MIC3002GML datasheet(28/65 Pages) MICREL

28 / 65 page

Micrel, Inc.

MIC3002

July 2007

M9999-073107-B

hbwhelp@micrel.com or (408) 955-1690

Figure 9. MIC3002 Power-On Timing (OE = 1)

Fault Comparators

In addition to detecting and reporting the events specified

in SFF-8472, the MIC3002 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

10. 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.

Figure 10. 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

saturation (Figure 11). 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 (tFLTTMR). Note that a saturation

comparator cannot be relied upon to meet certain eye-

safety standards that require 100ms 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

<100ms

timing

requirements.

The MIC3002 can also except and respond to fault inputs

from external devices. See “SHDN and TXFIN” section.

A similar comparator circuit monitors received signal

strength

and

asserts

RXLOS

when

loss-of-signal

detected (Figure 12). RXLOS will be asserted when and if

VRX drops below the level programmed in LOSFLT.

Hysteresis is implemented such that RXLOS will be de-

asserted when VRX subsequently rises above the level

programmed in LOSFLTn. The loss-of-signal comparator

may be disabled completely by setting the LOSDIS bit in

OEMCFG3. Once the LOS comparator is disabled, an

external 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.

programmable

digital-to-analog

converter

provides the comparator reference voltages for monitoring

MIC3002GML Datasheet(PDF) 28 Page - Micrel Semiconductor

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