MIC3002_10 Datasheet, PDF (22/65 Pages) Micrel Semiconductor – FOM Management IC with Internal Calibration

Micrel, Inc.
Calibration Modes
The default mode of calibration in the MIC3002 is
external calibration, for which INTCAL bit (bit 0 in
OEMCF3 register) is set to 0. The internal calibration
mode is selected by setting INTCAL bit to 1.
A/ External Calibration
The voltage and temperature values returned by the
MIC3002’s A/D converter are internally calibrated. The
binary values of TEMPh:TEMPl and VOLTh:VOLTl are in
the format called for by SFF-8472 under Internal
Calibration.
SFF-8472 calls for a set of calibration constants to be
stored by the transceiver OEM at specific non-volatile
memory locations; refer to SFF-8472 specifications for
memory map of calibration coefficient. The MIC3002
provides the non-volatile memory required for the
storage of these constants. The Digital Diagnostic
Monitoring Interface specification should be consulted
for full details. Slopes and offsets are stored for use with
voltage, temperature, bias current, and transmitted
power measurements. Coefficients for a fourth-order
polynomial are provided for use with received power
measurements. The host system can retrieve these
constants and use them to process the measured data.
Voltage
The voltage values returned by the MIC3002’s A/D
converter are internally calibrated. The binary values of
VOLTh:VOLTl are in the format called for by SFF-8472
under Internal Calibration. Since VINh:VINl requires no
processing, the corresponding slope should be set to
one and the offset to zero.
Temperature
The temperature values returned by the MIC3002’s A/D
converter are internally calibrated. The binary values of
TEMPh:TEMPl are in the format called for by SFF-8472
under Internal Calibration.
Bias Current
Bias current is sensed via an external sense resistor as
a voltage appearing between VILD+ and VILD-. The
value returned by the A/D is therefore a voltage
analogous to bias current. Bias current, IBIAS, is simply
V_VILD/R_SENSE. The binary value in IBIASh (IBIASl is
always zero) is related to bias current by:
(1)
MIC3002
The value of the least significant bit (LSB) of IBIASh is
given by:
(2)
Per SFF-8472, the value of the bias current LSB is 2µA.
The conversion factor, “slope”, needed is therefore:
The tolerance of the sense resistor directly impacts the
accuracy of the bias current measurement. It is
recommended that the sense resistor chosen be 1%
accurate or better. The offset correction, if needed, can
be determined by shutting down the laser, i.e., asserting
TXDISABLE, and measuring the bias current. Any non-
zero result gives the offset required. The offset will be
equal and opposite to the result of the “zero current”
measurement.
TX Power
Transmit power is sensed via a resistor carrying the
monitor photodiode current. In most applications, the
signal at VMPD will be feedback voltage on FB. The
VMPD voltage may be measured relative to GND or
V_DDA depending on the setting of the BIASREF bit in
OEMCFG1. The value returned by the A/D is therefore a
voltage analogous to transmit power. The binary value in
TXOPh (TXOPl is always zero) is related to transmit
power by:
(3)
For a given implementation, the value of R_SENSE is
known. It is either the value of the external resistor or the
chosen value of RFB used in the application. The
constant, K, will likely have to be determined through
experimentation or closed-loop calibration, as it depends
on the monitoring photodiode responsivity and coupling
efficiency.
It should be noted that the APC circuit acts to hold the
transmitted power constant. The value of transmit power
reported by the circuit should only vary by a small
amount as long as APC is functioning correctly.
July 2007
22
M9999-073107-B
[e-mail] or [phone]