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LT3844 Datasheet(PDF) 9 Page - Linear Technology |
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LT3844 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  LT3844 9 3844fb OPERATION (Refer to Functional Diagram) 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 LT3844 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 VCC or VFB and enabled by connecting the pin to SGND. When the required switch current, sensed via the VC 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 VC pin to continue to drop. When the voltage on VC drops 150mV below the 15% load level, switching is disabled, and the LT3844 shuts down most of its internal circuitry, reducing total quiescent current to 120μA. When the converter output begins to fall, the VC pin voltage begins to climb. When the voltage on the VC pin climbs back to the 15% load level, the IC returns to normal operation and switching resumes. An internal clamp on the VC 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 VIN pin current is 20μA and the VCC current is reduced to 100μA. If no external drive is provided for VCC, all VCC bias currents originate from the VIN pin, giving a total VIN current of 120μA. Burst current can be reduced further when VCC is driven using an output derived source, as the VCC componentofVIN 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 LT3844 and the switching supply, there are internal undervoltage lockout (UVLO) circuits with hysteresis on VIN, VCC and VBOOST, 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 con- dition. In start-up, for most applications, VCC is powered from VIN through the high voltage linear regulator of the LT3844. This requires VIN to be high enough to drive the VCC voltage above its undervoltage lockout threshold. VCC, 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 VCC through an external diode. Internal circuitry disables the high voltage regulator to conserve VIN supply current. Output voltages that are too low or too high to backdrive VCC require additional circuitry such as a voltage doubler or linear regulator. Once VCC is backdriven from a supply other than VIN, VIN can be reduced to 4V with normal operation maintained. Soft-Start The soft-start function controls the slew rate of the power supply output voltage during start-up. A controlled output voltage ramp minimizes output voltage overshoot, reduces inrush current from the VIN supply, and facilitates supply sequencing. A capacitor, CSS, connected from the CSS pin to SGND, programs the slew rate. The capacitor is charged from an internal 2μA current source producing a ramped voltage. The capacitor voltage overrides the internal refer- ence to the error amplifier. If the VFB pin voltage exceeds |
Similar Part No. - LT3844_15 |
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Similar Description - LT3844_15 |
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