MAX5942BCSE Maxim Integrated, MAX5942BCSE Datasheet - Page 20

• Manufacturer Part Number: MAX5942BCSE
• Description: Power Switch ICs - POE / LAN
• Manufacturer: Maxim Integrated

IEEE 802.3af Power-Over-Ethernet
Interface/PWM Controller for Power Devices
6) Choose R
7) Choose the inductor value so that the peak ripple
Table 3. Component Suppliers
20
Power FETS
Current-Sense Resistors
Diodes
Capacitors
Magnetics
where:
V
voltage (0.465V).
N
example).
I
this example):
current (LIR) in the inductor is between 10% and
20% of the maximum output current:
where V
age drop (0.5V) and LIR is the ratio of inductor rip-
ple current to DC output current:
OUTMAX
ILIM
S
______________________________________________________________________________________
/N
P
is the current-sense comparator trip threshold
is the secondary-side turns ratio (5/14 in this
D
SENSE
is the maximum DC output current (10A in
L
R
R
is the output Schottky diode forward-volt-
≥
COMPONENT
SENSE
SENSE
L
0 4 275
≥
.
( )
5 5
2
according to the following equation:
.
×
×
(
V
≤
≤
LIR
OUT
×
N
N
14
(
6
1 0 198
kHz
S
P
×
0 465
-
×
275
.
+
×
1 2 10
.
1 2 . I
V
×
.
D
kHz I
V
10
ILIM
V
×
)
×
)
×
A
OUTMAX
(
×
1
=
=
-
OUTMAX
D
4 01
90 4
.
MIN
.
µ
m
)
H
International Rectifier
Fairchild
Vishay-Siliconix
Dale-Vishay
IRC
ON Semi
General Semiconductor
Central Semiconductor
Sanyo
Taiyo Yuden
AVX
Coiltronics
Coilcraft
Pulse Engineering
Ω
SUPPLIERS
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.
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
V
capacitive ripple. Calculate the ESR ripple and
capacitive ripple as follows:
RIPPLE
RIPPLE,ESR
V
RIPPLE,C
V
RIPPLE
is the combined RMS output ripple due to
Layout Recommendations
V
www.irf.com
www.fairchildsemi.com
www.vishay.com/brands/siliconix/main.html
www.vishay.com/brands/dale/main.html
www.irctt.com/pages/index.cfm
www.onsemi.com
www.gensemi.com
www.centralsemi.com
www.sanyo.com
www.t-yuden.com
www.avxcorp.com
www.cooperet.com
www.coilcraft.com
www.pulseeng.com
, the ESR ripple, and V
RIPPLE,ESR
=
= I
RIPPLE
V
RIPPLE ESR
2
/(2 x π x 275kHz x C
= I
,
RIPPLE
WEBSITE
+
V
RIPPLE C
2
x ESR
RIPPLE,C
,
OUT
, the
)