MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controller in µMAX
12 ______________________________________________________________________________________
Diode Selection
The MAX1744/MAX1745’s high switching frequency
demands a high-speed rectifier. Schottky diodes, such
as the 1N5817–1N5822 family or surface-mount equiva-
lents, are recommended. Ultra-high-speed rectifiers
with reverse recovery times around 50ns or faster
should be used for high output voltages, where the
increased forward drop causes less efficiency degra-
dation. Make sure that the diode’s peak current rating
exceeds the peak current limit set by R
SENSE
, and that
its breakdown voltage exceeds V
IN
. Schottky diodes
are preferred for heavy loads due to their low forward
voltage, especially in low-voltage applications. For
high-temperature applications, some Schottky diodes
may be inadequate due to their high leakage currents.
In such cases, ultra-high-speed rectifiers are recom-
mended, although a Schottky diode with a higher
reverse voltage rating can often provide acceptable
performance.
Capacitor Selection
Choose filter capacitors to service input and output
peak currents with acceptable voltage ripple. ESR in
the output capacitor is a major contributor to output rip-
ple, so low-ESR capacitors are recommended. Low-
ESR tantalum, polymer, or ceramic capacitors are best.
Low-ESR aluminum electrolytic capacitors are tolera-
ble, but standard aluminum electrolytic capacitors are
not recommended.
Voltage ripple is the sum of contributions from ESR and
the capacitor value:
For tantalum capacitors, the ripple is determined by the
ESR, but for ceramic capacitors, the ripple is mostly
due to the capacitance. Voltage ripple as a conse-
quence of ESR is approximated by:
The ripple due to the capacitance is approximately:
Estimate input and output capacitor values for given
voltage ripple as follows:
where I
∆L
is the change in inductor current.
These equations are suitable for initial capacitor selec-
tion; final values should be set by testing a prototype or
evaluation kit. When using tantalum capacitors, use
good soldering practices to prevent excessive heat
from damaging the devices and increasing their ESR.
Also, ensure that the tantalum capacitors’ surge-current
ratings exceed the startup inrush and peak switching
currents.
Pursuing output ripple lower than the error compara-
tor’s hysteresis (0.6% of the output voltage) is not prac-
tical, since the MAX1744/MAX1745 will switch at slower
frequencies, increasing inductor ripple current thresh-
old. Choose an output capacitor with a working voltage
rating higher than the output voltage.
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
(16 strony)
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