UC1872
UC2872
UC3872
APPLICATION INFORMATION (cont.)
A unique protection feature incorporated in the UC3872
is the Open Lamp Detect circuit. An open lamp interrupts
the current feedback loop and causes very high second-
ary 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 amplifier’s 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 a battery. During shut-
down, the IC typically draws less than 1µA. The UC3872,
operating from 4.5V to 24V, is compatible with almost all
battery voltages used in portable computers and auto-
motive applications. Undervoltage lockout circuitry dis-
ables operation until sufficient supply voltage is
available, and a 1% voltage reference insures accurate
operation.
UDG-93018-2
Figure 1. Typical application.
5
TI [ TEXAS INSTRUMENTS ]
