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B0093-01

DISABLE

SDOWN

Inverter

Inverter

MUX

MUX

IBEN

IMODEN

MONP

MONB

I1

I0

I0

I1

VCC

I

PD

I

BIAS/68

1.25 V

2.8 V

Comparator

Comparator

Flipflop

CMOS

Buffer

Comparator

APCSET

MODSET

MODTC

IBMAX

APCSET

MODSET

MODTC

RES

T

Counter

RES

-

-

S

-

Q

Q

Q

Q

Q

Q

+

+

R

+

SHORT

START

IBMAX

FLTMODE

Short Circuit

to VCC or

GND Detect

+

-

+

-

ONET4201LD

SLLS677 – NOVEMBER 2005

DETAILED DESCRIPTION (continued)

Figure 2. Functional Representation of the Fault Detection Circuitry

A fault mode is produced if the laser cathode is grounded and the photocurrent causes MONP to exceed its

programmed threshold. Another fault mode can be produced if the laser diode end-of-life condition causes

excessive bias current and photodiode current that results in monitor voltages (MONP, MONB) being greater

than their programmed threshold. Other fault modes can occur if there are any I/O pin single-point failures (short

5

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