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LT8301H 查看數據表(PDF) - Linear Technology

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LT8301H Datasheet PDF : 24 Pages
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LT8301
APPLICATIONS INFORMATION
Selecting Actual RFB Resistor Value
The LT8301 uses a unique sampling scheme to regulate
the isolated output voltage. Due to the sampling nature,
the scheme contains repeatable delays and error sources,
which will affect the output voltage and force a re-evaluation
of the RFB resistor value. Therefore, a simple two-step
process is required to choose feedback resistor RFB.
Rearrangement of the expression for VOUT in the Output
Voltage section yields the starting value for RFB:
( ) RFB
=
NPS
• VOUT +
100µA
VF
VOUT = Output voltage
VF = Output diode forward voltage = ~0.3V
NPS = Transformer effective primary-to-secondary
turns ratio
Power up the application with the starting RFB value and
other components connected, and measure the regulated
output voltage, VOUT(MEAS). The final RFB value can be
adjusted to:
RFB(FINAL )
=
VOUT
VOUT(MEAS)
• RFB
Once the final RFB value is selected, the regulation accuracy
from board to board for a given application will be very
consistent, typically under ±5% when including device
variation of all the components in the system (assuming
resistor tolerances and transformer windings 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.
Output Power
A flyback converter has a complicated relationship between
the input and output currents compared to a buck or a
boost converter. A boost converter has a relatively constant
maximum input current regardless of input voltage and a
buck converter has a relatively constant maximum output
current regardless of input voltage. This is due to the
continuous non-switching behavior of the two currents. A
flyback converter has both discontinuous input and output
currents which make it similar to a non-isolated buck-boost
converter. The duty cycle will affect the input and output
currents, making it hard to predict output power. In ad-
dition, the winding ratio can be changed to multiply the
output current at the expense of a higher switch voltage.
The graphs in Figures 1 to 4 show the typical maximum
output power possible for the output voltages 3.3V, 5V,
12V, and 24V. The maximum output power curve is the
calculated output power if the switch voltage is 50V dur-
ing the switch-off time. 15V of margin is left for leakage
inductance voltage spike. To achieve this power level at
a given input, a winding ratio value must be calculated to
stress the switch to 50V, resulting in some odd ratio values.
The curves below the maximum output power curve are
examples of common winding ratio values and the amount
of output power at given input voltages.
One design example would be a 5V output converter with
a minimum input voltage of 8V and a maximum input volt-
age of 32V. A three-to-one winding ratio fits this design
example perfectly and outputs equal to 5.42W at 32V but
lowers to 2.71W at 8V.
The following equations calculate output power:
POUT = η• VIN •D •ISW(MAX) • 0.5
η = Efficiency = 85%
( ( ) ) D = DutyCycle =
VOUT + VF •NPS
VOUT + VF •NPS + VIN
ISW(MAX) = Maximum switch current limit = 1.2A (min)
8301f
10
For more information www.linear.com/LT8301

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