MIC3000 Datasheet, PDF (63/68 Pages) Micrel Semiconductor – SFP Management IC

MIC3000
b) A current, I_SET, is drawn out of a pin on the driver
IC. The modulation current delivered by the driver
is then some fixed multiple of I_SET. 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.
c) A voltage, V_SET, is applied to a pin on the driver IC.
This voltage may be referenced to GND or V_DD.
The MIC3000’s V_MOD+ output can supply this
voltage directly. If a voltage swing wider than V_REF
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 V_MOD
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 V_MOD 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, I_SET, through R_SET is therefore regulated as
I_SET = V_MOD+/R_SET (In this case, the DAC output and
therefore the op-amp output, are referenced to V_DDA.) The
SY88912’s current gain, I_MOD/I_SET, is 23. A modulation
current level of 60mA requires I_SET = 60mA/23 = 2.61mA; a
modulation current level of 5mA requires I_SET = 5mA/23 =
0.217mA. R_FLTR and C_FLTR are optional and act to eliminate
any noise that might be present on V_DDA or V_MOD. The values
shown give a 100µs 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 t_ON and t_OFF require-
ments. The values of R_FLTR and/or C_FLTR 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 R_FLTR and
C_FLTR. The values of R_FLTR and C_FLTR 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.)
MIC3000
V_DD(1)
Micrel
SY88912
MODDAC
V_DDA
RFLTR
1k
V_MOD+
CFLTR
100nF
R_SET
420
Q1
2N3906 _ISET
R_SET
GNDA
GND
×23
I_MOD
MODREF bit = 1
V_MOD–
SHDN
/EN
Optional - see text
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
0
19
127
V_DDA – V_MOD
0V
0.091V
0.61V
I_SET
0mA
0.216mA
1.45mA
I_MOD
0mA
4.98mA
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 V_MOD 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.
MIC3000
V_DD(1)
SY88932
MODDAC
MODREF bit = 0
V_DDA
V_CC
V_MOD+
R_FLTR
1k
GNDA
V_MOD–
V_CTRL
C_FLTR
100nF
GND
I_MOD
SHDN
/EN
Optional - see text
Note:
1. Bypass capacitors not shown for clarity.
Figure 30. Controlling the SY88932
Modulation Current
October 2004
63
M9999-101204