Datasheet LT3845A (Analog Devices) - 10

LT3845A
APPLICATIONS INFORMATION Overview
exceeds the VC derived threshold for the current sense The LT3845A is a high input voltage range step-down comparator and, in turn, disables the switch driver. If synchronous DC/DC converter controller IC that uses the current comparator threshold is not obtained for the a programmable constant frequency, current mode entire oscillator cycle, the switch driver is disabled at the architecture with external N-channel MOSFET switches. end of the cycle for 350ns, typical. This minimum off-time mode of operation assures regeneration of the BOOST The LT3845A has provisions for high efficiency, low load bootstrapped supply. operation for battery-powered applications. Burst Mode operation reduces total average input quiescent currents to
Power Requirements
120µA during no load conditions. A low current shutdown The LT3845A is biased using an internal linear regulator to mode can also be activated, reducing quiescent current to generate operational voltages from the V 10µA. Burst Mode operation can be disabled if desired. IN pin. Virtually all of the circuitry in the LT3845A is biased via this internal A reverse-current inhibit feature allows increased linear regulator output (VCC). This pin is decoupled with efficiencies during light loads through nonsynchronous a low ESR, 1µF capacitor to PGND. operation. This feature disables the synchronous switch if The V inductor current approaches zero. If full time synchronous CC regulator generates an 8V output provided there is ample voltage on the V operation is desired, this feature can be disabled. IN pin. The VCC regulator has approximately 1V of dropout, and will follow the VIN pin Much of the IC’s internal circuitry is biased from an with voltages below the dropout threshold. internal linear regulator. The output of this regulator is The LT3845A has a start-up requirement of V the V IN > 7.5V. This CC pin, allowing bypassing of the internal regulator. assures that the onboard regulator has ample headroom The associated internal circuitry can be powered from to bring the V the output of the converter, increasing overall converter CC pin above its UVLO threshold. The VCC regulator can only source current, so forcing the V efficiency. Using externally derived power also eliminates CC pin above its 8V regulated voltage allows use of externally the IC’s power dissipation associated with the internal VIN derived power for the IC, minimizing power dissipation to VCC regulator. in the IC. Using the onboard regulator for start-up, then
Theory of Operation (See Block Diagram)
deriving power for VCC from the converter output maximizes conversion efficiencies and is common practice. If VCC The LT3845A senses converter output voltage via the VFB is maintained above 6.5V using an external source, the pin. The difference between the voltage on this pin and LT3845A can continue to operate with VIN as low as 4V. an internal 1.231V reference is amplified to generate an error voltage on the V The LT3845A operates with 3mA quiescent current from C pin which is used as a threshold for the current sense comparator. the VCC supply. This current is a fraction of the actual VCC quiescent currents during normal operation. Additional During normal operation, the LT3845A internal oscillator current is produced from the MOSFET switching currents runs at the programmed frequency. At the beginning of for both the boosted and synchronous switches and are each oscillator cycle, the switch drive is enabled. The typically derived from the VCC supply. switch drive stays enabled until the sensed switch current 3845afa 10 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts