Datasheet LT8300 (Analog Devices) - 9

LT8300
APPLICATIONS INFORMATION Selecting Actual RFB Resistor Value Output Power
The LT8300 uses a unique sampling scheme to regulate A flyback converter has a complicated relationship the isolated output voltage. Due to the sampling nature, between the input and output currents compared to a the scheme contains repeatable delays and error sources, buck or a boost converter. A boost converter has a rela- which will affect the output voltage and force a re-evalua- tively constant maximum input current regardless of input tion of the RFB resistor value. Therefore, a simple two-step voltage and a buck converter has a relatively constant process is required to choose feedback resistor RFB. maximum output current regardless of input voltage. This Rearrangement of the expression for V is due to the continuous non-switching behavior of the OUT in the Output Voltage section yields the starting value for R two currents. A flyback converter has both discontinu- FB: ous input and output currents which make it similar to N ( ) a non-isolated buck-boost converter. The duty cycle will R PS • VOUT + VF FB = affect the input and output currents, making it hard to 100µA predict output power. In addition, the winding ratio can VOUT = Output voltage be changed to multiply the output current at the expense V of a higher switch voltage. F = Output diode forward voltage = ~0.3V N The graphs in Figures 1 to 4 show the typical maximum PS = Transformer effective primary-to-secondary turns ratio output power possible for the output voltages 3.3V, 5V, 12V, and 24V. The maximum output power curve is the Power up the application with the starting RFB value and calculated output power if the switch voltage is 120V dur- other components connected, and measure the regulated ing the switch-off time. 30V of margin is left for leakage output voltage, VOUT(MEAS). The final RFB value can be inductance voltage spike. To achieve this power level at adjusted to: a given input, a winding ratio value must be calculated V to stress the switch to 120V, resulting in some odd ratio R OUT FB(FINAL) = •RFB values. The curves below the maximum output power V OUT(MEAS) curve are examples of common winding ratio values and Once the final R the amount of output power at given input voltages. FB value is selected, the regulation accu- racy from board to board for a given application will be One design example would be a 5V output converter with very consistent, typically under ±5% when including a minimum input voltage of 36V and a maximum input device variation of all the components in the system voltage of 72V. A six-to-one winding ratio fits this design (assuming resistor tolerances and transformer windings example perfectly and outputs equal to 2.44W at 72V but matching within ±1%). However, if the transformer or lowers to 1.87W at 36V. the output diode is changed, or the layout is dramatically altered, there may be some change in V The following equations calculate output power: OUT. POUT = η • VIN •D •ISW(MAX) •0.5 η = Efficiency = 85% (V D ) •N = Duty Cycle = OUT + VF PS (V  OUT + VF ) •NPS + VIN ISW(MAX) = Maximum switch current limit = 260mA Rev. A For more information www.analog.com 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Operation Applications Information Typical Applications Package Description Typical Application Related Parts