鈥?/div>
External Oscillator
Synchronization Capability
BLOCK DIAGRAM
Note: Pin Connection shown for 14-pin Package
UDG-95011-1
DESCRIPTION
The UCC3581 voltage mode pulse width modulator is
designed to control low power isolated DC - DC convert-
ers in applications such as Subscriber Line Power (ISDN
I.430). Primarily used for single switch forward and
flyback converters, the UCC3581 features BiCMOS cir-
cuitry for low startup and operating current, while main-
taining the ability to drive power MOSFETs at
frequencies up to 100kHz. The UCC3581 oscillator al-
lows 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 con-
trol chip V
DD
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 exter-
nal 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 mini-
mum duty cycle clamp. When a duty cycle below the
minimum is called for, the modulator skips cycles to pro-
vide the correct average duty cycle required for output
regulation. This effectively reduces the switching fre-
quency, 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
鈭?5擄C
to +125擄C, the UCC2581 is speci-
fied for operation from
鈭?0擄C
to +85擄C, and the
UCC3581 is specified for operation from 0擄C to +70擄C.
SLUS295 - MARCH 1999