Datasheet

A unique protection feature incorporated in the UC3871
is the Open Lamp Detect circuit. An open lamp interrupts
the current feedback loop and causes very high secon-
dary voltage. Operation in this mode will usually break-
down the transformer’s insulation, causing permanent
damage to the converter. The open lamp detect circuit,
shown in Figure 3 senses the lamp current feedback sig-
nal at the error amplifiers input, and shuts down the out-
puts if insufficient signal is present. Soft-start circuitry
limits initial turn-on currents and blanks the open lamp
detect signal.
Other features are included to minimize external circuitry
requirements. A logic level enable pin shuts down the IC,
allowing direct connection to the battery. During shut-
down, the IC typically draws less than 1µA. The UC3871,
operating from 4.5V to 20V, is compatible with almost all
battery voltages used in portable computers. Under-volt-
age lockout circuitry disables operation until sufficient
supply voltage is available, and a 1% voltage reference
insures accurate operation. Both inputs to the LCD sup-
ply error amplifier are uncommitted, allowing positive or
negative supply loop closure without additional circuitry.
The LCD supply modulator also incorporates cycle-by-
cycle current limiting for added protection.
Figure 3
UC1871
UC2871
UC3871
UNITRODE INTEGRATED CIRCUITS
7 CONTINENTAL BLVD. • MERRIMACK, NH 03054
TEL. [phone] • FAX [phone]
APPLICATION INFORMATION (cont.)
Figure 2
UC1871 Open Lamp Detect Circuitry
UC3871 Oscillator Section
5