Datasheet LT3800 (Analog Devices) - 10

LT3800
APPLICATIONS INFORMATION Burst Mode
During Burst Mode operation, VIN pin current is 20μA and V The LT3800 employs low current Burst Mode functional- CC current is reduced to 80μA. If no external drive is provided for V ity to maximize effi ciency during no load and low load CC, all VCC bias currents originate from the V conditions. Burst Mode operation is enabled by shorting IN pin, giving a total VIN current of 100μA. Burst current can be reduced further when V the BURST_EN pin to SGND. Burst Mode operation can CC is driven using an output derived source, as the V be disabled by shorting BURST_EN to either V CC component of VIN current is FB or VCC. then reduced by the converter buck ratio. When the required switch current, sensed via the VC
Reverse-Current Inhibit
pin voltage, is below 15% of maximum, the Burst Mode operation is employed and that level of sense current is The LT3800 contains a reverse-current inhibit feature to latched onto the IC control path. If the output load requires maximize effi ciency during light load conditions. This less than this latched current level, the converter will mode of operation allows discontinuous operation, and overdrive the output slightly during each switch cycle. is sometimes referred to as “pulse-skipping” mode. Refer This overdrive condition is sensed internally and forces to Figure 2. the voltage on the VC pin to continue to drop. When the This feature is enabled with Burst Mode operation, and can voltage on VC drops 150mV below the 15% load level, also be enabled while Burst Mode operation is disabled switching is disabled and the LT3800 shuts down most by shorting the BURST_EN pin to V of its internal circuitry, reducing total quiescent current FB. to 100μA. When the converter output begins to fall, the When reverse-current inhibit is enabled, the LT3800 sense V amplifi er detects inductor currents approaching zero and C pin voltage begins to climb. When the voltage on the V disables the synchronous switch for the remainder of C pin climbs back to the 15% load level, the IC returns to normal operation and switching resumes. An internal the switch cycle. If the inductor current is allowed to go clamp on the V negative before the synchronous switch is disabled, the C pin is set at 100mV below the switch disable threshold, which limits the negative excursion of switch node could inductively kick positive with a high the pin voltage, minimizing the converter output ripple dv/dt. The LT3800 prevents this by incorporating a 10mV during Burst Mode operation. positive offset at the sense inputs. PULSE SKIP MODE FORCED CONTINUOUS IL IL DECREASING LOAD CURRENT 3800 F02
Figure 2. Inductor Current vs Mode
3800fc 10 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS FUNCTIONAL DIAGRAM APPLICATIONS INFORMATION TYPICAL APPLICATIONS PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATION RELATED PARTS