MIC3000_07 datasheet(21/68 Pages) MICREL

21 / 68 page

October 2004

M9999-101204

MIC3000

Micrel

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 VDD 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 A4h 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

tCONV. 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.

COMPm

nn 1

n 2

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.

COUNTER

tFLTTMR

FLTTMR

SATURATION_FAULT

Saturation Detector

VCOMP

5% VDDA

95% VDDA

VDDA

Figure 9. Saturation Detector

FLTDAC

LOSFLT

OEMCFG3

LOS

DIS

CNTRL

LOS

VRX

RXLOS

VDDA

Figure 10. RXLOS Comparator Logic

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