LT8301MP Просмотр технического описания (PDF) - Analog Devices
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LT8301MP Datasheet PDF : 24 Pages
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LT8301
APPLICATIONS INFORMATION
For higher voltage outputs, such as 12V and 24V, the
output diode temperature coefficient has a negligible
effect on the output voltage regulation. For lower voltage
outputs, such as 3.3V and 5V, however, the output diode
temperature coefficient does count for an extra 2% to 5%
output voltage regulation. For customers requiring tight
output voltage regulation across temperature, please refer
to other ADI parts with integrated temperature compensa-
tion features.
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-evalua-
tion 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 accu-
racy 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 rela-
tively 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 discontinu-
ous 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 addition, 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)
Rev. B
For more information www.analog.com
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