描述 Description

The UC3844B, UC3845B series are high performance fixed frequency current mode controllers. They are specifically designed for Off-Line and dc-to-dc converter applications offering the designer a cost-effective solution with minimal external components. These integrated circuits feature an oscillator, a temperature compensated reference, high gain error amplifier, current sensing comparator, and a high current totem pole output ideally suited for driving a power MOSFET. Also included are protective features consisting of input and reference undervoltage lockouts each with hysteresis, cycle-by-cycle current limiting, a latch for single pulse metering, and a flip-flop which blanks the output off every other oscillator cycle, allowing output deadtimes to be programmed from 50% to 70%. These devices are available in an 8-pin dual-in-line and surface mount (SO-8) plastic package as well as the 14-pin plastic surface mount (SO-14). The SO-14 package has separate power and ground pins for the totem pole output stage. The UCX844B has UVLO thresholds of 16 V (on) and 10 V (off), ideally suited for off-line converters. The UCX845B is tailored for lower voltage applications having UVLO thresholds of 8.5 V (on) and 7.6 V (off).

特性 Features

? Trimmed Oscillator for Precise Frequency Control
? Oscillator Frequency Guaranteed at 250 kHz
? Current Mode Operation to 500 kHz Output Switching Frequency
? Output Deadtime Adjustable from 50% to 70%
? Automatic Feed Forward Compensation
? Latching PWM for Cycle-By-Cycle Current Limiting
? Internally Trimmed Reference with Undervoltage Lockout
? High Current Totem Pole Output
? Undervoltage Lockout with Hysteresis
? Low Startup and Operating Current
? Pb-Free Packages are Available