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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

Undervoltage Lockout (UVLO)

A resistive divider from VIN to the EN/UVLO pin imple-

ments undervoltage lockout (UVLO). The EN/UVLO enable

falling threshold is set at 1.214V with 14mV hysteresis. In

addition, the EN/UVLO pin sinks 2.5µA when the voltage

on the pin is below 1.214V. This current provides user

programmable hysteresis based on the value of R1. The

programmable UVLO thresholds are:

VIN(UVLO+

)

=

1.228

V • (R1+

R2

R2)

+

2.5µA

•

R1

VIN(UVLO–

)

=

1.214V • (R1+

R2

R2)

Figure 7 shows the implementation of external shutdown

control while still using the UVLO function. The NMOS

grounds the EN/UVLO pin when turned on, and puts the

VIN

R1

EN/UVLO

LT8302/LT8302-3

R2

GND

RUN/STOP

CONTROL

(OPTIONAL)

8302 F07

Figure 7. Undervoltage Lockout (UVLO)

LT8302/LT8302-3 in shutdown with quiescent current

less than 2µA.

Minimum Load Requirement

The LT8302/LT8302-3 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 LT8302/LT8302-3 has

to turn on and off for a minimum amount of time and with

a minimum frequency. The LT8302/LT8302-3 delivers a

minimum amount of energy even during light load con-

ditions to ensure accurate 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

•

fMIN

LPRI = Transformer primary inductance

ISW(MIN) = Minimum switch current limit = 1.04A (MAX)

fMIN = Minimum switching frequency = 12.7kHz (MAX)

The LT8302/LT8302-3 typically needs less than 0.5% of

its full output power as minimum load. Alternatively, a

Zener diode with its breakdown of 10% higher than the

output voltage can serve as a minimum load if pre-loading

is not acceptable. For a 5V output, use a 5.6V Zener with

cathode connected to the output.

Output Short Protection

When the output is heavily overloaded or shorted to

ground, the reflected SW pin waveform rings longer

than the internal blanking time. After the 350ns minimum

switch-off time, the excessive ringing falsely triggers the

boundary mode detector and turns the power switch back

on again before the secondary current falls to zero. Under

this condition, the LT8302/LT8302-3 runs into continu-

ous conduction mode at 380kHz maximum switching fre-

quency. If the sampled RREF voltage is still less than 0.6V

after 11ms (typ) soft-start timer, the LT8302/LT8302-3

initiates a new soft-start cycle. If the sampled RREF voltage

is larger than 0.6V after 11ms, the switch current may

run away and exceed the 4.5A maximum current limit.

Once the switch current hits 7.2A over current limit, the

LT8302/LT8302-3 also initiates a new soft-start cycle.

Under either condition, the new soft-start cycle throttles

back both the switch current limit and switch frequency.

The output short-circuit protection prevents the switch

current from running away and limits the average output

diode current.

For more information www.analog.com

Rev. G

17

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