MAX5988BEVKIT# Maxim Integrated, MAX5988BEVKIT# Datasheet - Page 15

• Manufacturer Part Number: MAX5988BEVKIT#
• Description: Power Management IC Development Tools MAX5988B Eval Kit
• Manufacturer: Maxim Integrated

Specifications of MAX5988BEVKIT#

• Rohs: yes

The input capacitor reduces the current peaks drawn
from the input power supply and reduces switching noise
in the IC. The total input capacitance must be equal or
greater than the value given by the following equation
to keep the input-ripple voltage within specification and
minimize the high-frequency ripple current being fed
back to the input source:
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
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:
Table 1. Design Selection Table
Maxim Integrated
IEEE 802.3af-Compliant, High-Efficiency, Class 1/Class 2,
V
RIPPLE
OUTPUT
Powered Devices with Integrated DC-DC Converter
3.3
(V)
12
I
RIPPLE
5
RIPPLE
IN-RIPPLE
=
= V
OUT
RIPPLE(C)
C
is the input RMS ripple current.
IN_MIN
/V
I
LOAD
V
IN
is the maximum allowed input ripple
CC
) and T
Output Capacitor Selection
2.2FF/100V
2.2FF/100V
2.2FF/100V
CERAMIC
×
=
Input Capacitor Selection
+ V
D
V
S
V
RIPPLE(ESR)
×
OUT
IN RIPPLE
is the switching period (1/f
T
−
S
×
×
(
I
V
V
OUT
IN
CC
C
IN
−
+V
V
ELECTROLYTIC
RIPPLE(ESL)
OUT
10FF/63V
10FF/63V
10FF/63V
)
S
).
where the output ripple due to output capacitance, ESR,
and ESL is:
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
1 x 100FF/6.3V
1 x 100FF/6.3V
)
2 x 10FF/16V
=
MAX5988A/MAX5988B
CERAMIC
C
OUT
I
P P
V
V
V
or
V
−
RIPPLE(C)
RIPPLE(ESR)
RIPPLE(ESL)
RIPPLE(ESL)
V
CC
LOAD
=
=
f
=
S
−
=
×
220FH/0.8A
V
33FH/1.4A
47FH/1.6A
OUT
8 C
. Before the controller can
L
×
I
I
t
t
I
P P
P P
L
P P
OFF
ON
−
−
−
I
OUT
P P
×
−
×
×
×
V
ESR
V
ESL
OUT
ESL
×
CC
f
S
CLASS
1 or 2
1 or 2
1
15