FEATURES

High Voltage Operation: Up to 60V

Synchronizable Up to 600kHz

Adjustable Constant Frequency: 100kHz to 500kHz

Output Voltages Up to 36V

Stable Operation in Current Limit

Adaptive Nonoverlap Circuitry Prevents Switch Shoot-Through

Reverse Inductor Current Inhibit for Discontinuous Operation Improves Efficiency with Light Loads

Programmable Soft-Start

120µA No Load Quiescent Current

10µA Shutdown Supply Current

1% Regulation Accuracy

Standard Gate N-Channel Power MOSFETs

Reverse Overcurrent Protection

16-Lead Thermally Enhanced TSSOP Package

 

APPLICATIONS

12V and 42V Automotive and Heavy Equipment

48V Telecom Power Supplies

Avionics and Industrial Control Systems

Distributed Power Converters

 

DESCRIPTION

The LT®3845A is a high voltage, synchronous, current mode controller used for medium to high power, high efficiency supplies. It offers a wide 4V to 60V input range (7.5V minimum start-up voltage). An onboard regulator simplifies the biasing requirements by providing IC power directly from VIN.

Burst Mode® operation maintains high efficiency at light loads by reducing IC quiescent current to 120µA. Light load efficiency is also improved with the reverse inductor current inhibit function which supports discontinuous operation.

Additional features include adjustable fixed operating frequency that can be synchronized to an external clock for noise sensitive applications, gate drivers capable of driving large N-channel MOSFETs, a precision under voltage lockout, 10µA shutdown current, short-circuit protection and a programmable soft-start.

The LT3845A is available in a 16-lead thermally enhanced TSSOP package.

 

APPLICATIONS INFORMATION

Overview

The LT3845A is a high input voltage range step-down synchronous DC/DC converter controller IC that uses a programmable constant frequency, current mode architecture with external N-channel MOSFET switches.

The LT3845A has provisions for high efficiency, low load operation for battery-powered applications. Burst Mode operation reduces total average input quiescent currents to 120µA during no load conditions. A low current shutdown mode can also be activated, reducing quiescent current to 10µA. Burst Mode operation can be disabled if desired.

A reverse-current inhibit feature allows 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 IC’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 efficiency. 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 LT3845A 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 used as a threshold for the current sense comparator.

During normal operation, the LT3845A internal oscillator runs at the programmed frequency. 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 threshold is not obtained for the entire oscillator cycle, the switch driver is disabled at the end of the cycle for 350ns, typical. This minimum off-time mode of operation assures regeneration of the BOOST bootstrapped supply.

Power Requirements

The LT3845A is biased using an internal linear regulator to generate operational voltages from the VIN pin. Virtually all of the circuitry in the LT3845A is biased via this internal linear regulator output (VCC). This pin is decoupled with a low ESR, 1µF 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 LT3845A 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 maximizes conversion efficiencies and is common practice. If VCC is maintained above 6.5V using an external source, the LT3845A can continue to operate with VIN as low as 4V.

The LT3845A 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.