Datasheet LT3844 (Analog Devices) - 10

LT3844
operaTion (Refer to Functional Diagram)
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 LT3844 and the below its turn on threshold. switching supply, there are internal undervoltage lockout
Low Current Operation (Burst Mode Operation)
(UVLO) circuits with hysteresis on VIN, VCC and VBOOST , as shown in the Electrical Characteristics table. Start-up To increase low current load efficiency, the LT3844 is and continuous operation require that all three of these capable of operating in Linear Technology’s proprietary undervoltage lockout conditions be satisfied because the Burst Mode operation where the external MOSFET operates TG MOSFET driver is disabled during any UVLO fault con- intermittently based on load current demand. The Burst dition. In start-up, for most applications, VCC is powered Mode function is disabled by connecting the BURST_EN pin from VIN through the high voltage linear regulator of the to VCC or VFB and enabled by connecting the pin to SGND. LT3844. This requires VIN to be high enough to drive the When the required switch current, sensed via the V V C pin CC voltage above its undervoltage lockout threshold. voltage, is below 15% of maximum, Burst Mode operation VCC, in turn, has to be high enough to charge the BOOST is employed and that level of sense current is latched onto capacitor through an external diode so that the BOOST the IC control path. If the output load requires less than voltage is above its undervoltage lockout threshold. There this latched current level, the converter will overdrive the is an NPN switch that pulls the SW node to ground each output slightly during each switch cycle. This overdrive cycle during the TG power MOSFET off-time, ensuring the condition is sensed internally and forces the voltage on the BOOST capacitor is kept fully charged. Once the supply V is up and running, the output voltage of the supply can C pin to continue to drop. When the voltage on VC drops 150mV below the 15% load level, switching is disabled, backdrive VCC through an external diode. Internal circuitry and the LT3844 shuts down most of its internal circuitry, disables the high voltage regulator to conserve VIN supply reducing total quiescent current to 120µA. When the current. Output voltages that are too low or too high to converter output begins to fall, the V backdrive V C pin voltage begins CC require additional circuitry such as a voltage to climb. When the voltage on the V doubler or linear regulator. Once V C pin climbs back to CC is backdriven from the 15% load level, the IC returns to normal operation a supply other than VIN, VIN can be reduced to 4V with and switching resumes. An internal clamp on the V normal operation maintained. C pin is set at 100mV below the output disable threshold, which limits the negative excursion of the pin voltage, minimizing
Soft-Start
the converter output ripple during Burst Mode operation. The soft-start function controls the slew rate of the power During Burst Mode operation, the V supply output voltage during start-up. A controlled output IN pin current is 20µA and the V voltage ramp minimizes output voltage overshoot, reduces CC current is reduced to 100µA. If no external drive is provided for V inrush current from the VIN supply, and facilitates supply CC, all VCC bias currents originate from the V sequencing. A capacitor, CSS, connected from the CSS pin IN pin, giving a total VIN current of 120µA. Burst current can be reduced further when V to SGND, programs the slew rate. The capacitor is charged CC is driven using an output derived source, as the V from an internal 2µA current source producing a ramped CC component of VIN current is then reduced by the converter duty cycle ratio. voltage. The capacitor voltage overrides the internal refer- ence to the error amplifier. If the VFB pin voltage exceeds 3844fc 10 For more information www.linear.com/LT3844 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts