MAX1556_11 Datasheet, PDF (10/12 Pages) Maxim Integrated Products – 16μA IQ, 1.2A PWM Step-Down DC-DC Converters

16µA I_Q, 1.2A PWM DC-DC
Step-Down Converters
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
=
1.06
x 10^−2V
x
⎛
⎜
⎝
V_OUTPUT −
3.6V
0.75V
⎞
⎟
⎠
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:
ERROR
AMPLIFIER
REFERENCE
1.25V
OUTPUT
R2
OUT
R3
⎛ 0.75V − k ⎞
⎝⎜ V_OUTPUT ⎠⎟
=
R3
(^{R3 +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
The output capacitor, C_OUT, is required to keep the
output voltage ripple small and to ensure regulation
loop stability. C_OUT must have low impedance at the
switching frequency. A 22µF ceramic output capacitor
is recommended for most applications. If a larger out-
put capacitor is used, then paralleling smaller capaci-
tors 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 capaci-
tor, C_INP, should be kept very low at the switching fre-
quency. A minimum value of 10µF is recommended at
SS
Figure 6. Adjustable Output Voltage
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
100Ω resistor between INP and IN, and connect a
0.47µF capacitor from IN to GND.
Soft-Start Capacitor
The soft-start capacitor, C_SS, is required for proper
operation of the MAX1556/MAX1556A/MAX1557. The
recommended value of C_SS is discussed in the Soft-
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.
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