAX1557. The
recommended value of C
is discussed in the Soft-
SS
Start section. Soft-start times for various soft-start
capacitors are shown in the Typical Operating
Characteristics.
Output Capacitor
The output capacitor C
is required to keep the out-
OUT
put voltage ripple small and to ensure regulation loop
stability. C must have low impedance at the switch-
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.
OUT
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.
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
should be kept very low at the switching frequen-
cy. A minimum value of 10µF is recommended at INP for
C
INP
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