Datasheet

MAX1572
800mA, 2MHz, PWM DC-to-DC
Step-Down Converter with RESET
_______________________________________________________________________________________ 9
Soft-Start
Soft-start is used to prevent input-current overshoot dur-
ing startup. For most applications using a 10µF output
capacitor, connect a 1000pF capacitor from SS to GND.
If a larger output capacitor is used, then use the follow-
ing formula to find the value of the soft-start capacitor
needed to prevent input-current overshoot:
C
SS
= C
OUT
/10
4
During soft-start, the output voltage rises from 0 to
V
OUT(nom)
with a time constant equal to C
SS
times
100kΩ (see the Typical Operating Characteristics).
170ms RESET
RESET is an open-drain output with an internal 14kΩ
pullup resistor to OUT. During startup, RESET is held low
until 200ms (typ) after the output voltage reaches 90% of
its nominal regulation voltage. When the output voltage
drops below 90% of its nominal regulation voltage,
RESET pulls low again. See the Typical Operating
Characteristics section for RESET waveforms during
startup and shutdown.
Applications Information
Inductor Selection
A 2.2µH inductor with a saturation current of at least 1A
is recommended for full-load (800mA) applications. For
lower load currents, the inductor current rating may be
reduced. For most applications, use an inductor with a
current rating 1.25 times the maximum required output
current. For maximum efficiency, the inductor’s DC
resistance should be as low as possible. See Table 2
for recommended inductors and manufacturers.
Capacitor Selection
Ceramic 10µF input and output capacitors are recom-
mended for most applications. For output voltages
below 1.5V, output capacitance should be increased to
22µF. For best stability over a wide temperature range,
use capacitors with an X5R or better dielectric.
ABATT Input Filter
In normal applications, an RC filter on ABATT keeps
power-supply noise from entering the IC. Connect a
10Ω resistor between BATT and ABATT and connect a
0.1µF capacitor from ABATT to GND.
PC Board Layout and Routing
Due to fast-switching waveforms and high-current
paths, careful PC board layout is required. An evalua-
tion kit (MAX1572EVKIT) 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 PGND
and GND directly to the exposed paddle underneath the
IC. The ground connections of the input and output
capacitors should be as close together as possible.
I
OUT
V
OUT
(CONVENTIONAL)
V
OUT
(VOLTAGE POSITIONING)
Figure 2. Load Transient Response, With and Without Voltage
Positioning
CHANGE IN
OUTPUT
VOLTAGE (%)
NORMAL OPERATION
FORCED-PWM
V
IN
= 5.5V
V
IN
= 2.6V
+1
0
-1
-2
0200400 600
800
LOAD CURRENT (mA)
V
IN
= 3.6V
Figure 3. Voltage-Positioning Load Line