LT8302MPS8E 查看數據表（PDF） - Linear Technology

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LT8302MPS8E

42VIN Micropower No-Opto Isolated Flyback Converter with 65V/3.6A Switch

Linear Technology 

LT8302

Applications Information

Step 6: Select the RREF and RFB resistors.

Use the following equation to calculate the starting values

for RREF and RFB:

( ) ( ) RFB

=

RREF

•

NPS

• VOUT

VREF

+

VF

TO

RREF = 10k

Example:

RFB

=

10k

•

3

• (5V +

1.00V

0.3V)

=

159k

For 1% standard values, a 158k resistor is chosen.

Step 7: Adjust RFB resistor based on output voltage.

Build and power up the application with application com-

ponents and measure the regulated output voltage. Adjust

RFB resistor based on the measured output voltage:

RFB(NEW)

=

VOUT

VOUT(MEASURED)

• RFB

Example:

RFB

=

5V

5.14V

•

158k

=

154k

Step 8: Select RTC resistor based on output voltage

temperature variation.

Measure output voltage in a controlled temperature envi-

ronment like an oven to determine the output temperature

coefficient. Measure output voltage at a consistent load

current and input voltage, across the operating tempera-

ture range.

Calculate the temperature coefficient of VF:

– (δVF /δT )

=

VOUT

(T1) –

T1–

VOUT (T2)

T2

RTC

=

3.35mV/°C

– (δVF /δT )

•

⎛

⎝⎜

RFB

NPS

⎞

⎠⎟

Example:

– (δVF / δT )

=

5.189V

100°C

–

–

5.041V

(0°C)

=

1.48mV

/

°C

RTC

=

3.35mV/°C

1.48mV/°C

•

⎛⎝⎜

1534 ⎞⎠⎟

=

115k

Step 9: Select the EN/UVLO resistors.

Determine the amount of hysteresis required and calculate

R1 resistor value:

VIN(HYS) = 2.5µA • R1

Example:

Choose 2V of hysteresis, R1 = 806k

Determine the UVLO thresholds and calculate R2 resistor

value:

VIN(UVLO+)

=

1.228V

• (R1+

R2

R2)

+

2.5µA

•

R1

Example:

Set VIN UVLO rising threshold to 7.5V:

R2 = 232k

VIN(UVLO+) = 7.5V

VIN(UNLO–) = 5.5V

Step 10: Ensure minimum load.

The theoretical minimum load can be approximately

estimated as:

ILOAD(MIN)

=

9µH • (0.96A)2

2 • 5V

• 12.7kHz

=10.5mA

Remember to check the minimum load requirement in

real application. The minimum load occurs at the point

where the output voltage begins to climb up as the con-

verter delivers more energy than what is consumed at

the output. The real minimum load for this application is

about 10mA. In this example, a 500Ω resistor is selected

as the minimum load.

8302fa

20

For more information www.linear.com/LT8302

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