Datasheet LT8301 (Analog Devices) - 9

LT8301
APPLICATIONS INFORMATION
For higher voltage outputs, such as 12V and 24V, the
Output Power
output diode temperature coefficient has a negligible A flyback converter has a complicated relationship effect on the output voltage regulation. For lower voltage between the input and output currents compared to a outputs, such as 3.3V and 5V, however, the output diode buck or a boost converter. A boost converter has a rela- temperature coefficient does count for an extra 2% to 5% tively constant maximum input current regardless of input output voltage regulation. For customers requiring tight voltage and a buck converter has a relatively constant output voltage regulation across temperature, please refer maximum output current regardless of input voltage. This to other ADI parts with integrated temperature compensa- is due to the continuous non-switching behavior of the tion features. two currents. A flyback converter has both discontinu-
Selecting Actual R
ous input and output currents which make it similar to
FB Resistor Value
a non-isolated buck-boost converter. The duty cycle will The LT8301 uses a unique sampling scheme to regulate affect the input and output currents, making it hard to the isolated output voltage. Due to the sampling nature, predict output power. In addition, the winding ratio can the scheme contains repeatable delays and error sources, be changed to multiply the output current at the expense which will affect the output voltage and force a re-evalua- of a higher switch voltage. tion of the RFB resistor value. Therefore, a simple two-step process is required to choose feedback resistor R The graphs in Figures 1 to 4 show the typical maximum FB. output power possible for the output voltages 3.3V, 5V, Rearrangement of the expression for VOUT in the Output 12V, and 24V. The maximum output power curve is the Voltage section yields the starting value for RFB: calculated output power if the switch voltage is 50V dur- ing the switch-off time. 15V of margin is left for leakage N ( ) R PS • VOUT + VF inductance voltage spike. To achieve this power level at FB = 100µA a given input, a winding ratio value must be calculated V to stress the switch to 50V, resulting in some odd ratio OUT = Output voltage values. The curves below the maximum output power VF = Output diode forward voltage = ~0.3V curve are examples of common winding ratio values and N the amount of output power at given input voltages. PS = Transformer effective primary-to-secondary turns ratio One design example would be a 5V output converter with Power up the application with the starting R a minimum input voltage of 8V and a maximum input volt- FB value and other components connected, and measure the regulated age of 32V. A three-to-one winding ratio fits this design output voltage, V example perfectly and outputs equal to 5.42W at 32V but OUT(MEAS). The final RFB value can be adjusted to: lowers to 2.71W at 8V. V The following equations calculate output power: R OUT FB(FINAL) = •R V FB OUT(MEAS) POUT = η• VIN •D•ISW(MAX) •0.5 Once the final R η=Efficiency = ∼85% FB value is selected, the regulation accu- racy from board to board for a given application will be (VOUT +VF)•NPS very consistent, typically under ±5% when including D=DutyCycle = (V )•N device variation of all the components in the system OUT + VF PS + VIN (assuming resistor tolerances and transformer windings ISW(MAX) = Maximum switch current limit = 1.2A (min) matching within ±1%). However, if the transformer or the output diode is changed, or the layout is dramatically altered, there may be some change in VOUT. Rev. B 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 Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts