UCC3912
APPLICATION INFORMATION (cont.)
The overcurrent comparator senses both the DAC output
and a representation of the output current. When the out-
put current exceeds the programmed level the timing ca-
pacitor C
T
charges with 36µA of current. If the fault
occurs for the time it takes for C
T
to charge up to 1.5V,
the fault latch is set and the output switch is opened. The
output remains opened until C
T
discharges to 0.5V with a
1.2µA current source. Once the 0.5V is reached the out-
put is enabled and will either appear as a switch, if the
fault is removed, or a current source if the fault remains.
If the over current condition is still present then C
T
will
begin charging, starting the cycle over, resulting in ap-
proximately a 3% on time.
UDG-94019-1
Figure 3. UCC3912 on time control circuitry.
SAFETY RECOMMENDATIONS
Although the UCC3912 is designed to provide system
protection for all fault conditions, all integrated circuits
can ultimately fail short. For this reason, if the UCC3912
is intended for use in safety critical applications where
UL or some other safety rating is required, a redundant
safety device such as a fuse should be placed in series
with the device. The UCC3912 will prevent the fuse from
blowing virtually all fault conditions, increasing system
reliability and reducing maintainence cost, in addition to
providing the hot swap benefits of the device.
UDG-94101
Figure 4. RDS(on) vs temperature at 2A load current.
UNITRODE CORPORATION
7 CONTINENTAL BLVD. • MERRIMACK, NH 03054
TEL. (603) 424-2410 FAX (603) 424-3460
6
TI [ TEXAS INSTRUMENTS ]
