MIC3000 Datasheet, PDF (20/68 Pages) Micrel Semiconductor – SFP Management IC

MIC3000
Fault Comparators
In addition to detecting and reporting the events specified in
SFF-8472, the MIC3000 also monitors five fault conditions:
inadequate supply voltage, thermal diode faults, excessive
bias current, excessive transmit power, and APC op-amp
saturation. Comparators monitor these parameters in order
to respond quickly to fault conditions that could indicate link
failure or safety issues, see Figure 8. When a fault is detected,
the laser is shut down and TXFAULT is asserted. Each fault
source may be independently disabled using the FLTMSK
register. FLTMSK is non-volatile, allowing faults to be masked
only during calibration and testing or permanently.
V_DDA
Saturation Detector
95% V_DDA
5% V_DDA
V_COMP
t_FLTTMR
COUNTER
FLTTMR
IBFLT
FLTDAC
VILD
TXFAULT pin
VUVLO
V_DD
/LASER_SHUTDOWN
TXFLT bit
TXPFLT
FLTDAC
VMPD
DIODE_FAULT
Figure 8. Fault Comparator Logic
Thermal diode faults are detected within the temperature
measurement subsystem when an out-of-range signal is
detected. A window comparator circuit monitors the voltage
on the compensation capacitor to detect APC op-amp satu-
ration (Figure 9). Op-amp saturation indicates that some fault
has occurred in the control loop such as loss of feedback. The
saturation detector is blanked for a time, t_FLTTMR, following
laser turn-on since the compensation voltage will essentially
be zero at turn-on. The FLTTMR interval is programmable
from 0.5ms to 127ms (typical) in increments of 0.5ms
(φ_FLTTMR). Note that a saturation comparator cannot be relied
upon to meet certain eye-safety standards that require 100µs
response times. This is because the operation of a saturation
detector is limited by the loop bandwidth, i.e., the choice of
C_COMP. Even if the comparator itself was very fast, it would
be subject to the limited slew-rate of the APC op-amp. Only
the other fault comparator channels will meet <100µs timing
requirements.
A similar comparator circuit monitors received signal strength
and asserts RXLOS when loss-of-signal is detected (Figure
10). RXLOS will be asserted when and if VRX drops below the
level programmed in LOSFLT. The loss-of-signal comparator
may be disabled completely by setting the LOSDIS bit in
OEMCFG3. Once the LOS comparator is disabled, an exter-
Micrel
nal device may drive RXLOS. The state of the RXLOS pin is
reported in the CNTRL register regardless of whether it is
driven by the internal comparator or by an external device. A
programmable digital-to-analog converter provides the com-
parator reference voltages for monitoring received signal
strength, transmit power, and bias current. Glitches less than
4µs (typical) in length are rejected by the fault comparators.
Since laser bias current varies greatly with temperature,
there is a temperature compensation look-up table for the
bias current fault DAC value.
When a fault condition is detected, the laser will be immedi-
ately shutdown and TXFAULT will be asserted. The V_MOD,
V_BIAS, and SHDN (if enabled) outputs will be driven to their
shutdown state according to the state of the configuration
bits. The shutdown states of V_MOD, V_BIAS, and SHDN versus
the configuration bit settings are shown in Table 8, Table 9,
and Table 10.
Duty-Cycle Limiting
When a fault occurs and TXFAULT is asserted, an internal
timer starts. Operation cannot resume until this timer expires.
This limits the duty-cycle that can be achieved while a fault
condition is present, preventing the host from causing an eye-
unsafe condition by continually cycling the laser on and off.
Given that the fault comparator propagation delay is 95µs
and the restart delay is 200ms, the maximum duty-cycle that
can theoretically be achieved in the presence of a persistent
fault is on the order of 0.095/200ms ≈ 0.0475% (0.095s is the
maximum fault comparator propagation delay; 200ms is the
typical reset delay interval).
If a fault occurs and the host toggles TXDISABLE within
200ms, the MIC3000 will wait until the interval expires before
restarting the laser. If the restart delay has expired, i.e., it has
been at least 200ms since the last occurrence of a fault, then
the MIC3000 will begin the restart sequence immediately.
The operation of this timer is transparent to the host and does
not require any special action. The system will still meet the
300ms startup requirement called for in specifications such
as the SFP MSA. If the host toggles TXDISABLE more than
once during the 200ms interval, the timing remains the same.
The laser is restarted after the expiration of the 200ms timer.
Temperature Measurement
The temperature-to-digital converter for both internal and
external temperature data is built around a switched current
source and an eight-bit analog-to-digital converter. The tem-
perature is calculated by measuring the forward voltage of a
diode junction at two different bias current levels. An internal
multiplexer directs the current source’s output to either an
internal or external diode junction. The value of the ZONE bit
in OEMCFG1 determines whether readings are taken from
the on-chip sensor or from the XPN input. The external PN
junction may be embedded in an integrated circuit, or it may
be a diode-connected discrete transistor. This data is also
used as the input to the temperature compensation look-up
tables. Each time temperature is sampled and an updated
value acquired, new corrective values for I_MOD and the APC
setpoint are read from the corresponding tables, added to the
set values, and transferred to DACs.
M9999-101204
20
October 2004