MIC3000BML datasheet(20/68 Pages) MICREL

20 / 68 page

MIC3000

Micrel

M9999-101204

October 2004

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

only during calibration and testing or permanently.

FLTDAC

IBFLT

COUNTER

tFLTTMR

FLTTMR

VDDA

Saturation Detector

VCOMP

VILD

VUVLO

VDD

VMPD

DIODE_FAULT

TXFLT bit

/LASER_SHUTDOWN

TXFAULT pin

FLTDAC

TXPFLT

5% VDDA

95% VDDA

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, tFLTTMR, 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

(

FLTTMR). Note that a saturation comparator cannot be relied

upon to meet certain eye-safety standards that require 100

µs

response times. This is because the operation of a saturation

detector is limited by the loop bandwidth, i.e., the choice of

CCOMP. 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 <100

µs 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-

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

µs (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 VMOD,

VBIAS, and SHDN (if enabled) outputs will be driven to their

shutdown state according to the state of the configuration

bits. The shutdown states of VMOD, VBIAS, 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 IMOD and the APC

setpoint are read from the corresponding tables, added to the

set values, and transferred to DACs.

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