Datasheet LT8303 (Analog Devices) - 9

LT8303
applicaTions inForMaTion Selecting Actual RFB Resistor Value Output Power
The LT8303 uses a unique sampling scheme to regulate A flyback converter has a complicated relationship between the isolated output voltage. Due to the sampling nature, the input and output currents compared to a buck or a the scheme contains repeatable delays and error sources, boost converter. A boost converter has a relatively constant which will affect the output voltage and force a re-evaluation maximum input current regardless of input voltage and a of the RFB resistor value. Therefore, a simple two-step buck converter has a relatively constant maximum output process is required to choose feedback resistor RFB. current regardless of input voltage. This is due to the Rearrangement of the expression for V continuous non-switching behavior of the two currents. A OUT in the Output Voltage section yields the starting value for R flyback converter has both discontinuous input and output FB: currents which make it similar to a non-isolated buck-boost N ( ) converter. The duty cycle will affect the input and output R PS • VOUT + VF currents, making it hard to predict output power. In ad- FB = 100µA dition, the winding ratio can be changed to multiply the VOUT = Output voltage output current at the expense of a higher switch voltage. VF = Output diode forward voltage = ~0.3V The graphs in Figures 1 to 4 show the typical maximum N output power possible for the output voltages 3.3V, 5V, PS = Transformer effective primary-to-secondary turns ratio 12V, and 24V. The maximum output power curve is the calculated output power if the switch voltage is 120V dur- Power up the application with the starting RFB value and ing the switch-off time. 30V of margin is left for leakage other components connected, and measure the regulated inductance voltage spike. To achieve this power level at output voltage, VOUT(MEAS). The final RFB value can be a given input, a winding ratio value must be calculated adjusted to: to stress the switch to 120V, resulting in some odd ratio V values. The curves below the maximum output power R OUT FB(FINAL) = •R V FB curve are examples of common winding ratio values and OUT(MEAS) the amount of output power at given input voltages. Once the final RFB value is selected, the regulation accuracy One design example would be a 5V output converter with from board to board for a given application will be very a minimum input voltage of 30V and a maximum input consistent, typically under ±5% when including device voltage of 80V. A six-to-one winding ratio fits this design variation of all the components in the system (assuming example perfectly and outputs equal to 4.35W at 80V but resistor tolerances and transformer windings matching lowers to 2.95W at 30V. within ±1%). However, if the transformer or the output The following equations calculate output power: diode is changed, or the layout is dramatically altered, there may be some change in VOUT. POUT = η• VIN •D •ISW(MAX) • 0.5 η = Efficiency = 85% (V )•N D = DutyCycle = OUT + VF PS (V  OUT + VF ) • NPS + VIN ISW(MAX) = Maximum switch current limit = 450mA 8303fa For more information www.linear.com/LT8303 9 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