MIC3000 Datasheet, PDF (21/68 Pages) Micrel Semiconductor – SFP Management IC

MIC3000
V_DDA
Saturation Detector
95% V_DDA
5% V_DDA
SATURATION_FAULT
V_COMP
t_FLTTMR
COUNTER
FLTTMR
Figure 9. Saturation Detector
CNTRL
LOS
VRX
OEMCFG3
LOS
DIS
V_DDA
RXLOS
LOSFLT
FLTDAC
Micrel
Figure 10. RXLOS Comparator Logic
Diode Faults
The MIC3000 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 MIC3000
provides a facility for temperature compensation of the A.P.C.
loop setpoint, laser modulation current, bias current fault
comparator threshold, and bias current high alarm flag thresh-
old. Temperature compensation is performed using a look-up
table (LUT) that stores values corresponding to each mea-
sured temperature over a 128°C span. Four identical tables
reside at serial address A4_h as summarized in Table 11. The
range of temperatures spanned by the tables is program-
mable via the LUTOFF register. Each table entry is a signed
twos complement number that is used as an offset to the
parameter being compensated. The default value of all table
entries is zero, giving a flat response.
The A/D converter reports a new temperature sample each
t_CONV. This occurs at roughly 10Hz. To prevent temperature
oscillation due to thermal or electrical noise, sixteen succes-
sive temperature samples are averaged together and used to
index the LUTs. Temperature compensation results are
therefore updated at 16×t_CONV intervals, or about 1.6 sec-
onds. This can be expressed as shown in Equation5.
T_COMPm
=
T_n
+
T_n+1 +
T_{n + 2}
16
+
• • •T_n+15
(5)
Each time an updated average value is acquired, a new offset
value for the APC setpoint is read from the corresponding
look-up table (see Table 12) and transferred to the APC
circuitry. This is illustrated in Equation 6. In a same way, new
offset values are taken from similar look-up tables (see Table
13 and Table 14), added to the nominal values and trans-
ferred into the modulation and fault comparator DACs. The
bias current high alarm threshold, is compensated using a
fourth look-up table (see Table 15). This compensation
happens internally and does not affect any host-accessible
registers.
October 2004
21
M9999-101204