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LT3800EFE-PBF Datasheet(PDF) 8 Page - Linear Technology |
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LT3800EFE-PBF Datasheet(HTML) 8 Page - Linear Technology |
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8 / 24 page  LT3800 8 3800fb Overview The LT3800 is a high input voltage range step-down synchronous DC/DC converter controller IC that uses a 200kHz constant frequency, current mode architecture with external N-channel MOSFET switches. The LT3800 has provisions for high efficiency, low load operation for battery-powered applications. Burst Mode operation reduces total average input quiescent currents to 100 µAduringnoloadconditions.Alowcurrentshutdown mode can also be activated, reducing quiescent current to <10 µA. Burst Mode operation can be disabled if desired. The LT3800 also employs a reverse-current inhibit fea- ture, allowing increased efficiencies during light loads through nonsynchronous operation. This feature disables the synchronous switch if inductor current approaches zero. If full time synchronous operation is desired, this feature can be disabled. Much of the LT3800’s internal circuitry is biased from an internal linear regulator. The output of this regulator is the VCC pin, allowing bypassing of the internal regulator. The associated internal circuitry can be powered from the output of the converter, increasing overall converter effi- ciency. Using externally derived power also eliminates the IC’s power dissipation associated with the internal VIN to VCC regulator. Theory of Operation (See Block Diagram) The LT3800 senses converter output voltage via the VFB pin. The difference between the voltage on this pin and an internal 1.231V reference is amplified to generate an error voltage on the VC pin which is, in turn, used as a threshold for the current sense comparator. During normal operation, the LT3800 internal oscillator runs at 200kHz. At the beginning of each oscillator cycle, the switch drive is enabled. The switch drive stays enabled until the sensed switch current exceeds the VC derived threshold for the current sense comparator and, in turn, disables the switch driver. If the current comparator APPLICATIO S I FOR ATIO threshold is not obtained for the entire oscillator cycle, the switch driver is disabled at the end of the cycle for 450ns. This minimum off-time mode of operation assures regen- eration of the BOOST bootstrapped supply. Power Requirements The LT3800 is biased using a local linear regulator to generate internal operational voltages from the VIN pin. Virtually all of the circuitry in the LT3800 is biased via an internal linear regulator output (VCC). This pin is decoupled with a low ESR 1 µF capacitor to PGND. The VCC regulator generates an 8V output provided there is ample voltage on the VIN pin. The VCC regulator has approximately 1V of dropout, and will follow the VIN pin with voltages below the dropout threshold. The LT3800 has a start-up requirement of VIN > 7.5V. This assures that the onboard regulator has ample headroom to bring the VCC pin above its UVLO threshold. The VCC regulator can only source current, so forcing the VCC pin above its 8V regulated voltage allows use of externally derived power for the IC, minimizing power dissipation in the IC. Using the onboard regulator for start-up, then deriving power for VCC from the converter output maxi- mizes conversion efficiencies and is common practice. If VCC is maintained above 6.5V using an external source, the LT3800 can continue to operate with VIN as low as 4V. The LT3800 operates with 3mA quiescent current from the VCC supply. This current is a fraction of the actual VCC quiescent currents during normal operation. Additional current is produced from the MOSFET switching currents for both the boosted and synchronous switches and are typically derived from the VCC supply. Because the LT3800 uses a linear regulator to generate VCC, power dissipation can become a concern with high VIN voltages. Gate drive currents are typically in the range of 5mA to 15mA per MOSFET, so gate drive currents can create substantial power dissipation. It is advisable to derive VCC and VBOOST power from an external source whenever possible. |
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