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LT3845EFEPBF Datasheet(PDF) 11 Page - Linear Technology |
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LT3845EFEPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 26 page  LT3845 11 3845fd where QG(TOTAL) is the total MOSFET gate charge of the TG and BG. In applications where these conditions are exceeded, VCC must be derived from an external source after start-up. Maximum continuous regulator power dissipation may be exceeded for short duration VIN transients. In LT3845 converter applications with output voltages in the 9V to 20V range, back-feeding VCC and VBOOST from the converter output is trivial, accomplished by connect- ing diodes from the output to these supply pins. Deriving these supplies from output voltages greater than 20V will require additional regulation to reduce the feedback voltage. Outputs lower than 9V will require step-up techniques to increase the feedback voltage to something greater than the 8V VCC regulated output. Low power boost switchers are sometimes used to provide the step-up function, but a simple charge-pump can perform this function in many instances. Burst Mode The LT3845 employs low current Burst Mode functional- ity to maximize efficiency during no load and low load conditions. Burst Mode operation is enabled by shorting the BURST_EN pin to SGND. Burst Mode operation can be disabled by shorting BURST_EN to either VFB or VCC. When the required switch current, sensed via the VC pin voltage, is below 15% of maximum, the 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 LT3845 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 switch disable threshold, which limits the negative excursion of the pin voltage, minimizing the converter output ripple during Burst Mode operation. During Burst Mode operation, VIN pin current is 20μA and VCC current is reduced to 100μA. If no external drive is provided for VCC, all VCC bias currents originate from the APPLICATIONS INFORMATION VOUT 1μF B0520 B0520 1μF Si1555DL LT3845 Charge Pump Doubler Charge Pump Tripler VCC BG VOUT 1μF B0520 B0520 1μF 1μF Si1555DL Si1555DL LT3845 VCC BG B0520 3845 AI01 TG VOUT 3845 AI04 VCC SW BG N • • LT3845 Inductor Auxiliary Winding |
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