LT3757 datasheet(9/24 Pages) AD

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LT8300

Rev. A

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APPLICATIONS INFORMATION

Selecting Actual RFB Resistor Value

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,

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 + VF

(

)

100µA

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 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.

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.

The following equations calculate output power:



POUT = η • VIN •D •ISW(MAX) •0.5

η = Efficiency =

85%

D = Duty Cycle =

VOUT + VF

(

) •NPS

VOUT + VF

(

) •NPS + VIN

ISW(MAX) = Maximum switch current limit = 260mA

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