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| 型号： | MIC3000BMLTR |
| 厂商： | MICREL INC |
| 元件分类： | 数字传输电路 |
| 英文描述： | ATM/SONET/SDH SUPPORT CIRCUIT, QCC24 |
| 封装： | 4 X 4 MM, MLF-24 |
| 文件页数： | 59/68页 |
| 文件大小： | 444K |
| 代理商： | MIC3000BMLTR |
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MIC3000
Micrel
M9999-101204
62
October 2004
Choosing CCOMP
The APC loop is compensated by a capacitor, CCOMP,
connected from COMP to either VDDA or GNDA. This
capacitor adjusts the slew rate and bandwidth of the loop as
follows:
SlewRate
dV dt
I
C
BW
G
2C
SLEW
COMP
M
COMP
==
=
/
π
where:
ISLEW = 64A,
GM = 125Mho
these relationships are shown graphically in Figure 27 and
Figure 28.
0
10
20
30
40
50
60
70
1
6 11 16 21 26 31 36 41 46 51
SLEW
RATE
(mV/
s)
C
COMP (nF)
Figure 27. Slew Rate vs. CCOMP Value
0
0.50
1.00
1.50
2.00
2.50
10 20 30 40 50 60 70 80 90 100
BANDWIDTH
(kHz)
C
COMP (nF)
Figure 28. Open Loop Unity-Gain Bandwidth
vs. CCOMP
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 CCOMP. The
capacitor’s voltage rating must exceed VDDA. Some typical
values are shown in Table 20.
Application
CCOMP (nF)
8b/10b encoding, 1Gbps, tON - 1ms
10
SONET (62b/64b encoding), 1Gbps
22
155Mbps, tON - 1ms
22
155Mbps
100
Table 20. Typical Values for CCOMP
While there is no theoretical upper limit on the size of CCOMP,
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 1
F is recommended. If laser turn-
on time is not a factor, a value between 100nF and 1
F 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),
CCOMP should be returned to GNDA when the VBIAS output
is sourcing current, e.g., driving an NPN transistor
(SRCE bit = 1). CCOMP should be returned to VDDA when the
VBIAS 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 VREF/4 or about 300mV.
Figure 25 and Figure 26 illustrate the typical implementation
of this function. Note that VILD– is always connected to the
circuit’s reference potential: VDD 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 VDD and the
emitter of Q1 on Figure 25 or between the emitter of Q1 an
GND on Figure 26.
Interfacing To Laser Drivers
In order for the MIC3000 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, ISET, is sourced into a pin on the driver
IC. The modulation current delivered by the driver
is then some fixed multiple of ISET. The SY88912
is an example of this type of driver. A simple circuit
can be used to create a current source controlled
by the VMOD outputs. The circuit is based on an
external bipolar transistor and a current sensing
resistor.
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