MIC3001 Datasheet, PDF (68/74 Pages) Micrel Semiconductor – SFP Management IC with Internal Calibration

MIC3001
Choosing C_COMP
The APC loop is compensated by a capacitor, C_COMP,
connected from COMP to either V_DDA or GNDA. This
capacitor adjusts the slew rate and bandwidth of the loop as
follows:
SlewRate = dV / dt = ^ISLEW
C_COMP
BW = ^GM
2πC_COMP
where:
I_SLEW = 64µA,
G_M = 125µMho
these relationships are shown graphically in Figure 28 and
Figure 29.
70
60
50
40
30
20
10
0
1 6 11 16 21 26 31 36 41 46 51
C_COMP (nF)
Figure 28. Slew Rate vs. C_COMP Value
2.50
2.00
1.50
1.00
0.50
0
10 20 30 40 50 60 70 80 90 100
C_COMP (nF)
Figure 29. Open Loop Unity-Gain Bandwidth
vs. C_COMP
The loop response should be tailored to the data rate,
encoding format and maximum run-lengths, and required
laser turn-on time. Higher data rates and/or shorter maximum
run lengths and/or faster turn-on times call for smaller capaci-
tors. Lower data rates and/or longer maximum run lengths
and/or slower turn-on times call for larger capacitors. In order
to meet the SFP/GBIC turn-on requirement of 1ms, for
example, do not employ a capacitor larger than 20nF. Low
ESR capacitors such as ceramics will give the best results.
Excessive ESR will reduce the effectiveness of C_COMP. The
capacitor’s voltage rating must exceed V_DDA. Some typical
values are shown in Table 22.
Micrel
Application
8b/10b encoding, ≥1Gbps, t_ON ≤ 1ms
SONET (62_b/64_b encoding), ≥1Gbps
≥155Mbps, t_ON ≤ 1ms
≥155Mbps
C_COMP (nF)
10
22
22
100
Table 22. Typical Values for C_COMP
While there is no theoretical upper limit on the size of C_COMP,
it is desirable for the loop to be able to track the changes
resulting from periodic temperature compensation. The typi-
cal temperature compensation update period is 1.6s. There-
fore, a maximum size of 1µF is recommended. If laser turn-
on time is not a factor, a value between 100nF and 1µF can
be used for virtually any typical application. The tradeoff is
that higher value capacitors have a larger physical size and
cost.
In order to maximize the power supply rejection ratio (PSRR),
C_COMP should be returned to GNDA when the V_BIAS output
is sourcing current, e.g., driving an NPN transistor
(SRCE bit = 1). C_COMP should be returned to V_DDA when the
V_BIAS output is sinking current, e.g., driving a PNP transistor
(SRCE bit =0).
Measuring Laser Bias Current
VILD+ and VILD– form a pair of pseudo-differential A/D
inputs for measuring laser diode bias current via a sense
resistor. The signal applied to these inputs is converted to a
single-ended, ground-referenced signal for input into the
ADC and bias current fault comparator. These inputs have
limited common-mode voltage range. The full-scale differen-
tial input range is V_REF/4 or about 300mV.
Figure 26 and Figure 27 illustrate the typical implementation
of this function. Note that VILD– is always connected to the
circuit’s reference potential: V_DD in the case of a common-
anode transmitter optical sub-assembly (TOSA) and GND in
the case of a common-cathode TOSA. Note that the monitor
photodiode current will also flow in the sense resistor. This
will result in a small offset in the measured bias current. The
APC function will hold this term constant, so it can be
corrected for in the external calibration constants. The sens-
ing resistor could also be connected between V_DD and the
emitter of Q1 on figure 26 or between the emitter of Q1 an
GND on figure 27.
Interfacing To Laser Drivers
In order for the MIC3001 to control the modulation current of
the laser diode, an interface circuit may be required depend-
ing on the method used by the driver to set its modulation
current level. Generally, most laser diode driver ICs use one
of three methods:
a) A current, I_SET, is sourced into a pin on the driver
IC. The modulation current delivered by the driver
is then some fixed multiple of I_SET. The SY88912
is an example of this type of driver. A simple circuit
can be used to create a current source controlled
by the V_MOD outputs. The circuit is based on an
external bipolar transistor and a current sensing
resistor.
M9999-082404
68
August 2004