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LTC1921CS8

Dual –48V Supply and Fuse Monitor

Linear Technology 

LTC1921

PI FU CTIO S

VA (Pin 1): Supply to be Monitored. The voltage at this pin

is compared to the valid supply voltage window and the

result is output at OUT A (Pin 5). Supply current is drawn

from VA as well as from VB (Pin 8).

FUSE A (Pin 2): This pin monitors the state of a fuse by

comparing the voltage at this pin to the voltage at VA

(Pin␣ 1). The result is output at OUT F (Pin 4).

RTN (Pin 3): Supply Return Reference. This pin must be

at an equal or higher potential than the other pins and

should be wired to the – 48V return.

OUT F (Pin 4): This pin indicates the state of the external

fuses by ORing the comparisons made to the FUSE A and

FUSE B pins. If VFUSEA ≅ VA (VFUSEA is within the specified

window around VA) and VFUSEB ≅ VB, then OUT F will

exhibit a high internal impedance to the RTN pin. If

VFUSEA ≠ VA or VFUSEB ≠ VB, then OUT F is shorted

internally to the RTN pin and can shunt enough current to

turn off an optoisolator or LED wired between these pins.

OUT F should be clamped externally so that it cannot be

driven more than 8V below RTN. This is done automati-

cally by the optoisolator or LED diodes shown in the

application circuits.

OUT A (Pin 5): Indicates the state of VA. If VA is within the

specified voltage window (neither undervoltage nor over-

voltage), OUT A will exhibit a high internal impedance to the

OUT F pin. If VA is outside the specified overvoltage or

undervoltage limits, then OUT A is shorted internally to the

OUT F pin and can shunt enough current to turn off an

optoisolator or LED wired between these pins. OUT A should

be clamped externally so that it cannot be driven more than

8V below RTN. This is done automatically by the

optoisolator or LED diodes shown in the application circuits.

OUT B (Pin 6): Indicates the state of VB. If VB is within the

specified voltage window (neither undervoltage nor over-

voltage), OUT B will exhibit a high internal impedance to the

OUT A pin. If VB is outside the specified overvoltage or

undervoltage limits, then OUT B is shorted internally to the

OUT A pin and can shunt enough current to turn off an

optoisolator or LED wired between these pins. OUT B should

be clamped externally so that it cannot be driven more than

8V below RTN. This is done automatically by the

optoisolator or LED diodes shown in the application circuits.

FUSE B (Pin 7): This pin monitors the state of a fuse by

comparing the voltage at this pin to the voltage at VB

(Pin␣ 8). The result is output at OUT F (Pin 4).

VB (Pin 8): Supply to be Monitored. The voltage at this pin

is compared to the valid supply voltage window and the

result is output at OUT B (Pin 6). Supply current is drawn

from VB as well as from VA (Pin 1).

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