LT3724_1 Datasheet, PDF (10/26 Pages) Linear Technology – High Voltage, Current Mode Switching Regulator Controller Thermal Shutdown

LT3724
OPERATIONS (Refer to Functional Diagram)
either the start of the next cycle or until the bootstrapped
capacitor is fully charged.
MOSFET Driver
The LT3724 contains a high speed boosted driver to turn
on and off an external N-channel MOSFET switch. The
MOSFET driver derives its power from the boost capacitor
which is referenced to the SW pin and the source of the
MOSFET. The driver provides a large pulse of current to
turn on the MOSFET fast and minimize transition times.
Multiple MOSFETs can be paralleled for higher current
operation.
To eliminate the possibility of shoot through between the
MOSFET and the internal SW pull-down switch, an adap-
tive nonoverlap circuit ensures that the internal pull-down
switch does not turn on until the gate of the MOSFET is
below its turn on threshold.
Low Current Operation (Burst Mode Operation)
To increase low current load efficiency, the LT3724 is
capable of operating in Linear Technology’s proprietary
Burst Mode operation where the external MOSFET operates
intermittently based on load current demand. The Burst
Mode function is disabled by connecting the BURST_EN
pin to V_CC and enabled by connecting the pin to SGND.
When the required switch current, sensed via the V_C pin
voltage, is below 15% of maximum, Burst Mode operation
is employed and that level of sense current is latched onto
the IC control path. If the output load requires less than
this latched current level, the converter will overdrive the
output slightly during each switch cycle. This overdrive
condition is sensed internally and forces the voltage on the
V_C pin to continue to drop. When the voltage on V_C drops
150mV below the 15% load level, switching is disabled,
and the LT3724 shuts down most of its internal circuitry,
reducing total quiescent current to 100µA. When the
converter output begins to fall, the V_C pin voltage begins
to climb. When the voltage on the V_C pin climbs back to
the 15% load level, the IC returns to normal operation and
switching resumes. An internal clamp on the V_C pin is set
at 100mV below the output disable threshold, which limits
the negative excursion of the pin voltage, minimizing the
converter output ripple during Burst Mode operation.
During Burst Mode operation, the V_IN pin current is 20µA
and the V_CC current is reduced to 80µA. If no external drive
is provided for V_CC, all V_CC bias currents originate from the
V_IN pin, giving a total V_IN current of 100µA. Burst current
can be reduced further when V_CC is driven using an output
derived source, as the V_CC component of V_IN current is
then reduced by the converter duty cycle ratio.
Start-Up
The following section describes the start-up of the supply
and operation down to 4V once the step-down supply is
up and running. For the protection of the LT3724 and the
switching supply, there are internal undervoltage lockout
(UVLO) circuits with hysteresis on V_IN, V_CC and V_BOOST,
as shown in the Electrical Characteristics table. Start-up
and continuous operation require that all three of these
undervoltage lockout conditions be satisfied because
the TG MOSFET driver is disabled during any UVLO fault
condition. In startup, for most applications, V_CC is powered
from V_IN through the high voltage linear regulator of the
LT3724. This requires V_IN to be high enough to drive the
V_CC voltage above its undervoltage lockout threshold.
V_CC, in turn, has to be high enough to charge the BOOST
capacitor through an external diode so that the BOOST
voltage is above its undervoltage lockout threshold. There
is an NPN switch that pulls the SW node to ground each
cycle during the TG power MOSFET off-time, ensuring the
BOOST capacitor is kept fully charged. Once the supply
is up and running, the output voltage of the supply can
backdrive V_CC through an external diode. Internal circuitry
disables the high voltage regulator to conserve V_IN supply
current. Output voltages that are too low or too high to
backdrive V_CC require additional circuitry such as a voltage
doubler or linear regulator. Once V_CC is backdriven from
a supply other than V_IN, V_IN can be reduced to 4V with
normal operation maintained.
3724fd
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