LT8301MP 查看數據表（PDF） - Analog Devices

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LT8301MP

42VIN Micropower No-Opto Isolated Flyback Converter with 65V/1.2A Switch

Analog Devices 

LT8301

APPLICATIONS INFORMATION

Minimum Load Requirement

The LT8301 samples the isolated output voltage from

the primary-side flyback pulse waveform. The flyback

pulse occurs once the primary switch turns off and the

secondary winding conducts current. In order to sample

the output voltage, the LT8301 has to turn on and off at

least for a minimum amount of time and with a minimum

frequency. The LT8301 delivers a minimum amount of

energy even during light load conditions to ensure accu-

rate output voltage information. The minimum energy

delivery creates a minimum load requirement, which can

be approximately estimated as:

ILOAD(MIN)

=

LPRI

•ISW(MIN)2

2 • VOUT

•

f

MIN

LPRI = Transformer primary inductance

ISW(MIN) = Minimum switch current limit = 360mA

(max)

fMIN = Minimum switching frequency = 10.6kHz (max)

The LT8301 typically needs less than 0.5% of its full out-

put power as minimum load. Alternatively, a Zener diode

with its breakdown of 20% higher than the output volt-

age can serve as a minimum load if pre-loading is not

acceptable. For a 5V output, use a 6V Zener with cathode

connected to the output.

worst-case scenario where the output is directly shorted

to ground through a long wire and the huge ring after fold-

ing back still falsely triggers the boundary mode detec-

tor, a secondary overcurrent protection ensures that the

LT8301 can still function properly. Once the switch cur-

rent hits 2.2A overcurrent limit, a soft-start cycle initiates

and throttles back both switch current limit and switching

frequency very hard. This output short protection pre-

vents the switch current from running away and limits

the average output diode current.

Design Example

Use the following design example as a guide to design

applications for the LT8301. The design example involves

designing a 5V output with a 500mA load current and an

input range from 8V to 32V.

VIN(MIN) = 8V, VIN(NOM) = 12V, VIN(MAX) = 32V,

VOUT = 5V, IOUT = 500mA

Step 1: Select the Transformer Turns Ratio.

NPS

<

65V

−

VIN(MAX) − VLEAKAGE

VOUT + VF

VLEAKAGE = Margin for transformer leakage spike = 15V

VF = Output diode forward voltage = ~0.3V

Example:

Output Short-Circuit Protection

When the output is heavily overloaded or shorted, the

reflected SW pin waveform rings longer than the internal

blanking time. If no protection scheme is applied, after

the 450ns minimum switch-off time, the excessive ring

might falsely trigger the boundary mode detector and turn

the power switch back on again before the secondary cur-

rent falls to zero. The part then runs into continuous con-

duction mode at maximum switching frequency, and the

switch current may run away. To prevent the switch cur-

rent from running away under this condition, the LT8301

gradually folds back both maximum switch current limit

and switching frequency as the output voltage drops

from regulation. As a result, the switch current remains

below 1.375A (typ) maximum switch current limit. In the

NPS

<

65V − 32V −15V

5V + 0.3V

=

3.4

The choice of transformer turns ratio is critical in deter-

mining output current capability of the converter. Table 4

shows the switch voltage stress and output current capa-

bility at different transformer turns ratio.

Table 4. Switch Voltage Stress and Output Current Capability

vs Turns Ratio

VSW(MAX) at

IOUT(MAX) at

NPS

VIN(MAX) (V) VIN(MIN) (mA) DUTY CYCLE (%)

1:1

37.3

330

14-40

2:1

42.6

470

25-57

3:1

47.9

540

33-67

Since only NPS = 3 can meet the 500mA output current

requirement, NPS = 3 is chosen in this example.

Rev. B

For more information www.analog.com

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