Datasheet LT3575 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | Isolated Flyback Converter without an Opto-Coupler |
| Pages / Page | 24 / 10 — APPLICATIONS INFORMATION. Table 2. Common Resistor Values for 2:1 … |
| File Format / Size | PDF / 251 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Table 2. Common Resistor Values for 2:1 Transformers. VOUT (V). NPS. RFB (kΩ). RREF (kΩ). RTC (kΩ)
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Text Version of Document
LT3575
APPLICATIONS INFORMATION Table 2. Common Resistor Values for 2:1 Transformers
predict output power. In addition, the winding ratio can
VOUT (V) NPS RFB (kΩ) RREF (kΩ) RTC (kΩ)
be changed to multiply the output current at the expense 3.3 2.00 37.4 6.04 18.7 of a higher switch voltage. 5 2.00 56 6.04 28 The graphs in Figures 1-3 show the maximum output 12 2.00 130 6.04 66.5 power possible for the output voltages 3.3V, 5V, and 12V. 15 2.00 162 6.04 80.6 The maximum power output curve is the calculated output power if the switch voltage is 50V during the off-time. To
Table 3. Common Resistor Values for 3:1 Transformers
achieve this power level at a given input, a winding ratio
VOUT (V) NPS RFB (kΩ) RREF (kΩ) RTC (kΩ)
value must be calculated to stress the switch to 50V, 3.3 3.00 56.2 6.04 20 resulting in some odd ratio values. The curves below are 5 3.00 80.6 6.04 28.7 examples of common winding ratio values and the amount 10 3.00 165 6.04 54.9 of output power at given input voltages. One design example would be a 5V output converter with
Table 4. Common Resistor Values for 4:1 Transformers
a minimum input voltage of 20V and a maximum input
VOUT (V) NPS RFB (kΩ) RREF (kΩ) RTC (kΩ)
voltage of 30V. A three-to-one winding ratio fi ts this design 3.3 4.00 76.8 6.04 19.1 example perfectly and outputs close to ten watts at 30V 5 4.00 113 6.04 28 but lowers to eight watts at 20V.
Output Power TRANSFORMER DESIGN CONSIDERATIONS
A fl yback converter has a complicated relationship between Transformer specifi cation and design is perhaps the most the input and output current compared to a buck or a critical part of successfully applying the LT3575. In addition boost. A boost has a relatively constant maximum input to the usual list of caveats dealing with high frequency current regardless of input voltage and a buck has a isolated power supply transformer design, the following relatively constant maximum output current regardless of information should be carefully considered. input voltage. This is due to the continuous nonswitching behavior of the two currents. A fl yback converter has both Linear Technology has worked with several leading magnetic discontinuous input and output currents which makes it component manufacturers to produce pre-designed fl yback similar to a nonisolated buck-boost. The duty cycle will transformers for use with the LT3575. Table 5 shows the affect the input and output currents, making it hard to details of several of these transformers. 14 14 14 MAXIMUM MAXIMUM MAXIMUM MAX P MAX P OUT OUT MAX P OUTPUT OUTPUT OUTPUT OUT 12 12 12 7:1 POWER 4:1 POWER POWER 10:1 7:1 2:1 5:1 10 10 5:1 3:1 10 4:1 3:1 8 3:1 8 2:1 8 1:1 6 2:1 6 6 1:1 OUTPUT POWER (W) 4 OUTPUT POWER (W) 4 OUTPUT POWER (W) 4 1:1 2 2 2 0 0 0 0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 30 35 40 INPUT VOLTAGE (V) INPUT VOLTAGE (V) INPUT VOLTAGE (V) 3575 F01 3573 F02 3573 F03
Figure 1. Output Power for 3.3V Output Figure 2. Output Power for 5V Output Figure 3. Output Power for 12V Output
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APPLICATIONS INFORMATION Table 2. Common Resistor Values for 2:1 Transformers
predict output power. In addition, the winding ratio can
VOUT (V) NPS RFB (kΩ) RREF (kΩ) RTC (kΩ)
be changed to multiply the output current at the expense 3.3 2.00 37.4 6.04 18.7 of a higher switch voltage. 5 2.00 56 6.04 28 The graphs in Figures 1-3 show the maximum output 12 2.00 130 6.04 66.5 power possible for the output voltages 3.3V, 5V, and 12V. 15 2.00 162 6.04 80.6 The maximum power output curve is the calculated output power if the switch voltage is 50V during the off-time. To
Table 3. Common Resistor Values for 3:1 Transformers
achieve this power level at a given input, a winding ratio
VOUT (V) NPS RFB (kΩ) RREF (kΩ) RTC (kΩ)
value must be calculated to stress the switch to 50V, 3.3 3.00 56.2 6.04 20 resulting in some odd ratio values. The curves below are 5 3.00 80.6 6.04 28.7 examples of common winding ratio values and the amount 10 3.00 165 6.04 54.9 of output power at given input voltages. One design example would be a 5V output converter with
Table 4. Common Resistor Values for 4:1 Transformers
a minimum input voltage of 20V and a maximum input
VOUT (V) NPS RFB (kΩ) RREF (kΩ) RTC (kΩ)
voltage of 30V. A three-to-one winding ratio fi ts this design 3.3 4.00 76.8 6.04 19.1 example perfectly and outputs close to ten watts at 30V 5 4.00 113 6.04 28 but lowers to eight watts at 20V.
Output Power TRANSFORMER DESIGN CONSIDERATIONS
A fl yback converter has a complicated relationship between Transformer specifi cation and design is perhaps the most the input and output current compared to a buck or a critical part of successfully applying the LT3575. In addition boost. A boost has a relatively constant maximum input to the usual list of caveats dealing with high frequency current regardless of input voltage and a buck has a isolated power supply transformer design, the following relatively constant maximum output current regardless of information should be carefully considered. input voltage. This is due to the continuous nonswitching behavior of the two currents. A fl yback converter has both Linear Technology has worked with several leading magnetic discontinuous input and output currents which makes it component manufacturers to produce pre-designed fl yback similar to a nonisolated buck-boost. The duty cycle will transformers for use with the LT3575. Table 5 shows the affect the input and output currents, making it hard to details of several of these transformers. 14 14 14 MAXIMUM MAXIMUM MAXIMUM MAX P MAX P OUT OUT MAX P OUTPUT OUTPUT OUTPUT OUT 12 12 12 7:1 POWER 4:1 POWER POWER 10:1 7:1 2:1 5:1 10 10 5:1 3:1 10 4:1 3:1 8 3:1 8 2:1 8 1:1 6 2:1 6 6 1:1 OUTPUT POWER (W) 4 OUTPUT POWER (W) 4 OUTPUT POWER (W) 4 1:1 2 2 2 0 0 0 0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 30 35 40 INPUT VOLTAGE (V) INPUT VOLTAGE (V) INPUT VOLTAGE (V) 3575 F01 3573 F02 3573 F03
Figure 1. Output Power for 3.3V Output Figure 2. Output Power for 5V Output Figure 3. Output Power for 12V Output
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