MIC3003GML datasheet(29/75 Pages) MICREL

29 / 75 page

Micrel, Inc.

MIC3003

November 2009

M9999-111209-C

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

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

127.5ms (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 MIC3003 can also except and respond to fault inputs

from external devices. See the “SHDN and TXFIN”

section.

A similar comparator circuit monitors received signal

strength and asserts RXLOS when loss-of-signal is

detected (Figure 12). RXLOS will be asserted 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 received signal strength, transmit power, and

bias current. 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

shutdown immediately and TXFAULT will be asserted.

The VMOD, VBIAS, and SHDN (if enabled by setting

OEMCFG5 bit 7 to 1) 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

12, Table 13, and Table 14.

SHDN and TXFIN

SHDN and TXFIN are optional functions of pin 7. SHDN

is an output function and is designed to drive a redundant

safety switch in the laser current path. TXFIN is an input

function and serves as an input for fault signals from

external devices that must be reported to the host via

TXFAULT. The SHDN function is designed for applications

in which the MIC3003 is performing all APC and laser

management tasks. The TXFIN function is for situations in

which an external device such as a laser diode driver IC is

performing laser management tasks, including fault

detection.

If the TXFIN bit in OEMCFG3 is zero (the default mode),

SHDN will be activated anytime the laser is off. Thus, it will

be active if 1) TXDISABLE is asserted, 2) STXDIS in the

CNTRL register, is set, or 3) a fault is detected. SHDN is a

push-pull logic output. Its polarity is programmable via the

SPOL bit in OEMCFG1.

If TXFIN bit is set to one, pin 7 serves as an input that

accepts fault signals from external devices such as laser

diode driver ICs. Multiple TXFAULT signals cannot simply

be wire-ORed together as they are open-drain and active

high. The input polarity is programmable via the TXFPOL

bit in OEMCFG3. TXFIN is logically ORed with the

MIC3003’s internal fault sources to produce TXFAULT and

determine the value of the transmit fault bit in CNTRL. See

Figure 10.

Figure 11. Saturation Detector

Figure 12. RXLOS Comparator Logic

MIC3003GML 数据表(PDF) 29 Page - Micrel Semiconductor

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