MIC3002_10 Datasheet, PDF (29/65 Pages) Micrel Semiconductor – FOM Management IC with Internal Calibration

Micrel, Inc.
MIC3002
received signal strength, transmit power, and bias
current. Glitches less than 10ms (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
shutdown immediately and TXFAULT will be asserted.
The V_MOD, V_BIAS, and SHDN if enabled, OEMCFG5-7 is
set to 1, 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 13, Table 14,
and Table 15.
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 MIC3002 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
CNTRL, 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 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
MIC3002’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
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/nine-bit analog-to-digital converter.
The temperature 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 the DACs.
Diode Faults
The MIC3002 is designed to respond in a failsafe manner to
hardware faults in the temperature sensing circuitry. If the
connection to the sensing diode is lost or the sense line is
shorted to V_DD or ground, the temperature data reported by
the A/D converter will be forced to its full-scale value
(+127°C). The diode fault flag, DFLT, will be set in
OEMCFG1, TXFAULT will be asserted, and the high
temperature alarm and warning flags will be set. The
reported temperature will remain +127°C until the fault
condition is cleared. Diode faults may be reset by toggling
TXDISABLE, as with any other fault. Diode faults will not be
detected at power up until the first A/D conversion cycle is
completed. Diode faults are not reported while TXDISABLE
is asserted.
Temperature Compensation
Since the performance characteristics of laser diodes and
photodiodes change with operating temperature, the
July 2007
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M9999-073107-B
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