application
INFO
UCC1581
UCC2581
UCC3581
available
Micropower Voltage Mode PWM
BLOCK DIAGRAM
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
UDG-95011-1
Note: Pin Connection shown for 14-pin Package
DESCRIPTION
The UCC3581 voltage mode pulse width modulator is de- A linear preregulator driver in conjunction with an exter-
signed to control low power isolated DC - DC converters nal depletion mode N-MOSFET provides initial controller
in applications such as Subscriber Line Power (ISDN power. Once the bootstrap supply is functional, the
I.430). Primarily used for single switch forward and preregulator is shut down to conserve power. During light
flyback converters, the UCC3581 features BiCMOS cir- load, power is saved by providing a programmable mini-
cuitry for low startup and operating current, while main- mum duty cycle clamp. When a duty cycle below the
taining the ability to drive power MOSFETs at minimum is called for, the modulator skips cycles to pro-
frequencies up to 100kHz. The UCC3581 oscillator al- vide the correct average duty cycle required for output
lows the flexibility to program both the frequency and the regulation. This effectively reduces the switching fre-
maximum duty cycle with two resistors and a capacitor. A quency, saving significant gate drive and power stage
TTL level input is also provided to allow synchronization losses.
to an external frequency source.
The UCC3581 is available in 14-pin plastic and ceramic
The UCC3581 includes programmable soft start circuitry, dual-in-line packages and in a 14-pin narrow body small
overcurrent detection, a 7.5V linear preregulator to con- outline IC package (SOIC). The UCC1581 is specified for
trol chip VDD during startup, and an on-board 4.0V logic
supply.
operation from −55°C to +125°C, the UCC2581 is speci-
fied for operation from −40°C to +85°C, and the
UCC3581 is specified for operation from 0°C to +70°C.
The UCC3581 provides functions to maximize light load
efficiency that are not normally found in PWM controllers.
MARCH 1999 - REVISED MARCH 2003 - SLUS295B