LTC3765_15 Datasheet, PDF (11/24 Pages) Linear Technology – Active Clamp Forward Controller and Gate Driver

LTC3765
OPERATION
core by a resistor from the R_CORE pin to ground. The mag-
netizing 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, even when starting into a pre-biased output,
without compromising transient response.
Additional Protection Features
The LTC3765 contains additional features to protect the
circuit in the event of a persistent abnormal condition.
Overcurrent and overtemperature monitors ensure reliable
operation even in abnormal conditions.
The overcurrent monitor is implemented with an external
sense resistor in series with the source of the primary
NMOS. When the differential voltage between the current-
sense pins, I_S+ and I_S–, exceeds 150mV, the primary NMOS
is immediately turned off and a fault is initiated.
The internal overtemperature monitor 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 a
failure or abnormal condition. If the internal temperature
exceeds this level, switching immediately stops and a
fault is flagged.
Fault Indicator
A fault is initiated when any of the following conditions
are encountered: overcurrent trip, overtemperature trip,
communication loss with the LTC3766, V_CC falling be-
low its UVLO threshold, or the RUN pin falling below its
threshold. The SSFLT pin is used to indicate these faults,
to communicate the fault in a polyphase system, and to
optionally lockout on a fault.
When a fault occurs, switching stops immediately and the
SSFLT pin is rapidly pulled up to above 5.75V as an indica-
tor. The LTC3766 will detect that switching has stopped
and will also fault and restart. As soon as the fault clears,
the voltage on the SSFLT pin will slowly discharge to al-
low time for the LTC3766 to prepare for a restart. When
the SSFLT voltage reaches 0.7V, the pin is momentarily
grounded, and the soft-start sequence begins again. A
fault can optionally be “locked out” by adding a 5.6V Zener
diode from SSFLT to ground. This will inhibit the restart
until the SSFLT pin is externally grounded, the diode clamp
is removed, or the input supply collapses.
In a PolyPhase^® application, the SSFLT pins of the LTC3765s
should all be tied together. This not only ensures that all of
the LTC3765 phases begin their open-loop start-up simul-
taneously, but also provides a means for communicating
a fault condition. If one LTC3765 detects a fault, it pulls
the combined SSFLT node to above 6V. When the voltage
rises above 5V, the other LTC3765s detect this and stop
switching until the common SSFLT pin has discharged.
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