Datasheet

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 Overvoltage or Short to V
CC
Condition
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
V
CC
in resistor setting mode)
Alarm if shorted to V
CC
(the alarm is valid for
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 V
CC
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
Resistor mode: tied to V
CC
RPAV Voltage mode: fault state occurs Fault state occurs Voltage mode: fault state occurs
Resistor mode: tied to V
CC
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: tied to V
CC
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