Datasheet LT3574 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | Isolated Flyback Converter Without an Opto-Coupler |
| Pages / Page | 24 / 10 — Table 2. Common Resistor Values for 2:1 Transformers. VOUT (V). NPS. RFB … |
| File Format / Size | PDF / 296 Kb |
| Document Language | English |
Table 2. Common Resistor Values for 2:1 Transformers. VOUT (V). NPS. RFB (kΩ). RREF (kΩ). RTC (kΩ)
Model Line for this Datasheet
Text Version of Document
LT3574 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 of a higher switch voltage.
VOUT (V) NPS RFB (kΩ) RREF (kΩ) RTC (kΩ)
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
V
value must be calculated to stress the switch to 50V,
OUT (V) NPS RFB (kΩ) RREF (kΩ) RTC (kΩ)
resulting in some odd ratio values. The curves below are 3.3 4.00 76.8 6.04 19.1 examples of common winding ratio values and the amount 5 4.00 113 6.04 28 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 2.5W at 30V but input current regardless of input voltage and a buck has a lowers to 2W at 20V. 3.5 3.5 3.5 3.0 3.0 3.0 2.5 2.5 2.5 2.0 2.0 2.0 1.5 1.5 1.5 OUTPUT POWER (W) 1.0 OUTPUT POWER (W) 1.0 OUTPUT POWER (W) 1.0 0.5 0.5 0.5 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 45 INPUT VOLTAGE (V) 3574 F01 INPUT VOLTAGE (V) 3574 F02 INPUT VOLTAGE (V) 3574 F03 MAX POWER OUTPUT MAX POWER OUTPUT MAX POWER OUTPUT 1:1 5:1 1:1 4:1 1:1 2:1 7:1 2:1 5:1 2:1 3:1 10:1 3:1 7:1 3:1 4:1
Figure 1. Output Power for 3.3V Output Figure 2. Output Power for 5V Output Figure 3. Output Power for 12V Output
3574f 0 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 Typical Application Related Parts
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 of a higher switch voltage.
VOUT (V) NPS RFB (kΩ) RREF (kΩ) RTC (kΩ)
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
V
value must be calculated to stress the switch to 50V,
OUT (V) NPS RFB (kΩ) RREF (kΩ) RTC (kΩ)
resulting in some odd ratio values. The curves below are 3.3 4.00 76.8 6.04 19.1 examples of common winding ratio values and the amount 5 4.00 113 6.04 28 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 2.5W at 30V but input current regardless of input voltage and a buck has a lowers to 2W at 20V. 3.5 3.5 3.5 3.0 3.0 3.0 2.5 2.5 2.5 2.0 2.0 2.0 1.5 1.5 1.5 OUTPUT POWER (W) 1.0 OUTPUT POWER (W) 1.0 OUTPUT POWER (W) 1.0 0.5 0.5 0.5 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 45 INPUT VOLTAGE (V) 3574 F01 INPUT VOLTAGE (V) 3574 F02 INPUT VOLTAGE (V) 3574 F03 MAX POWER OUTPUT MAX POWER OUTPUT MAX POWER OUTPUT 1:1 5:1 1:1 4:1 1:1 2:1 7:1 2:1 5:1 2:1 3:1 10:1 3:1 7:1 3:1 4:1
Figure 1. Output Power for 3.3V Output Figure 2. Output Power for 5V Output Figure 3. Output Power for 12V Output
3574f 0 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 Typical Application Related Parts
