Datasheet

LTC3766
16
3766fa
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operaTion
greater than the minimum controllable on-time. Under
short-circuit conditions, for example, the LTC3766 limits
the current using a separate overcurrent comparator. When
this overcurrent comparator is tripped, the LTC3766 gener
-
ates a fault followed by a soft-start retry. This hiccup mode
overcurrent protection is highly effective at minimizing
power losses under short-cir
cuit conditions.
Direct Flux Limit
In active clamp forward converters, it is essential to es
-
tablish an accurate limit to the transformer flux density
in order to avoid core saturation during load transients or
when starting up into a pre-biased output. Although the
active clamp
technique
provides a suitable reset voltage
during steady-state operation, the sudden increase in duty
cycle caused in response to a load step can cause the
transformer flux to accumulate or “walk,” potentially lead
-
ing to saturation. This occurs because the reset voltage on
the active clamp capacitor cannot keep up with the rapidly
changing duty cycle. This effect is most pronounced at low
input voltage, where the voltage loop demands a greater
increase in
duty cycle due to the lower voltage available
to ramp up the current in the output inductor.
Traditionally, transformer core saturation has been avoided
either by limiting the maximum duty cycle of the converter
or by slowing down the loop to limit the rate at which the
duty cycle changes. Limiting the maximum duty cycle does
help the converter avoid saturation for a load step at low
input voltage, since the duty cycle maximum is clamped;
however, transformer saturation can also easily occur at
higher input voltage where the maximum duty cycle clamp
is ineffective. Limiting the rate of duty cycle change such
that the active clamp capacitor can sufficiently track the
duty cycle change also helps to prevent saturation in many
situations, but results in a very poor transient response.
Neither of these traditional techniques is guaranteed to
prevent the transformer from saturating in all situations.
For example, saturation can easily occur using these
traditional techniques when starting up into a pre-biased
output, where the duty cycle can quickly change from 0%
to 75%. Moreover, neither of these traditional techniques is
able to prevent saturation in the negative direction, which
can result from sudden decreases in duty cycle.
The LTC3765 and LTC3766 implement a new unique system
for monitoring and directly limiting the flux accumula
-
tion in the transformer core. During a reset cycle, when
the active clamp PMOS is on, the magnetizing current is
directly measured and limited through a sense resistor
in
series
with the PMOS source. This prevents saturation
in the negative direction. When the PMOS turns off and
the main NMOS switch turns on, the LTC3765 generates
an accurate internal estimate of the magnetizing current
based on the sensed input voltage on the LTC3765 RUN
pin and transformer core parameters customized to the
particular core by a resistor from the LTC3765 R
CORE
pin
to ground. The magnetizing current is then limited during
the on-time by this accurate internal approximation. Unlike
previous methods, the direct flux limit directly measures
and monitors flux accumulation and guarantees that the
transformer will not saturate in either direction, even
when starting into a pre-biased output. This technique
also provides the best possible transient response, as it
will temporarily allow very high duty cycles, only limiting
the duty cycle when absolutely necessary. Moreover, this
technique prevents overcurrent damage to the active clamp
PMOS, which is a potentially significant weakness in many
active clamp forward converter designs.
Additional Protection Features
The LTC3766 contains a wide array of protection features,
which protect the DC/DC converter in the event that ab
-
normal conditions persist. In general, protections features
are either classified as a fault or a limit. When a fault is
detected, all switching stops and the LTC3766 initiates a
soft-start retr
y. Faults of this nature include overcurrent,
overtemperature, differential amplifier miswire and
communication-lock fault.
An overcurrent fault occurs if the peak current exceeds
approximately 133% of its normal value during current
limit. Note that when inductor ripple cancellation is used,
the value of the peak current during current limit will vary
with inductor current ripple. The overtemperature fault is
set at 165°C, with 20°C of hysteresis. This is helpful for
limiting the temperature of the DC/DC converter in the
event of some external device failure or other abnormal
condition. The differential amplifier wiring fault is gener
-