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参数资料
| 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： | 60/68页 |
| 文件大小： | 444K |
| 代理商： | MIC3000BMLTR |
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October 2004
63
M9999-101204
MIC3000
Micrel
b) A current, ISET, is drawn out of a pin on the driver
IC. The modulation current delivered by the driver
is then some fixed multiple of ISET. 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.
c) A voltage, VSET, is applied to a pin on the driver IC.
This voltage may be referenced to GND or VDD.
The MIC3000’s VMOD+ output can supply this
voltage directly. If a voltage swing wider than VREF
is needed, gain can be applied with a pair of
external resistors. The SY88932, SY88982, and
SY89307 are examples of this type of driver.
SY88912 3.3V 3.2Gbps SONET/SDH Laser Driver
The modulation level of the SY88912 driver is controlled by
the current sourced into the RSET pin (Type (a) above). The
circuit shown in Figure 29 allows the MIC3000’s VMOD
outputs to control the SY88912’s modulation current from its
minimum value, 5mA, to its maximum value, 60mA. The
circuit operates as a DAC-controlled current source. The
current source is formed by the VMOD buffer amplifier, exter-
nal transistor, and current sense resistor. The op-amp acts to
force the voltage drop across RSET to be equal to the DAC
output voltage.
The current, ISET, through RSET is therefore regulated as
ISET = VMOD+/RSET (In this case, the DAC output and
therefore the op-amp output, are referenced to VDDA.) The
SY88912’s current gain, IMOD/ISET, is 23. A modulation
current level of 60mA requires ISET = 60mA/23 = 2.61mA; a
modulation current level of 5mA requires ISET = 5mA/23 =
0.217mA. RFLTR and CFLTR are optional and act to eliminate
any noise that might be present on VDDA or VMOD. The values
shown give a 100
s time constant. Note that the time con-
stant is present whenever the laser is turned on or turned off.
This must be taken into account when designing to system
specifications such as the SFP MSA’s tON and tOFF require-
ments. The values of RFLTR and/or CFLTR may need to be
adjusted accordingly. The impact of the filter time constant on
the turn off time can be eliminated by using the MIC3000’s
SHDN signal to drive the SY88912’s enable input, /EN.
The use of the SHDN signal is completely optional. The main
benefit to using SHDN, however, is that it shuts down the
driver very quickly and irrespective of the values of RFLTR and
CFLTR. The values of RFLTR and CFLTR can therefore be
increased, enhancing their effect without incurring any turn-
off time penalty. Depending on the polarity chosen for SHDN
using the SPOL bit, an inversion may be required between
the MIC3000’s SHDN output and the driver’s /EN input. (The
SHDN output may also be used to drive a redundant safety
switch and the same polarity may not be appropriate for both
functions.)
MODDAC
MODREF bit = 1
MIC3000
SY88912
RSET
420
ISET
IMOD
×23
RFLTR
1k
SHDN
/EN
GND
RSET
VMOD–
Optional - see text
VDDA
VMOD+
GNDA
CFLTR
100nF
Q1
2N3906
VDD
(1)
Notes:
1.
Bypass capacitors not shown for clarity.
Figure 29. Controlling the SY88912
Modulation Current
For the circuit of Figure 29, the modulation current control
range and corresponding DAC values are shown in Table 21
below.
DAC VALUE
VDDA – VMOD
ISET
IMOD
0
0V
0mA
19
0.091V
0.216mA
4.98mA
127
0.61V
1.45mA
33.4mA
255
1.22V
2.91mA
66.8mA
Table 21. Control Range of SY88912
Modulation Control Circuit
SY88932 3.3V 3.2Gbps SONET/SDH Laser Driver
The modulation level of the SY88932 driver is controlled by
the voltage applied to the VCTRL pin (Type (c) above). The
circuit shown in Figure 30 allows the MIC3000’s VMOD output
to control the SY88932’s modulation current. The circuit
operates as a DAC-controlled voltage source. VCTRL is
simply the DAC output voltage. See section above on SY88912
for RFLTR, CFLTR and SHDN.
MODDAC
MODREF bit = 0
MIC3000
SY88932
IMOD
RFLTR
1k
SHDN
/EN
GND
VCTRL
VCC
VMOD–
Optional - see text
VDDA
VMOD+
GNDA
CFLTR
100nF
VDD
(1)
Note:
1.
Bypass capacitors not shown for clarity.
Figure 30. Controlling the SY88932
Modulation Current
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