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LT8300

8300f

Minimum Load Requirement

The LT8300 samples the isolated output voltage from the

primary-side flyback pulse waveform. The flyback pulse

occursoncetheprimaryswitchturnsoffandthesecondary

winding conducts current. In order to sample the output

voltage, the LT8300 has to turn on and off at least for a

minimum amount of time and with a minimum frequency.

The LT8300 delivers a minimum amount of energy even

duringlightloadconditionstoensureaccurateoutputvolt-

age information. The minimum energy delivery creates a

minimum load requirement, which can be approximately

estimated as:

ILOAD(MIN) =

LPRI •ISW(MIN)

2 • fMIN

2 • VOUT

LPRI = Transformer primary inductance

ISW(MIN) = Minimum switch current limit = 52mA

fMIN = Minimum switching frequency = 7.5kHz

The LT8300 typically needs less than 0.5% of its full output

power as minimum load. Alternatively, a Zener diode with its

breakdown of 20% higher than the output voltage can serve

as a minimum load if pre-loading is not acceptable. For a 5V

output,usea6VZenerwithcathodeconnectedtotheoutput.

Output Short Protection

When the output is heavily overloaded or shorted, the

reflected SW pin waveform rings longer than the internal

blanking time. After the 350ns minimum switch-off time,

the excessive ring falsely trigger the boundary mode

detector and turn the power switch back on again before

the secondary current falls to zero. Under this condition,

the LT8300 runs into continuous conduction mode at

750kHz maximum switching frequency. Depending on the

VIN supply voltage, the switch current may run away and

exceed 260mA maximum current limit. Once the switch

current hits 520mA over current limit, a soft-start cycle

initiates and throttles back both switch current limit and

switchfrequency.Thisoutputshortprotectionpreventsthe

switch current from running away and limits the average

output diode current.

applicaTions inForMaTion

Design Example

Use the following design example as a guide to design

applications for the LT8300. The design example involves

designing a 12V output with a 120mA load current and an

input range from 36V to 72V.

VIN(MIN) = 36V, VIN(NOM) = 48V, VIN(MAX) = 72V,

VOUT = 12V, IOUT = 120mA

Step 1: Select the Transformer Turns Ratio.

NPS <

150V

− VIN(MAX) − VLEAKAGE

VOUT + VF

VLEAKAGE = Margin for transformer leakage spike = 30V

VF = Output diode forward voltage = ~0.3V

Example:

NPS <

150V

− 72V − 30V

12V

+ 0.3V

= 3.9

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

NPS

VSW(MAX) at

VIN(MAX) (V)

IOUT(MAX) at

VIN(MIN) (mA)

DUTY CYCLE (%)

1:1

84.3

15-25

2:1

96.6

135

25-41

3:1

108.9

168

34-51

Since both NPS=2andNPS=3canmeetthe120mAoutput

current requirement, NPS = 2 is chosen in this example

to allow more margin for transformer leakage inductance

voltage spike.

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