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LTC1921CMS8

Dual –48V Supply and Fuse Monitor

Linear Technology 

LTC1921

APPLICATIO S I FOR ATIO

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

conjunction with VA and VB for simple window compari-

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

supply A (VA) and a diode connected between OUT B and

OUT A will indicate the condition of supply B (VB). A

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.

SUPPLY A

–48V

SUPPLY B

–48V

–48V

RETURN

F1

F2

R1 R2

100k 100k

1

VA

3

RTN

4

OUT F

8

VB

LTC1921

2

FUSE A

7

5

FUSE B OUT A

47k

5V

FUSE

STATUS

MOC207

47k

5V

SUPPLY A

STATUS

MOC207

47k

5V

SUPPLY B

STATUS

6

OUT B

D1

D2

1921 F02

MOC207

R3

47k

1/4W

–48V OUT

= LOGIC COMMON

VA

OK

OK

UV OR OV

UV OR OV

VB

OK

UV OR OV

OK

UV OR OV

SUPPLY A

STATUS

0

0

1

1

OK: WITHIN SPECIFICATION

OV: OVERVOLTAGE

UV: UNDERVOLTAGE

SUPPLY B

STATUS

0

1

0

1

VFUSE A

= VA

= VA

≠ VA

≠ VA

VFUSE B

= VB

≠ VB

= VB

≠ VB

FUSE STATUS

0

1

1

1*

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

Figure 2.

1921f

8

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