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LT8302HS8E-3-WPBF

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

Analog Devices 

LT8302/LT8302-3

APPLICATIONS INFORMATION

The equations below calculate output power:

POUT = η • VIN • D • ISW(MAX) • 0.5

η = Efficiency = ~85%

D

=

Duty

Cycle

=

(

(VOUT +

VOUT + VF

VF ) •NPS

) •NPS + VIN

ISW(MAX) = Maximum switch current limit = 3.6A (MIN)

Primary Inductance Requirement

The LT8302/LT8302-3 obtains output voltage information

from the reflected output voltage on the SW pin. The con-

duction of secondary current reflects the output voltage on

the primary SW pin. The sample-and-hold error amplifier

needs a minimum 350ns to settle and sample the reflected

output voltage. In order to ensure proper sampling, the

secondary winding needs to conduct current for a mini-

mum of 350ns. The following equation gives the minimum

value for primary-side magnetizing inductance:

LPRI

≥

tOFF(MIN)

•NPS • (VOUT

ISW(MIN)

+

VF

)

tOFF(MIN) = Minimum switch-off time = 350ns (TYP)

ISW(MIN) = Minimum switch current limit = 0.87A (TYP)

In addition to the primary inductance requirement for the

minimum switch-off time, the LT8302/LT8302-3 has mini-

mum switch-on time that prevents the chip from turning on

the power switch shorter than approximately 160ns. This

minimum switch-on time is mainly for leading-edge blank-

ing the initial switch turn-on current spike. If the inductor

current exceeds the desired current limit during that time,

oscillation may occur at the output as the current control

loop will lose its ability to regulate. Therefore, the following

equation relating to maximum input voltage must also be

followed in selecting primary-side magnetizing inductance:

LPRI

≥

tON(MIN) • VIN(MAX)

ISW(MIN)

tON(MIN) = Minimum switch-on time = 160ns (TYP)

In general, choose a transformer with its primary mag-

netizing inductance about 40% to 60% larger than the

minimum values calculated above. A transformer with

much larger inductance will have a bigger physical size

and may cause instability at light load.

Selecting a Transformer

Transformer specification and design is perhaps the most

critical part of successfully applying the LT8302/LT8302-

3. In addition to the usual list of guidelines dealing with

high frequency isolated power supply transformer design,

the following information should be carefully considered.

Analog Devices has worked with several leading magnetic

component manufacturers to produce pre-designed fly-

back transformers for use with the LT8302/LT8302-3.

Table 1 shows the details of these transformers.

Table 1. Predesigned Transformers–Typical Specifications

TRANSFORMER

PART NUMBER

DIMENSIONS

(W × L × H) (mm)

LPRI LLKG

(µH) (µH) NP:NS

750311625

17.75 × 13.46 × 12.70 9 0.35 4:1

750311564

17.75 × 13.46 × 12.70 9 0.12 3:1

750313441

15.24 × 13.34 x 11.43 9

0.6 2:1

750311624

17.75 × 13.46 × 12.70 9 0.18 3:2

12387-TO79

15.5 × 12.5 × 11.5

9

0.5 1:1:1

750313445

15.24 × 13.34 × 11.43 9 0.25 1:2

750313457

15.24 × 13.34 × 11.43 9 0.25 1:4

750313460

15.24 × 13.34 × 11.43 12 0.7 4:1

750311342

15.24 × 13.34 × 11.43 15 0.44 2:1

750313439

15.24 × 13.34 × 11.43 12 0.6 2:1

750313442

15.24 × 13.34 × 11.43 12 0.75 3:2

RPRI

(mΩ)

43

36

75

34

55

85

85

85

85

115

150

RSEC

(mΩ)

6

7

18

21

90

190

770

11

22

28

53

VENDOR

Wurth Elektronik

Wurth Elektronik

Wurth Elektronik

Wurth Elektronik

Sumida

Wurth Elektronik

Wurth Elektronik

Wurth Elektronik

Wurth Elektronik

Wurth Elektronik

Wurth Elektronik

14

For more information www.analog.com

TARGET APPLICATION

VIN (V)

8 to 32

VOUT (V)

3.3

IOUT (A)

2.1

8 to 32

5

1.5

8 to 32

5

1.3

8 to 32

8

0.9

8 to 36

±12

0.3

8 to 36

24

0.3

8 to 36

48

0.15

4 to 18

5

0.9

4 to 18

12

0.4

18 to 42

3.3

2.1

18 to 42

5

1.6

Rev. G

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