Datasheet

MAX5942A/MAX5942B
IEEE 802.3af Power-Over-Ethernet
Interface/PWM Controller for Power Devices
20 ______________________________________________________________________________________
COMPONENT SUPPLIERS WEBSITE
International Rectifier www.irf.com
Fairchild www.fairchildsemi.com
Power FETS
Vishay-Siliconix www.vishay.com/brands/siliconix/main.html
Dale-Vishay www.vishay.com/brands/dale/main.html
Current-Sense Resistors
IRC www.irctt.com/pages/index.cfm
ON Semi www.onsemi.com
General Semiconductor www.gensemi.com
Diodes
Central Semiconductor www.centralsemi.com
Sanyo www.sanyo.com
Taiyo Yuden www.t-yuden.comCapacitors
AVX www.avxcorp.com
Coiltronics www.cooperet.com
Coilcraft www.coilcraft.com
Magnetics
Pulse Engineering www.pulseeng.com
Table 3. Component Suppliers
6) Choose R
SENSE
according to the following equation:
where:
V
ILIM
is the current-sense comparator trip threshold
voltage (0.465V).
N
S
/N
P
is the secondary-side turns ratio (5/14 in this
example).
I
OUTMAX
is the maximum DC output current (10A in
this example):
7) Choose the inductor value so that the peak ripple
current (LIR) in the inductor is between 10% and
20% of the maximum output current:
where V
D
is the output Schottky diode forward-volt-
age drop (0.5V) and LIR is the ratio of inductor rip-
ple current to DC output current:
8) The size and ESR of the output filter capacitor deter-
mine the output ripple. Choose a capacitor with a
low ESR to yield the required ripple voltage.
Use the following equations to calculate the peak-to-
peak output ripple:
where:
V
RIPPLE
is the combined RMS output ripple due to
VRIPPLE,ESR
, the ESR ripple, and V
RIPPLE,C
, the
capacitive ripple. Calculate the ESR ripple and
capacitive ripple as follows:
V
RIPPLE,ESR
= I
RIPPLE
x ESR
V
RIPPLE,C
= I
RIPPLE
/(2 x π x 275kHz x C
OUT
)
Layout Recommendations
All connections carrying pulsed currents must be very
short, be as wide as possible, and have a ground plane
as a return path. The inductance of these connections
must be kept to a minimum due to the high di/dt of the
currents in high-frequency switching power converters.
Current loops must be analyzed in any layout pro-
posed, and the internal area kept to a minimum to
reduce radiated EMI. Ground planes must be kept as
intact as possible.
VV V
RIPPLE
RIPPLEESR RIPPLE C
=+
,,
22
L
-
≥
()
×
()
××
=µ
5 5 1 0 198
0 4 275 10
401
..
.
.
kHz A
H
L
V-
OUT
≥
+
()
×
()
×× ×
VD
LIR kHz I
DMIN
OUTMAX
1
2 275
R
SENSE
≤
××
= Ω
0 465
6
14
12 10
90 4
.
.
.
V
m
R
V
N
N
SENSE
ILIM
S
P
≤
××12.I
OUTMAX