LT1786F

The LT1786F is a fixed frequency, current mode, switching regulator that provides the control function for Cold Cathode Fluorescent Lighting (CCFL). The IC includes an efficient high current switch, an oscillator, output drive logic, control circuitry and a micropower 6-bit 100µA full-scale current output DAC. The DAC provides simple “bits-to-lamp-current control” and communicates using the 2-wire SMBus serial interface. The LT1786F acts as an SMBus slave device using one of two selectable SMBus addresses set by the address pin ADR.

On Power-up, the DAC output current assumes midrange or zero scale, depending on the logic state of the ADR pin.The entire IC can be shut down through the SMBSUS pin or by setting the SHDN bit = 1 in the SMBus command byte. Digital data for the DAC output current is retained internally and the supply current drops to 40µA for standby operation. The active low SHDN pin disables the CCFL control circuitry, but keeps the DAC alive. Supply current in this operating mode drops to 150µA.

The LT1786F control circuitry operates from a logic supply voltage of 3.3V or 5V. The IC also has a battery supply pin that operates from 4.5V to 30V. The LT1786F draws 6mA typical quiescent current. A 200kHz switching frequency minimizes magnetic component size. Current mode switching techniques with cycle-by-cycle limiting gives high reliability and simple loop frequency compensation. The LT1786F is available in a 16-pin narrow SO package.

Applications

• Notebook and Palmtop Computers
• Portable Instruments
• Personal Digital Assistants