LT3758 datasheet(16/24 Pages) AD

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LT8301

Rev. B

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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) = 450ns

tON(MIN) = 170ns

ISW(MIN) = 290mA (typ)

Example:

LPRI ≥

450ns•3•(5V

+0.3V)

290mA

= 25µH

LPRI ≥

170ns•32V

290mA

= 19µH

Most transformers specify primary inductance with a tol-

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

ered, choose a transformer with its primary inductance

30% larger than the minimum values calculated above.

LPRI = 40µH 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:

(5V

+0.3V)•3

(5V

+0.3V)•3+12V

= 0.57

ISW =

5V •0.5A •2

0.85•12V •0.57

= 0.86A

fSW =199kHz

The transformer also needs to be rated for the correct

saturation current level across line and load conditions.

A saturation current rating larger than 2A is necessary

to work with the LT8301. The 750313974 from Würth 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) = 4.125A

Next calculate reverse voltage requirement using maxi-

mum VIN:

VREVERSE = VOUT +

VIN(MAX)

NPS

Example:

VREVERSE = 5V+

32V

= 15.6V

The CMSH5-20 (5A, 20V diode) from Central

Semiconductor is chosen.

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