Datasheet

LT8302
10
Rev E
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OPERATION
Boundary conduction mode returns the secondary current
to zero every cycle, so parasitic resistive voltage drops
do not cause load regulation errors. Boundary conduc
-
tion mode also allows the use of smaller transformers
compared to continuous conduction mode and does not
exhibit subharmonic oscillation.
Discontinuous Conduction Mode Operation
As the load gets lighter, boundary conduction mode
increases the switching frequency and decreases the
switch peak current at the same ratio. Running at a higher
switching frequency up to several MHz increases switch-
ing and gate charge losses. To avoid this scenario, the
LT8302 has an additional internal oscillator, which clamps
the maximum switching frequency to be less than 380kHz.
Once the switching frequency hits the internal frequency
clamp, the part starts to delay the switch turn-on and
operates in discontinuous conduction mode.
Low Ripple Burst Mode Operation
Unlike traditional flyback converters, the LT8302 has to
turn on and off at least for a minimum amount of time
and with a minimum frequency to allow accurate sampling
of the output voltage. The inherent minimum switch cur-
rent limit and minimum switch-off time are necessary to
guarantee the correct operation of specific applications.
As the load gets very light, the LT8302 starts to fold
back the switching frequency while keeping the mini
-
mum switch current limit. So the load current is able to
decrease while still allowing minimum switch-off time for
the sample-and-hold error amplifier. Meanwhile, the part
switches between sleep mode and active mode, thereby
reducing the effective quiescent current to improve light
load efficiency. In this condition, the LT8302 runs in low
ripple Burst Mode operation. The typical 12kHz minimum
switching frequency determines how often the output volt-
age is sampled and also the minimum load requirement.
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