The UCC3581 voltage mode pulse width modulator is designed to control low power isolated DC - DC converters in applications such as Subscriber Line Power (ISDN I.430). Primarily used for single switch forward and flyback converters, the UCC3581 features BiCMOS circuitry for low startup and operating current, while maintaining the ability to drive power MOSFETs at frequencies up to 100kHz. The UCC3581 oscillator allows the flexibility to program both the frequency and the maximum duty cycle with two resistors and a capacitor. A TTL level input is also provided to allow synchronization to an external frequency source.

The UCC3581 includes programmable soft start circuitry, overcurrent detection, a 7.5V linear preregulator to control chip VDD during startup, and an on-board 4.0V logic supply.

The UCC3581 provides functions to maximize light load efficiency that are not normally found in PWM controllers.

A linear preregulator driver in conjunction with an external depletion mode N-MOSFET provides initial controller power. Once the bootstrap supply is functional, the preregulator is shut down to conserve power. During light load, power is saved by providing a programmable minimum duty cycle clamp. When a duty cycle below the minimum is called for, the modulator skips cycles to provide the correct average duty cycle required for output regulation. This effectively reduces the switching frequency, saving significant gate drive and power stage losses.

The UCC3581 is available in 14-pin plastic and ceramic dual-in-line packages and in a 14-pin narrow body small outline IC package (SOIC). The UCC1581 is specified for operation from –55°C to +125°C, the UCC2581 is specified for operation from –40°C to +85°C, and the UCC3581 is specified for operation from 0°C to +70°C.

• Low 85µA Startup Current
• Low 300µA Operating Current
• Automatically Disabled Startup Preregulator
• Programmable Minimum Duty Cycle with Cycle Skipping
• Programmable Maximum Duty Cycle
• Output Current 1A Peak Source and Sink
• Programmable Soft Start
• Programmable Oscillator Frequency
• External Oscillator Synchronization Capability