Datasheet LT3573 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | Isolated Flyback Converter without an Opto-Coupler |
| Pages / Page | 26 / 10 — APPLICATIONS INFORMATION. Table 2. Common Resistor Values for 2:1 … |
| File Format / Size | PDF / 282 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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LT3573
APPLICATIONS INFORMATION Table 2. Common Resistor Values for 2:1 Transformers
relatively constant maximum output current regardless of
VOUT (V) NPS RFB (kΩ) RREF (kΩ) RTC (kΩ)
input voltage. This is due to the continuous nonswitching 3.3 2.00 37.4 6.04 18.7 behavior of the two currents. A flyback converter has both 5 2.00 56 6.04 28 discontinuous input and output currents which makes it 12 2.00 130 6.04 66.5 similar to a nonisolated buck-boost. The duty cycle will 15 2.00 162 6.04 80.6 affect the input and output currents, making it hard to predict output power. In addition, the winding ratio can
Table 3. Common Resistor Values for 3:1 Transformers
be changed to multiply the output current at the expense
VOUT (V) NPS RFB (kΩ) RREF (kΩ) RTC (kΩ)
of a higher switch voltage. 3.3 3.00 56.2 6.04 20 The graphs in Figures 1-3 show the maximum output 5 3.00 80.6 6.04 28.7 power possible for the output voltages 3.3V, 5V, and 12V. 10 3.00 165 6.04 54.9 The maximum power output curve is the calculated output power if the switch voltage is 50V during the off-time. To
Table 4. Common Resistor Values for 4:1 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 4.00 76.8 6.04 19.1 resulting in some odd ratio values. The curves below are 5 4.00 113 6.04 28 examples of common winding ratio values and the amount of output power at given input voltages.
Output Power
One design example would be a 5V output converter with A flyback converter has a complicated relationship be- a minimum input voltage of 20V and a maximum input tween the input and output current compared to a buck voltage of 30V. A three-to-one winding ratio fits this design or a boost. A boost has a relatively constant maximum example perfectly and outputs close to six watts at 30V input current regardless of input voltage and a buck has a but lowers to five watts at 20V. 8 8 8 MAXIMUM MAXIMUM N = 5:1 N = 2:1 MAXIMUM 7 OUTPUT 7 OUTPUT 7 POWER OUTPUT POWER POWER 6 6 6 N = 7:1 N = 10:1 N = 3:1 5 N = 7:1 N = 4:1 5 N = 3:1 5 4 4 4 N = 1:1 N = 3:1 N = 2:1 3 3 3 OUTPUT POWER (W) OUTPUT POWER (W) OUTPUT POWER (W) 2 2 2 1 1 1 0 0 0 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 30 35 40 45 INPUT VOLTAGE (V) INPUT VOLTAGE (V) INPUT VOLTAGE (V) 3573 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
3573fd 10 For more information www.linear.com/LT3573 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts
APPLICATIONS INFORMATION Table 2. Common Resistor Values for 2:1 Transformers
relatively constant maximum output current regardless of
VOUT (V) NPS RFB (kΩ) RREF (kΩ) RTC (kΩ)
input voltage. This is due to the continuous nonswitching 3.3 2.00 37.4 6.04 18.7 behavior of the two currents. A flyback converter has both 5 2.00 56 6.04 28 discontinuous input and output currents which makes it 12 2.00 130 6.04 66.5 similar to a nonisolated buck-boost. The duty cycle will 15 2.00 162 6.04 80.6 affect the input and output currents, making it hard to predict output power. In addition, the winding ratio can
Table 3. Common Resistor Values for 3:1 Transformers
be changed to multiply the output current at the expense
VOUT (V) NPS RFB (kΩ) RREF (kΩ) RTC (kΩ)
of a higher switch voltage. 3.3 3.00 56.2 6.04 20 The graphs in Figures 1-3 show the maximum output 5 3.00 80.6 6.04 28.7 power possible for the output voltages 3.3V, 5V, and 12V. 10 3.00 165 6.04 54.9 The maximum power output curve is the calculated output power if the switch voltage is 50V during the off-time. To
Table 4. Common Resistor Values for 4:1 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 4.00 76.8 6.04 19.1 resulting in some odd ratio values. The curves below are 5 4.00 113 6.04 28 examples of common winding ratio values and the amount of output power at given input voltages.
Output Power
One design example would be a 5V output converter with A flyback converter has a complicated relationship be- a minimum input voltage of 20V and a maximum input tween the input and output current compared to a buck voltage of 30V. A three-to-one winding ratio fits this design or a boost. A boost has a relatively constant maximum example perfectly and outputs close to six watts at 30V input current regardless of input voltage and a buck has a but lowers to five watts at 20V. 8 8 8 MAXIMUM MAXIMUM N = 5:1 N = 2:1 MAXIMUM 7 OUTPUT 7 OUTPUT 7 POWER OUTPUT POWER POWER 6 6 6 N = 7:1 N = 10:1 N = 3:1 5 N = 7:1 N = 4:1 5 N = 3:1 5 4 4 4 N = 1:1 N = 3:1 N = 2:1 3 3 3 OUTPUT POWER (W) OUTPUT POWER (W) OUTPUT POWER (W) 2 2 2 1 1 1 0 0 0 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 30 35 40 45 INPUT VOLTAGE (V) INPUT VOLTAGE (V) INPUT VOLTAGE (V) 3573 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
3573fd 10 For more information www.linear.com/LT3573 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts
