Datasheet

LT8302
17
Rev E
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APPLICATIONS INFORMATION
Undervoltage Lockout (UVLO)
A resistive divider from V
IN
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:
V
IN(UVLO
+
)
=
1.228V • R1+R2
( )
R2
+ 2.5µA •R
1
V
IN(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.
LT8302
GND
EN/UVLO
R1
RUN/STOP
CONTROL
(OPTIONAL)
R2
V
IN
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 sec-
ondary 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
voltage information. The minimum energy delivery cre-
ates a minimum load requirement, which can be approxi-
mately estimated as:
I
LOAD(MIN)
=
L
P
2
RI
•I
SW(MIN)
• f
MIN
2• V
OUT
L
PRI
= Transformer primary inductance
I
SW(MIN)
= Minimum switch current limit = 1.04A (MAX)
f
MIN
= Minimum switching frequency = 12.7kHz (MAX)
The LT8302 typically needs less than 0.5% of its full out-
put 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 accept-
able. For a 5V output, use a 5.6V Zener with cathode con-
nected 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 runs into continuous conduc-
tion mode at 380kHz maximum switching frequency. If the
sampled R
REF
voltage is still less than 0.6V after 11ms
(typ) soft-start timer, the LT8302 initiates a new soft-start
cycle. If the sampled R
REF
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.
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