MAX5003EEE-T Maxim Integrated Products, MAX5003EEE-T Datasheet - Page 12

• Manufacturer Part Number: MAX5003EEE-T
• Description: Voltage Mode PWM Controllers PWM Power-Supply Controller
• Manufacturer: Maxim Integrated Products

Specifications of MAX5003EEE-T

• Topology: Flyback, Forward
• Output Current: 1000 mA
• Switching Frequency: 300 KHz
• Duty Cycle (max): 75 %
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Package / Case: QSOP-16
• Mounting Style: SMD/SMT
• Synchronous Pin: Yes

3) The main factors influencing the choice of the turns
High-Voltage PWM
Power-Supply Controller
Figure 2. Application Example 1: Nonisolated +48V to +5V Converter
12
0V
V
IN
ratio are the switch breakdown voltage and the duty
cycle. With a smaller turns ratio, the secondary
reflected voltage and the maximum voltage seen by
the switch during flyback are reduced, which is
favorable. On the other hand, a smaller turns ratio
will shorten the duty cycle and increase the primary
RMS current, which can impact efficiency. A good
starting figure is the ratio of the input voltage to the
output voltage, rounding to the nearest integer. To
keep the flyback voltage under control, choose an 8-
to-1 ratio for the 48V to 5V system. The maximum
duty cycle allowed without putting the device in con-
tinuous-conduction mode can be found using the fol-
lowing formula:
where:
N = N
V
DC
39k
1M
(36V TO 72V)
SEC
______________________________________________________________________________________
MAX
+48V
0.1µF
= Secondary voltage
P
/N
= Maximum duty cycle
S
DC
= Turns ratio
0.1µF
MAX
=



470nF
V
62k
SEC
V
MIN
1
×
N

 +

0.1µF
1
1
2
3
4
5
6
7
8
V+
INDIV
ES
FREQ
SS
REF
CON
COMP
390pF
C
F
MAX5003
MAXTON
200k
PGND
AGND
NDRV
R
V
V
F
CS
FB
DD
CC
16
15
14
13
12
11
10
9
10µF
51k
4) Assuming 80% efficiency, a 6.25W input is needed
XFACOILTRCTX03
L
PRI
V
For a 48V to 5V system with an 8-to-1 turns ratio, the
maximum duty cycle before putting the device in
discontinuous mode is 55%. Assume that V
36V (minimum input voltage, neglecting drops in the
power switch and in the resistance of the primary
coil) and V
drop). The MAX5003 maximum duty cycle is internal-
ly limited to 75%. Generally this parameter must fall
between 45% to 65% to obtain a balance between
efficiency and flyback voltage while staying out of
continuous conduction. If the value exceeds these
bounds, adjust the turns ratio.
to produce a 5W output. Set an operating duty cycle
around 12% below the maximum duty cycle to allow
for component variation: 55% - 12% = 43%. Use the
following formula to calculate the primary inductance:
33µF
MIN
=
100Ω
2
= Minimum power-line voltage
×
65µH
(
0.1Ω
0.1µF
R
DC V
PWR
L
CS
P
2
5
IRFD620S
×
SEC
IN
MIN
× ƒ
is 5.4V (5V plus a Schottky diode
)
8
7
11, 12
2
9, 10
SW
CMSD4448
MBRS130L
=
2 6 25
×
(
4.7µF
0 43 36
.
.
22µF
W
×
×
300
V
22µF
)
2
kHz
IN
≅
65
min is
R
41.2k
R
17.4k
A
B
µ
+5V
1A
H