LTC1921
7
1921f
Supply Monitoring
The LTC1921 is designed to monitor dual –48V power
supplies. This is accomplished with precision window
comparators and an accurate bandgap reference, as well
as internal level shifting circuitry. The comparators are
preset to standard voltage thresholds in order to accu-
rately verify the status of each supply. These comparators
also include precision hysteresis which allows accurate
determination of voltage recovery. Status of the two
supplies are indicated by the OUT A and OUT B pins.
The supply window comparison works in a straightfor-
ward way (Figure 1). As long as each supply magnitude
remains in the valid supply window (38.5V to 70V), the
outputs will indicate a valid supply condition by exhibiting
a high internal impedance. If a supply magnitude falls
below the undervoltage threshold (38.5V), then its respec-
tive output will short internally (OUT A to OUT F or OUT B
to OUT A) until that supply reaches the undervoltage
recovery threshold, which is preset to –43V. At this time,
the output will return to a high impedance state. If a supply
magnitude rises above the overvoltage threshold (70V),
then its respective output will short internally, just as with
an undervoltage condition. The output will return to its
nominal state when the supply overcomes the overvoltage
hysteresis.
Monitoring for each supply, V
A
and V
B
, is independent of
the condition of the other supply. The LTC1921 can be
powered equally from either V
A
, V
B
or both supplies. This
allows the LTC1921 to provide correct information at its
outputs as long as at least one supply is functional,
whether or not the fuses are intact (see Figure 2).
Undervoltage Lockout
If both supplies are active and their magnitude falls below
13V, or if only one supply is active and its magnitude falls
below 19V, the LTC1921 will lock all outputs into a fault
condition by closing all three output switches. This state
will be held until one supply magnitude is driven above
19V or both are driven above 13V.
Fuse Monitoring
In addition to monitoring two supplies, the LTC1921 can
monitor the condition of two supply fuses via the FUSE A
(Pin 2) and FUSE B (Pin 7) inputs. Fuse monitoring is
accomplished by comparing the potential at FUSE A to the
potential at supply V
A
and comparing the potential at
FUSE␣ B to the potential at supply V
B
. If V
FUSEA
is within the
specified voltage window around V
A
and V
FUSEB
is within
the specified voltage window around V
B
, the OUT F pin will
indicate that the fuses are intact by exhibiting a high
Figure 1. Supply Comparison
APPLICATIO S I FOR ATIO
WUUU
0
–38.5
–43
–48
–68.7
–70
1921 F01
TIME
NOMINAL
VOLTAGE
UNDERVOLTAGE
RECOVERY
OVERVOLTAGE
FAULT
OVERVOLTAGE
RECOVERY
UNDERVOLTAGE
FAULT
SUPPLY VOLTAGE (V)
