FEATURES
• 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

DESCRIPTION
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.