MAX8543 MAXIM [Maxim Integrated Products], MAX8543 Datasheet - Page 18

• Manufacturer Part Number: MAX8543
• Description: Step-Down Controllers with Prebias Startup, Lossless Sensing, Synchronization, and OVP
• Manufacturer: MAXIM [Maxim Integrated Products]

Step-Down Controllers with Prebias Startup,
Lossless Sensing, Synchronization, and OVP
Figure 4. Inductor-Current Waveform
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
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.
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.
18
______________________________________________________________________________________
R
FSYNC
=
⎛
⎜
⎝
2
Switching Frequency and
Power-Good Signal (POK)
1
f
S
−
240
ns
TIME
⎞
⎟
⎠
⎛
⎜
⎝
Synchronization
14 18
1
.
k
FSYNC
Ω
ns
⎞
⎟
⎠
I
I
I
PEAK
LOAD
VALLEY
such that
Thermal-overload protection limits total power dissipation
in the MAX8543/MAX8544. When the junction tempera-
ture exceeds T
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.
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
close to the IC as possible.
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.
Figure 5. Setting the Output Voltage with a Resistor Voltage-
Divider
FB
MAX8543/
MAX8544
= 0.8V. R1 and R2 should be placed as
J
= +160°C, an internal thermal sensor
R
Thermal-Overload Protection
1
=
Setting the Output Voltage
R
LX
FB
2
×
⎛
⎜
⎝
Design Procedure
V
V
OUT
FB
Inductor Selection
−
1
⎞
⎟
⎠
R1
R2