lt3724 Linear Technology Corporation, lt3724 Datasheet - Page 10

• Manufacturer Part Number: lt3724
• Description: High Voltage, Current Mode Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

LT3724
OPERATIO
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 to 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 oper-
ates intermittently based on load current demand. The
Burst Mode function is disabled by connecting the
BURST_EN pin to V
to SGND.
When the required switch current, sensed via the V
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
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
to climb. When the voltage on the V
15% load level, the IC returns to normal operation and
10
C
pin to continue to drop. When the voltage on V
U
CC
(Refer to Functional Diagram)
and enabled by connecting the pin
C
pin climbs back to the
C
pin voltage begins
C
drops
C
pin
switching resumes. An internal clamp on the V
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
and the V
is provided for V
V
can be reduced further when V
derived source, as the V
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
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
ered from V
the LT3724. This requires V
the V
V
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
disables the high voltage regulator to conserve V
current. Output voltages that are too low or too high to
backdrive V
doubler or linear regulator. Once V
supply other than V
normal operation maintained.
IN
CC
, in turn, has to be high enough to charge the BOOST
pin, giving a total V
CC
voltage above its undervoltage lockout threshold.
CC
CC
current is reduced to 80μA. If no external drive
IN
CC
through the high voltage linear regulator of
require additional circuitry such as a voltage
through an external diode. Internal circuitry
CC
, all V
IN
, V
IN
CC
CC
current of 100μA. Burst current
IN
bias currents originate from the
IN
component of V
can be reduced to 4V with
CC
to be high enough to drive
is driven using an output
CC
IN
IN
is backdriven from a
, V
pin current is 20μA
CC
and V
IN
C
CC
current is
BOOST
IN
pin is set
is pow-
supply
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, as