Datasheet LT3724 (Analog Devices) - 10

LT3724
OPERATIONS (Refer to Functional Diagram)
either the start of the next cycle or until the bootstrapped switching resumes. An internal clamp on the VC pin is set capacitor is fully charged. at 100mV below the output disable threshold, which limits the negative excursion of the pin voltage, minimizing the
MOSFET Driver
converter output ripple during Burst Mode operation. The LT3724 contains a high speed boosted driver to turn During Burst Mode operation, the VIN pin current is 20µA on and off an external N-channel MOSFET switch. The and the VCC current is reduced to 80µA. If no external drive MOSFET driver derives its power from the boost capacitor is provided for VCC, all VCC bias currents originate from the which is referenced to the SW pin and the source of the VIN pin, giving a total VIN current of 100µA. Burst current MOSFET. The driver provides a large pulse of current to can be reduced further when VCC is driven using an output turn on the MOSFET fast and minimize transition times. derived source, as the VCC component of VIN current is Multiple MOSFETs can be paralleled for higher current then reduced by the converter duty cycle ratio. operation. To eliminate the possibility of shoot through between the
Start-Up
MOSFET and the internal SW pull-down switch, an adap- The following section describes the start-up of the supply tive nonoverlap circuit ensures that the internal pull-down and operation down to 4V once the step-down supply is switch does not turn on until the gate of the MOSFET is up and running. For the protection of the LT3724 and the below its turn on threshold. switching supply, there are internal undervoltage lockout (UVLO) circuits with hysteresis on VIN, VCC and VBOOST,
Low Current Operation (Burst Mode Operation)
as shown in the Electrical Characteristics table. Start-up To increase low current load efficiency, the LT3724 is and continuous operation require that all three of these capable of operating in Linear Technology’s proprietary undervoltage lockout conditions be satisfied because Burst Mode operation where the external MOSFET operates the TG MOSFET driver is disabled during any UVLO fault intermittently based on load current demand. The Burst condition. In startup, for most applications, VCC is powered Mode function is disabled by connecting the BURST_EN from VIN through the high voltage linear regulator of the pin to V LT3724. This requires V CC and enabled by connecting the pin to SGND. IN to be high enough to drive the V When the required switch current, sensed via the V CC voltage above its undervoltage lockout threshold. C pin V voltage, is below 15% of maximum, Burst Mode operation CC, in turn, has to be high enough to charge the BOOST capacitor through an external diode so that the BOOST is employed and that level of sense current is latched onto voltage is above its undervoltage lockout threshold. There the IC control path. If the output load requires less than is an NPN switch that pulls the SW node to ground each this latched current level, the converter will overdrive the cycle during the TG power MOSFET off-time, ensuring the output slightly during each switch cycle. This overdrive BOOST capacitor is kept fully charged. Once the supply condition is sensed internally and forces the voltage on the is up and running, the output voltage of the supply can VC pin to continue to drop. When the voltage on VC drops backdrive V 150mV below the 15% load level, switching is disabled, CC through an external diode. Internal circuitry disables the high voltage regulator to conserve V and the LT3724 shuts down most of its internal circuitry, IN supply current. Output voltages that are too low or too high to reducing total quiescent current to 100µA. When the backdrive V converter output begins to fall, the V CC require additional circuitry such as a voltage C pin voltage begins doubler or linear regulator. Once V to climb. When the voltage on the V CC is backdriven from C pin climbs back to a supply other than V the 15% load level, the IC returns to normal operation and IN, VIN can be reduced to 4V with normal operation maintained. 3724fd 10