Linear Technology Analog Devices LT8301 Datasheet page 9

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 R Resistor Value
FB
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 R resistor value. Therefore, a simple two-step
FB
process is required to choose feedback resistor R
Rearrangement of the expression for V
Voltage section yields the starting value for R
(
N • V + V
PS OUT
R
=
FB
100µA
V = Output voltage
OUT
V = Output diode forward voltage = ~0.3V
F
N = Transformer effective primary-to-secondary
PS
turns ratio
Power up the application with the starting R
other components connected, and measure the regulated
output voltage, V
OUT(MEAS)
adjusted to:
V
OUT
R
=
FB(FINAL)
V
OUT(MEAS)
Once the final R value is selected, the regulation accu-
FB
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 V
FB
in the Output
OUT
:
FB
)
F
value and
FB
. The final R value can be
FB
• R
FB
.
OUT
For more information
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:
P = η • V
OUT IN
η = Efficiency = 85%
D = Duty Cycle =

I = Maximum switch current limit = 1.2A (min)
SW(MAX)
www.analog.com
• D •I • 0.5
SW(MAX)
( )
V + V • N
OUT F PS
( )
V + V • N + V
F IN
OUT PS
LT8301
Rev. A
9