ADN2872

Rev. 0 | Page 16 of 20

ALARMS

The ADN2872 has a latched active high monitoring alarm

(FAIL). The FAIL alarm output is an open drain in conformance

with SFP MSA specification requirements.

The ADN2872 has a three-fold alarm system that covers:



Use of a bias current higher than expected, likely as a result

of laser aging.



Out-of-bounds average voltage at the monitor photodiode

(MPD) input, indicating an excessive amount of laser

power or a broken loop.



Undervoltage in the IBIAS pin (laser diode cathode) that

increases the laser power.

The bias current alarm trip point is set by selecting the value of

resistor on the IBMON pin to GND. The alarm is triggered

when the voltage on the IBMON pin goes above 1.2 V.

FAIL is activated when the single-point faults in Table 5 occur.

Table 5. ADN2872 Single-Point Alarms

Alarm Type

Mnemonic

Undervoltage or Short to

GND Condition

Bias Current

IBMON

Alarm if > 1.2 V

Ignore

MPD Current

PAVSET

Alarm if > 2.0 V

Alarm if < 0.4 V

Crucial Nodes

ERREF (the ERRREF is designed tied to

voltage setting mode only)

Alarm if shorted to GND

IBIAS

Ignore

Alarm if < 0.6 V

Table 6. ADN2872 Response to Various Single-Point Faults in AC-Coupled Configuration, as Shown in Figure 32

Mnemonic

Short to GND

Open

CCBIAS

Fault state occurs

Fault state occurs

Does not increase laser average power

PAVSET

Fault state occurs

Fault state occurs

Fault state occurs

PAVREF

Voltage mode: fault state occurs

Fault state occurs

Fault state occurs

RPAV

Voltage mode: fault state occurs

Fault state occurs

Voltage mode: fault state occurs

Resistor mode: does not increase

average power

ERCAP

Does not increase laser average power

Does not increase laser average power

Does not increase laser average power

PAVCAP

Fault state occurs

Fault state occurs

Fault state occurs

DATAP

Does not increase laser average power

Does not increase laser average power

Does not increase laser average power

DATAN

Does not increase laser average power

Does not increase laser average power

Does not increase laser average power

ALS

Output currents shut off

Normal currents

Output currents shut off

ERSET

Does not increase laser average power

Does not increase laser average power

Does not increase laser average power

IMMON

Does not affect laser power

Does not increase laser average power

Does not increase laser average power

ERREF

Voltage mode: fault state occurs

Voltage mode: does not increase

average power

Does not increase laser average power

Resistor mode: fault state occurs

IBMON

Fault state occurs

Does not increase laser average power

Does not increase laser average power

FAIL

Fault state occurs

Does not increase laser average power

Does not increase laser average power

IMODP

Does not increase laser average power

Does not increase laser average power

Does not increase laser average power

IMODN

Does not increase laser average power

Does not increase laser average power

Does not increase laser power

IBIAS

Fault state occurs

Fault state occurs

Fault state occurs