UC1903
UC2903
UC3903
OPERATION AND APPLICATION INFORMATION
(continued)
Figure 8.
The OV and UV comparators on the UC1903 trigger respective fault delay circuits when one or more of the sense inputs
move outside the fault window. Input clamps insure proper operation under extreme fault conditions. Terminating the UV delay ca-
pacitor to V
REF
assures correct logic at power up.
Start Latch and Supply Under-Voltage Sense Allow
Predictable Power-Up
The supply under-voltage sense and start-latch circuitry
on the UC1903 prevents fault indications during start-up
or low input supply (+V
IN
) conditions. When the input sup-
ply voltage is below the supply under-voltage threshold
the OV and UV fault outputs are disabled and the
POWER OK output is active low. The POWER OK output
will remain active until the input supply drops below ap-
proximately 3.0V. With +V
IN
below this level, all of the
open collector outputs will be off.
When the input supply is low, the under-voltage sense cir-
cuitry resets the start-latch. With the start-latch reset, the
UV fault output will remain disabled until the input supply
rises to its normal operating level (8-40V), and all of the
sense inputs are above the under-voltage threshold. This
allows slow starting, or supply sequencing, without an ar-
tificial under-voltage fault indication. Once the latch is set,
the UV fault output will respond if any of the sense inputs
drop below the under-voltage threshold.
UNITRODE CORPORATION
7 CONTINENTAL BLVD.
•
MERRIMACK, NH 03054
TEL. (603) 424-2410
•
FAX (603) 424-3460
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