Datasheet

MAX8543/MAX8544
Step-Down Controllers with Prebias Startup,
Lossless Sensing, Synchronization, and OVP
18 ______________________________________________________________________________________
Switching Frequency and
Synchronization
The MAX8543/MAX8544 have an adjustable internal
oscillator that can be set to any frequency from 200kHz
to 1MHz. To set the switching frequency, connect a
resistor from FSYNC to GND. Calculate the resistor
value from the following equation:
The MAX8543/MAX8544 can also be synchronized to an
external clock by connecting the clock signal to FSYNC.
When using an external clock, select R
FSYNC
such that
the free-running frequency is within ±30% of the clock fre-
quency. In addition, the MAX8544 has a synchronization
output (SYNCO) that provides a clock signal that is 180°
out-of-phase with the MAX8544 switching. SYNCO is
used to synchronize a second controller 180° out-of-
phase with the first by connecting SYNCO of the first con-
troller to FSYNC of the second when the first controller
operates in free-running mode. When the first controller is
synchronized to an external clock, the external clock is
inverted to generate SYNCO.
Power-Good Signal (POK)
POK is an open-drain output on the MAX8544 that moni-
tors the output voltage. When the output is above 91% of
its nominal regulation voltage, POK is high impedance.
When the output drops below 91% of its nominal regula-
tion voltage, POK is pulled low. POK is also pulled low
when the MAX8544 is shut down. To use POK as a logic-
level signal, connect a pullup resistor from POK to the
logic-supply rail.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipation
in the MAX8543/MAX8544. When the junction tempera-
ture exceeds T
J
= +160°C, an internal thermal sensor
shuts down the device, allowing the IC to cool. The ther-
mal sensor turns the IC on again after the junction tem-
perature cools by 15°C, resulting in a pulsed output
during continuous thermal-overload conditions.
Design Procedure
Setting the Output Voltage
To set the output voltage for the MAX8543/MAX8544,
connect FB to the center of an external resistor-divider
from the output to GND (Figure 5). Select R2 between
8kΩ and 24kΩ; then calculate R1 with the following
equation:
where V
FB
= 0.8V. R1 and R2 should be placed as
close to the IC as possible.
Inductor Selection
There are several parameters that must be examined
when determining which inductor is to be used: input volt-
age, output voltage, load current, switching frequency,
and LIR. LIR is the ratio of peak-to peak inductor current
ripple to maximum DC load current. A higher LIR value
allows for a smaller inductor, but results in higher losses
and higher output ripple.
RR
V
V
OUT
FB
12 1=× −
⎛
⎝
⎜
⎞
⎠
⎟
R
f
ns
k
ns
FSYNC
S
=−
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
1
2
240
1
14 18
Ω
.
TIME
INDUCTOR CURRENT
I
VALLEY
I
LOAD
I
PEAK
Figure 4. Inductor-Current Waveform
MAX8543/
MAX8544
R1
R2
LX
FB
Figure 5. Setting the Output Voltage with a Resistor Voltage-
Divider