General Description

The MIC38HC4x family are fixed frequency current-mode PWM controllers with 1A drive current capability. Micrel’s BiCMOS devices are pin-compatible with 384x bipolar devices. Their high output drive, with fast rise and fall times, combined with low startup current make it an ideal PWM controller where high efficiency is required.

Undervoltage lockout circuitry allows the ’42 and ’44 versions to start up at 14.5V and operate down to 9V, and the ’43 and ’45 versions start at 8.4V with operation down to 7.6V. All versions operate up to 20V.

When compared to bipolar UC384x devices operating from a 15V supply, start-up current has been reduced to 50µA typical and operating current has been reduced to 4.0 mA typical. Decreased output rise and fall times drive larger MOSFETs, and rail-to-rail output capability increases efficiency, especially at lower supply voltages. The MIC38HC4x also features a trimmed oscillator discharge current and bandgap reference.

Features

• Fast 20ns output rise and 15ns output fall times

• –40°C to +85°C temperature range exceeds UC284x specifications

• High-performance, low-power BiCMOS Process

• Ultralow start-up current (50µA typical)

• Low operating current (4mA typical)

• High output drive (1A peak current, HC version)

• CMOS outputs with rail-to-rail swing

• Current-mode operation up to 500kHz

• Trimmed 5V bandgap reference

• Pin-for-pin compatible with UC3842/3843/3844/3845(A)

• Trimmed oscillator discharge current

• UVLO with hysteresis

• Low cross-conduction currents

Applications

• Current-mode, off-line, switched-mode power supplies

• Current-mode, dc-to-dc converters.

• Step-down “buck” regulators

• Step-up “boost” regulators

• Flyback, isolated regulators

• Forward converters

• Synchronous FET converters