6

LTC1921

1921i

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.

APPLICATIO S I FOR ATIO

the condition of the other supply. The LTC1921 can be

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

indicate that the fuses are intact by exhibiting a high

0

0

R3

–38.5

–43

–48

–68.7

–70

1921 F01

TIME

NOMINAL

VOLTAGE

UNDERVOLTAGE

RECOVERY

OVERVOLTAGE

FAULT

OVERVOLTAGE

RECOVERY

UNDERVOLTAGE

FAULT

Figure 1. Supply Comparison