LT3575EFE#PBF Linear Technology, LT3575EFE#PBF Datasheet - Page 10

• Manufacturer Part Number: LT3575EFE#PBF
• Description: IC FLYBACK CONVERTER 16TSSOP
• Manufacturer: Linear Technology
• Type: Flyback, Isolatedr

Specifications of LT3575EFE#PBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adj to 60V
• Current - Output: 2.5A
• Frequency - Switching: 1MHz
• Voltage - Input: 3 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: *
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Voltage - Supply: 3 V ~ 40 V
• Frequency-max: 1MHz
• Pwm Type: Regulator
• Buck: No
• Boost: No
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: Yes
• Primary Input Voltage: 40V
• No. Of Outputs: 1
• Output Current: 3.5A
• No. Of Pins: 16
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 1 - Unlimited
• Peak Reflow Compatible (260 C): Yes
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Duty Cycle: -

APPLICATIONS INFORMATION
LT3575
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 between
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
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
10
V
V
V
14
10
Figure 1. Output Power for 3.3V Output
12
OUT
OUT
OUT
8
6
4
2
0
3.3
3.3
3.3
12
15
10
0
5
5
5
(V)
(V)
(V)
5
10
10:1
MAX P
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
PS
PS
PS
OUT
20
7:1
25
5:1
4:1
30
R
R
R
3:1
FB
FB
FB
37.4
56.2
80.6
76.8
130
162
165
113
56
2:1
35
MAXIMUM
OUTPUT
POWER
1:1
(kΩ)
(kΩ)
(kΩ)
40
3575 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)
14
10
12
8
6
4
2
0
Figure 2. Output Power for 5V Output
0
R
R
R
5
TC
TC
TC
18.7
66.5
80.6
28.7
54.9
19.1
28
20
28
MAX P
5:1
(kΩ)
(kΩ)
(kΩ)
10
7:1
INPUT VOLTAGE (V)
OUT
15
20
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 ten watts at 30V
but lowers to eight watts at 20V.
TRANSFORMER DESIGN CONSIDERATIONS
Transformer specifi cation and design is perhaps the most
critical part of successfully applying the LT3575. In addition
to the usual list of caveats dealing with high frequency
isolated power supply transformer design, the following
information should be carefully considered.
Linear Technology has worked with several leading magnetic
component manufacturers to produce pre-designed fl yback
transformers for use with the LT3575. Table 5 shows the
details of several of these transformers.
25
4:1
3:1
30
2:1
MAXIMUM
OUTPUT
POWER
35
1:1
40
3573 F02
45
14
10
12
Figure 3. Output Power for 12V Output
8
6
4
2
0
0
MAXIMUM
OUTPUT
POWER
5
10
INPUT VOLTAGE (V)
15
3:1
20
2:1
25
30
MAX P
1:1
35
OUT
3573 F03
3575f
40