MAX5941ACSE Maxim Integrated, MAX5941ACSE Datasheet - Page 19

• Manufacturer Part Number: MAX5941ACSE
• Description: Power Switch ICs - POE / LAN
• Manufacturer: Maxim Integrated

4) The reset winding turns ratio (N
5) Choose the tertiary winding turns ratio (N
low enough to guarantee that the entire energy in
the transformer is returned to V+ within the off cycle
at the maximum duty cycle. Use the following equa-
tion to determine the reset winding turns ratio:
where:
N
D
Round N
The turns ratio of the reset winding (N
mines the peak voltage across the N-channel MOS-
FET.
Use the following equation to determine the maxi-
mum drain-source voltage across the N-channel
MOSFET:
V
V
Choose MOSFETs with appropriate avalanche
power ratings to absorb any leakage energy.
the minimum input voltage provides the minimum
operating voltage at V
equation to calculate the tertiary winding turns ratio:
where:
V
V
DSMAX
IN_MAX
R
MAX
DDMIN
DDMAX
/N
P
’ = Maximum value of maximum duty cycle:
IEEE 802.3af-Compliant Power-Over-Ethernet
= Reset winding turns ratio.
Interface/PWM Controller for Power Devices
is the minimum V
= Maximum MOSFET drain-source voltage.
R
V
= Maximum input voltage:
is the maximum V
V
DSMAX
to the nearest smallest integer.
DSMAX
______________________________________________________________________________________
N
N
V
V
R
≥
R
DDMIN
DDMAX
≥
≤
67
≤
V
V
V
IN_MIN
IN_MAX
1
N
IN_MAX
V
4
P
×
×
DD
×
+
+



1- 0.5
0 7
1-D
DD
1 +
0 7
0.5
D
.
DD
(13V). Use the following
.
MAX
×
supply voltage (13V).
MAX
×
×
supply voltage (30V).



N
14
14
=
1 +
N
′
P
14
P
′
R

 =

≤
/N
N
N
N
134
T
P
P
R
) needs to be
≤



R
V
T
/N
/N
P
P
) so that
) deter-
6) Choose R
7) Choose the inductor value so that the peak ripple
8) The size and ESR of the output filter capacitor deter-
Choose N
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:
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:
V
V
design example).
N
N
OUTMAX
ILIM
IN_MIN
IN_MAX
S
P
T
/N
is the number of turns of the tertiary winding:
is the number of turns of the primary winding.
P
is the current-sense comparator trip threshold
V
is the secondary side turns ratio (5/14 in this
RIPPLE
D
is the minimum input voltage (30V).
L
SENSE
is the maximum input voltage (67V in this
is the maximum DC output current (10A in
T
R
R
is the output Schottky diode forward volt-
≥
= 7.
SENSE
SENSE
L
0 4 275
( )
.
≥
=
5 5
.
6 39
2
×
13 7
according to the following equation:
30
.
×
V
(
.
≤
V
×
RIPPLE ESR
≤
LIR
2
OUT
(
≤
N
N
14
1 0 198
×
kHz
6
-
N
1
S
P
×
0 465
4
T
×
275
×
.
.
+
≤
,
1 2 10
×
≤
1 2 . I
V
.
N
10
7 67
D
V
kHz I
T
.
ILIM
V
×
)
)
A
×
+
≤
×
OUTMAX
=
V
(
36 7
×
1
RIPPLE C
2
67
4 01
=
-
OUTMAX
.
D
.
90 4
MIN
×
µ
.
14
H
,
m
)
Ω
19