Datasheet

ADN2871
Rev. A | Page 18 of 20
ALARMS
The ADN2871 has a latched, active high monitoring alarm
(FAIL). The FAIL alarm output is an open drain in conformance
to SFP MSA specification requirements.
The ADN2871 has a three-fold alarm system that covers
• Use of a bias current higher than expected, probably 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 node (laser diode cathode) that
would increase 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. The
circuit in
Figure 37 can be used to indicate that FAIL has been
activated while allowing the bias and modulation currents to
remain on. The transistor’s V
BE
clamps the FAIL voltage to below
1.3 V disabling the automatic shutdown of bias and modulation
currents. If an alarm has triggered and FAIL is activated, ALS
can be brought high and then low to clear the alarm.
05228-041
ADN2871
FAIL
R1
10kΩ
R2
330Ω
Q1
NPN
V
CC
LED
D1
Figure 37. FAIL Indication Circuit
Table 5. ADN2871 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 typical (±10% tolerance) Ignore
MPD Current PAVSET Alarm if > threshold (typical threshold: 1.5 V to 2.1 V) Alarm if < threshold (typical threshold: 0.6 V to 1.1 V)
Crucial Nodes
ERREF (the ERRREF
designed tied to
V
CC
in resistor
setting mode)
Alarm if shorted to V
CC
(the alarm is valid for
voltage setting mode only)
Ignore
IBIAS Ignore Alarm if shorted to GND
Table 6. ADN2871 Response to Various Single-Point Faults in AC-Coupled Configuration (as shown in
Figure 35)
Pin 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
Circuit designed to tie to V
CC
in resistor
setting mode, so no open case
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
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