lt3573 Linear Technology Corporation, lt3573 Datasheet - Page 10

• Manufacturer Part Number: lt3573
• Description: Isolated Flyback Converter Without An Opto-coupler
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3573
Table 2. Common Resistor Values for 2:1 Transformers
Table 3. Common Resistor Values for 3:1 Transformers
Table 4. Common Resistor Values for 4:1 Transformers
Output Power
A fl yback converter has a complicated relationship be-
tween the input and output current compared to a buck
or a boost. A boost has a relatively constant maximum
input current regardless of input voltage and a buck has a
10
Figure 1. Output Power for 3.3V Output
V
V
V
OUT
OUT
OUT
8
6
7
5
4
3
2
1
0
3.3
3.3
3.3
12
15
10
5
5
5
0
N = 3:1
(V)
(V)
(V)
5
10
N = 1:1
INPUT VOLTAGE (V)
15
2.00
2.00
2.00
2.00
3.00
3.00
3.00
4.00
4.00
N
N
N
N = 2:1
PS
PS
PS
20
25
30
R
R
R
MAXIMUM
OUTPUT
POWER
FB
FB
FB
37.4
56.2
80.6
76.8
130
162
165
113
56
35
(kΩ)
(kΩ)
(kΩ)
40
3573 F01
45
R
R
R
REF
REF
REF
6.04
6.04
6.04
6.04
6.04
6.04
6.04
6.04
6.04
(kΩ)
(kΩ)
(kΩ)
Figure 2. Output Power for 5V Output
8
6
5
4
7
3
2
1
0
0
N = 7:1
R
R
R
5
TC
TC
TC
18.7
66.5
80.6
28.7
54.9
19.1
28
20
28
(kΩ)
(kΩ)
(kΩ)
10
N = 5:1
INPUT VOLTAGE (V)
15
N = 2:1
20
N = 3:1
relatively constant maximum output current regardless of
input voltage. This is due to the continuous nonswitching
behavior of the two currents. A fl yback converter has both
discontinuous input and output currents which makes it
similar to a nonisolated buck-boost. The duty cycle will
affect the input and output currents, making it hard to
predict output power. In addition, the winding ratio can
be changed to multiply the output current at the expense
of a higher switch voltage.
The graphs in Figures 1-3 show the maximum output
power possible for the output voltages 3.3V, 5V, and 12V.
The maximum power output curve is the calculated output
power if the switch voltage is 50V during the off-time. To
achieve this power level at a given input, a winding ratio
value must be calculated to stress the switch to 50V,
resulting in some odd ratio values. The curves below are
examples of common winding ratio values and the amount
of output power at given input voltages.
One design example would be a 5V output converter with
a minimum input voltage of 20V and a maximum input
voltage of 30V. A three-to-one winding ratio fi ts this design
example perfectly and outputs close to six watts at 30V
but lowers to fi ve watts at 20V.
25
30
MAXIMUM
OUTPUT
POWER
35
40
3573 F02
45
Figure 3. Output Power for 12V Output
8
6
5
4
7
3
2
1
0
0
N = 3:1
5
10
N = 2:1
INPUT VOLTAGE (V)
15
N = 1:1
20
25
30
MAXIMUM
OUTPUT
POWER
35
40
3573 F03
45
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