LT3757 datasheet(16/24 Pages) AD

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LT8300

Rev. A

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APPLICATIONS INFORMATION

Step 2: Determine the Primary Inductance.

Primary inductance for the transformer must be set above

a minimum value to satisfy the minimum switch-off and

switch-on time requirements:

LPRI ≥

tOFF(MIN) •NPS • VOUT + VF

(

)

ISW(MIN)

LPRI ≥

tON(MIN) • VIN(MAX)

ISW(MIN)

tOFF(MIN) = 350ns

tON(MIN) = 160ns

ISW(MIN) = 52mA

Example:

LPRI ≥

350ns • 2 •(12V + 0.3V)

52mA

= 166µH

LPRI ≥

160ns • 72V

52mA

= 222µH

Most transformers specify primary inductance with a tol-

erance of ±20%. With other component tolerance consid-

ered, choose a transformer with its primary inductance

20% to 40% larger than the minimum values calculated

above. LPRI = 300µH is then chosen in this example.

Once the primary inductance has been determined, the

maximum load switching frequency can be calculated as:

fSW =

tON + tOFF

LPRI •ISW

VIN

LPRI •ISW

NPS •(VOUT + VF)

ISW =

VOUT •IOUT •2

η • VIN •D

Example:

D =

(12V + 0.3V)• 2

(12V + 0.3V)• 2+ 48V

= 0.34

ISW =

12V • 0.12A • 2

0.85 • 48V • 0.34

= 0.21A

fSW = 260kHz

The transformer also needs to be rated for the correct

saturation current level across line and load conditions. A

saturation current rating larger than 400mA is necessary

to work with the LT8300. The 10396-T022 from Sumida

is chosen as the flyback transformer.

Step 3: Choose the Output Diode.

Two main criteria for choosing the output diode include

forward current rating and reverse voltage rating. The

maximum load requirement is a good first-order guess

as the average current requirement for the output diode.

A conservative metric is the maximum switch current limit

multiplied by the turns ratio,

IDIODE(MAX) = ISW(MAX) • NPS

Example:

IDIODE(MAX) = 0.52A

Next calculate reverse voltage requirement using maxi-

mum VIN:

VREVERSE = VOUT +

VIN(MAX)

NPS

Example:

VREVERSE = 12V +

72V

= 48V

The SBR0560S1 (0.5A, 60V diode) from Diodes Inc.

is chosen.

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