Datasheet LT3800 (Analog Devices) - 8

LT3800
APPLICATIONS INFORMATION Overview
threshold is not obtained for the entire oscillator cycle, the switch driver is disabled at the end of the cycle for The LT3800 is a high input voltage range step-down 450ns. This minimum off-time mode of operation assures synchronous DC/DC converter controller IC that uses a regeneration of the BOOST bootstrapped supply. 200kHz constant frequency, current mode architecture with external N-channel MOSFET switches.
Power Requirements
The LT3800 has provisions for high effi ciency, low load The LT3800 is biased using a local linear regulator to operation for battery-powered applications. Burst Mode generate internal operational voltages from the V operation reduces total average input quiescent currents to IN pin. Virtually all of the circuitry in the LT3800 is biased via an 100μA during no load conditions. A low current shutdown internal linear regulator output (V mode can also be activated, reducing quiescent current to CC). This pin is decoupled with a low ESR 1μF capacitor to PGND. <10μA. Burst Mode operation can be disabled if desired. The V The LT3800 also employs a reverse-current inhibit feature, CC regulator generates an 8V output provided there is ample voltage on the V allowing increased effi ciencies during light loads through IN pin. The VCC regulator has approximately 1V of dropout, and will follow the V nonsynchronous operation. This feature disables the IN pin with voltages below the dropout threshold. synchronous switch if inductor current approaches zero. If full time synchronous operation is desired, this feature The LT3800 has a start-up requirement of VIN > 7.5V. This can be disabled. assures that the onboard regulator has ample headroom to bring the V Much of the LT3800’s internal circuitry is biased from an CC pin above its UVLO threshold. The VCC regulator can only source current, so forcing the V internal linear regulator. The output of this regulator is CC pin above its 8V regulated voltage allows use of externally the VCC pin, allowing bypassing of the internal regulator. derived power for the IC, minimizing power dissipation The associated internal circuitry can be powered from in the IC. Using the onboard regulator for start-up, then the output of the converter, increasing overall converter deriving power for V effi ciency. Using externally derived power also eliminates CC from the converter output maxi- mizes conversion effi ciencies and is common practice. If the IC’s power dissipation associated with the internal VIN V to V CC is maintained above 6.5V using an external source, CC regulator. the LT3800 can continue to operate with VIN as low as 4V.
Theory of Operation (See Block Diagram)
The LT3800 operates with 3mA quiescent current from the V The LT3800 senses converter output voltage via the V CC supply. This current is a fraction of the actual VCC FB quiescent currents during normal operation. Additional pin. The difference between the voltage on this pin and current is produced from the MOSFET switching currents an internal 1.231V reference is amplifi ed to generate an for both the boosted and synchronous switches and are error voltage on the VC pin which is, in turn, used as a typically derived from the V threshold for the current sense comparator. CC supply. Because the LT3800 uses a linear regulator to generate During normal operation, the LT3800 internal oscilla- V tor runs at 200kHz. At the beginning of each oscillator CC, power dissipation can become a concern with high V cycle, the switch drive is enabled. The switch drive stays IN voltages. Gate drive currents are typically in the range of 5mA to 15mA per MOSFET, so gate drive currents can enabled until the sensed switch current exceeds the VC create substantial power dissipation. It is advisable to derived threshold for the current sense comparator and, in derive V turn, disables the switch driver. If the current comparator CC and VBOOST power from an external source whenever possible. 3800fc 8 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