LTC1921

8

1921f

internal impedance to the return (RTN) pin. The applica-

tion must be designed so that an open fuse condition will

force the fuse input (FUSE A or FUSE B) to be outside the

specified window around the supply pins, such as with a

weak pull-up resistor to RTN, so that the LTC1921 can

properly indicate a fault at OUT F. If supply diodes that

exhibit high reverse leakage, such as Schottky diodes, are

used, then the values of the pull-up resistors must be

reduced accordingly.

The FUSE A and FUSE B pins may also be used in

son, provided that one of the circuit nodes to be compared

can provide the small amount of supply current required

to bias the IC. The LTC1921 is ideally suited for compari-

son functions in a circuit where only high supply voltages

are available.

Output Pins

The output pins in the LTC1921 are designed to shunt

external optoisolator diodes or LEDs during a supply or

fuse fault condition. Up to three diodes may be used in

series, one for each output. In this configuration

(Figure 2), a diode connected between OUT F and RTN will

indicate the condition of both fuses. A diode connected

between OUT A and OUT F will indicate the condition of

resistor connected from OUT B to the negative supply is

required to bias the diodes. The LTC1921 is designed to

allow the current from this resistor to flow through the

diodes during normal supply conditions and intact fuses,

and will shunt this current away from the proper diodes

during a fault condition. These diodes will further clamp

the output pin potentials to RTN in order to keep the

outputs within rated voltages. If LEDs are used instead of

optoisolators, they should be green since they will be lit

when the supplies are within the proper voltage range.

The LTC1921 may be connected in such a way as to OR

various outputs to allow the use of fewer optocouplers or

LEDs (Figures 3a and 3b). One and two diode circuits are

shown.

APPLICATIO S I FOR ATIO

Figure 2.

MOC207

MOC207

MOC207

FUSE

STATUS

SUPPLY A

STATUS

1921 F02

5V

47k

5V

47k

5V

47k

R3

47k

1/4W

SUPPLY B

STATUS

OK: WITHIN SPECIFICATION

OV: OVERVOLTAGE

UV: UNDERVOLTAGE

–48V OUT

= LOGIC COMMON

0: LED/PHOTODIODE ON

1: LED/PHOTODIODE OFF

*IF BOTH FUSES (F1 AND F2) ARE OPEN,

ALL STATUS OUTPUTS WILL BE HIGH

SINCE R3 WILL NOT BE POWERED

OUT F

–48V

RETURN

3

4

5

7

2

8

1

6

FUSE A

F1

D1

D2

F2

RTN

LTC1921

FUSE B

OUT A

OUT B

SUPPLY A

–48V

SUPPLY B

–48V

R1

100k

R2

100k

SUPPLY A

STATUS

0

0

1

1

OK

UV OR OV

OK

UV OR OV

OK

OK

UV OR OV

UV OR OV

SUPPLY B

STATUS

0

1

0

1

FUSE STATUS

0

1

1

1*