MAX5986A MAXIM [Maxim Integrated Products], MAX5986A Datasheet - Page 14

• Manufacturer Part Number: MAX5986A
• Description: IEEE 802.3af-Compliant, High-Efficiency, Class 1,Powered Devices with Integrated DC-DC Converter
• Manufacturer: MAXIM [Maxim Integrated Products]

where V
voltage across the input capacitors and is recommended
to be less than 2% of the minimum input voltage. D is the
duty cycle (V
f
The impedance of the input capacitor at the switching
frequency should be less than that of the input source so
high-frequency switching currents do not pass through
the input source, but are instead shunted through the
input capacitor. The input capacitor must meet the ripple
current requirement imposed by the switching currents.
The RMS input ripple current is given by:
where I
The key selection parameters for the output capacitor are
capacitance, ESR, ESL, and voltage-rating requirements.
These affect the overall stability, output ripple voltage,
and transient response of the DC-DC converter. The out-
put ripple occurs due to variations in the charge stored in
the output capacitor, the voltage drop due to the capaci-
tor’s ESR, and the voltage drop due to the capacitor’s
ESL. Estimate the output-voltage ripple due to the output
capacitance, ESR, and ESL:
where the output ripple due to output capacitance, ESR,
and ESL is:
Powered Devices with Integrated DC-DC Converter
S
).
V
RIPPLE
IEEE 802.3af-Compliant, High-Efficiency, Class 1,
RIPPLE
IN-RIPPLE
= V
OUT
RIPPLE(C)
is the input RMS ripple current.
/V
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is the maximum allowed input ripple
IN
) and T
Output Capacitor Selection
+ V
RIPPLE(ESR)
S
is the switching period (1/
+V
RIPPLE(ESL)
or whichever is larger. The peak-to-peak inductor current
(I
Use these equations for initial output capacitor selec-
tion. Determine final values by testing a prototype or an
evaluation circuit. A smaller ripple current results in less
output-voltage ripple. Since the inductor ripple current is
a factor of the inductor value, the output-voltage ripple
decreases with larger inductance. Use ceramic capaci-
tors for low ESR and low ESL at the switching frequency
of the converter. The ripple voltage due to ESL is negli-
gible when using ceramic capacitors.
Load-transient response depends on the selected output
capacitance. During a load transient, the output instantly
changes by ESR x I
respond, the output deviates further, depending on the
inductor and output capacitor values. After a short time,
the controller responds by regulating the output voltage
back to its predetermined value. The controller response
time depends on the closed-loop bandwidth. A higher
bandwidth yields a faster response time, preventing the
output from deviating further from its regulating value.
P-P
)
MAX5986A/MAX5987A
LOAD
. Before the controller can