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LT8302MPS8E

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

Linear Technology 

LT8302

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+

)

=

1.228V •(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

LT8302 in shutdown with quiescent current less than 2µA.

VIN

R1

EN/UVLO

LT8302

R2

GND

RUN/STOP

CONTROL

(OPTIONAL)

8302 F07

Figure 7. Undervoltage Lockout (UVLO)

Minimum Load Requirement

The LT8302 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 has to turn on and off for a

minimum amount of time and with a minimum frequency.

The LT8302 delivers a minimum amount of energy even

during light load conditions to ensure accurate output volt-

age 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 = 0.96A (MAX)

fMIN = Minimum switching frequency = 12.7kHz (MAX)

The LT8302 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 in-

ternal 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 runs into continuous conduction

mode at 380kHz maximum switching frequency. If the

sampled RREF voltage is still less than 0.6V after 11ms

(typ) soft-start timer, the LT8302 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 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.linear.com/LT8302

8302fa

17

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