Datasheet

where:
DC = Duty cycle. Set to calculated minimum duty
cycle at V
MIN.
PWR
IN
= Input power, at maximum output power
This gives an inductance value (L
PRI
) of approxi-
mately 65µH.
5) The other parameter that defines the transformer is
peak current. This is given by:
The peak secondary current is the peak primary cur-
rent multiplied by the turns ratio, or 0.8A · 8 = 6.4A.
Calculating the minimum duty cycle:
With these numbers, the transformer manufacturer
can choose a core.
6) For this application, the MAX5003 must be pro-
grammed for a maximum duty cycle of 55% at 36V.
The MAX5003 will automatically scale the limit with
the reciprocal of the input voltage as it changes. The
duty-cycle limit for an input voltage of 72V will be
27% (half of 55%). The duty cycle needed to stay out
of continuous conduction at 72V is 37%, so there is a
10% margin. The maximum duty time scales with the
voltage at the undervoltage lockout pin, V
INDIV
. The
voltage at INDIV is set by selecting the power line
undervoltage lockout trip point. The trip point for this
system, running from 36V to 72V, is 32V. Then INDIV
must be connected to the center point of a divider
with a ratio of 32/1.25, connected between the
power line and ground. Then R
MAXTON
is:
R
V
V
kHz
DC V
k
V
V
kHz
kHz
kk
MAXTON
MIN
UVL SW
MAX MIN
=






ƒ






()






=


















=
%
%
%
100
75
200
36
32
100
300
55
75
200 55
Ω
ΩΩ
DC DC
V
V
V
V
MIN MAX
IN MIN
IN MAX
() ( )
()
()
% =× =×=43
36
72
I
2 PWR
L
2 6.25W
65 H 300kHz
0.8A
PRI
IN
PRI SW
=
×
ƒ
==
×
×
×µ
MAX5003
High-Voltage PWM
Power-Supply Controller
______________________________________________________________________________________ 13
10µF
0.1µF
51k
1.3k
R
CS
0.1Ω
0.1µF
0.1µF
470nF
62k
V+
INDIV
ES
FREQ
SS
REF
CON
COMP
8
7
6
5
4
3
2
1
9
10
11
12
13
14
15
16
V
DD
V
CC
NDRV
PGND
CS
AGND
MDC217
IRFD620S
L
P
65µH
MAXTON
FB
R2
39k
R1
1M
33µF
0.1µF
100Ω
+5V
1A
-
51Ω
680Ω
240k
6
7
5
2
1
22µF
0.01µF
3900pF
24.9k
24.9k
TL431
4.7µF
22µF
MAX5003
V
IN
0V
XFACOILTRCTX03
CMSD4448
MBRS130L
8
2
5
7
9, 10
11, 12
-48V
-36V TO -72V
Figure 3. Application Example 2: Isolated -48V to +5V Converter