MIC3003 Datasheet, PDF (22/75 Pages) Micrel Semiconductor – FOM Management IC with Internal Calibration

Micrel, Inc.
Calibration Modes
The default mode of calibration in the MIC3003 is
external calibration, for which the INTCAL bit (bit 0 in
OEMCFG3 register) is set to 0. The internal calibration
mode is selected by setting INTCAL to 1.
A/ External Calibration
The voltage and temperature values returned by the
MIC3003’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 the SFF-8472 specifications
for the memory map of the calibration coefficients. The
MIC3003 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 MIC3003’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 MIC3003’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.
The temperature value may be offset by storing a value
in A6:74(4Ah). The temperature offset is a six-bit signed
quantity with .5 degrees C resolution.
The temperature offset coefficient at A6:74(4Ah) is used
in the same way in both internal and external calibration
modes.
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:
MIC3003
(1)
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 necessary conversion factor, “slope”, 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
selected internal value of RFB. The constant, K, will
likely have to be determined through experimentation or
closed-loop calibration, as it depends on the monitoring
photodiode responsively 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 the APC is functioning correctly.
November 2009
22
M9999-111209-C
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