LT8300
applicaTions inForMaTion
Selecting Actual R Resistor Value
Output Power
FB
The LT8300 uses a unique sampling scheme to regulate
the isolated output voltage. Due to the sampling nature,
the scheme contains repeatable delays and error sources,
whichwillaffecttheoutputvoltageandforceare-evaluation
Aflybackconverterhasacomplicatedrelationshipbetween
the input and output currents compared to a buck or a
boostconverter.Aboostconverterhasarelativelyconstant
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
flybackconverterhasbothdiscontinuousinputandoutput
currentswhichmakeitsimilartoanon-isolatedbuck-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.
of the R resistor value. Therefore, a simple two-step
FB
process is required to choose feedback resistor R .
FB
Rearrangement of the expression for V
in the Output
FB
OUT
Voltage section yields the starting value for R :
NPS • V
+ VF
(
)
OUT
RFB =
100µA
V
OUT
= Output voltage
V = Output diode forward voltage = ~0.3V
F
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 120V dur-
ing the switch-off time. 30V 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 120V, 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.
N
PS
= Transformer effective primary-to-secondary
turns ratio
Power up the application with the starting R value and
FB
other components connected, and measure the regulated
output voltage, V
adjusted to:
. The final R value can be
OUT(MEAS)
FB
VOUT
VOUT(MEAS)
RFB(FINAL)
=
•RFB
OncethefinalR valueisselected,theregulationaccuracy
FB
One design example would be a 5V output converter with
a minimum input voltage of 36V and a maximum input
voltage of 72V. A six-to-one winding ratio fits this design
example perfectly and outputs equal to 2.44W at 72V but
lowers to 1.87W at 36V.
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,
The following equations calculate output power:
there may be some change in V
.
OUT
POUT = η • VIN •D•ISW(MAX) • 0.5
η = Efficiency = 85%
V
+ V •N
F
PS
(
)
OUT
D = DutyCycle =
V
+ V •N + V
F IN
PS
(
)
OUT
I
= Maximum switch current limit = 260mA
SW(MAX)
8300f
10
Linear [ Linear ]
