Datasheet
SLUS431A – MAY 2000 – REVISED DECEMBER 2000
2
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
description
The UCC3961 advanced primary-side startup controller is a unique solution that provides all the primary-side
functions required for a single-ended, isolated, switch-mode power converter incorporating secondary-side
PWM control. It is usable with a wide range of secondary control circuits and is especially well suited for systems
where sophisticated handling of overload conditions is required.
Secondary-side control assumes that output voltage and current measurements are interfaced directly to an
output ground-referenced PWM stage that develops the power switch command for the supply. This digital
PWM command can then be transmitted to the primary-side power switch through a simple and low-cost
isolating pulse transformer. With secondary-side control, it is much easier to monitor and control the system load
with tightly coupled analog control loops. Load-oriented features such as output current sharing and
synchronous rectification are implemented more easily.
The UCC3961 provides all the circuitry required on the primary side of a secondary-side controlled power
supply. It features a free running 60-kHz to 360-kHz oscillator which is synchronizable to the secondary-side
PWM signal and also has the ability to accept start/stop PWM commands from the isolating pulse-edge
transformer (PET). The use of an extremely small and low-cost pulse transformer allows for higher converter
bandwidth. This also eliminates the loop-gain variations due to initial accuracy and aging of an opto-coupler
feedback element or the size penalty of a gate transformer. It also includes an undervoltage lockout circuit with
2-V hysteresis, a low-current startup with active low during UVLO, a soft-start capability, a 5-V reference and
a high-current power output.
Advanced features of UCC3961 allows implementation of initial startup with optional high-voltage disconnect
after starting, and input voltage monitoring with turnoff for either undervoltage or overvoltage conditions. Other
features include power-switch current protection, pulse-by-pulse current limiting, shutdown after a
programmable delay and continuous input volt*second clamp. The UCC3961 also provides a multi purpose,
bidirectional shutdown pin (SD), that can be used in conjunction with the devices on the START pin, to modify
the converters behavior in overload conditions. The possible configurations include continuous-peak current
limiting, delayed or immediate shutdown with full cycle soft restart or fully latched overcurrent shutdown.
In a non-typical use, the UCC3961 can accommodate an analog feedback signal through an opto-isolator where
the UCC3961 then can operate in voltage-mode control mode with primary-side peak current limiting.
The UCC3961 and the UCC2961 are available in the 14-pin SOIC (D) and PDIP (P) packages. For applications
where only startup and control are desired, a simplified version of this product is offered in an 8-pin package
as the UCC2960 and UCC3960.
AVAILABLE OPTIONS
PACKAGED DEVICES
TA
SOIC–14
SMALL OUTLINE
(D)
PDIP–14
PLASTIC DIP
(N)
–40C to 85C UCC2961D UCC2961N
0C to 70C UCC3961D UCC3961N
†
The SOIC (D) packages are available taped and reeled. Add an R suffix
to the device type (e.g., UCC2961DR) to order quantities of 2500
devices per reel.
VDD
START
UVS
OUT
PGND
VS
CS
1
2
3
4
5
6
7
14
13
12
11
10
9
8
SD
OVS
REF
AGND
SS
FB
RT
D OR N PACKAGES
(TOP VIEW)