MIC3000 Datasheet, PDF (64/68 Pages) Micrel Semiconductor – SFP Management IC

MIC3000
SY89307 5.0V/ 3.3V 2.5Gbps VCSEL Driver
The modulation level of the SY89307 driver is controlled by
the voltage applied to the VCTRL pin (Type (c) above). The
circuit shown in Figure 31 allows the MIC3000’s V_MOD output
to control the SY89307’s output swing. VCTRL is simply the
DAC output voltage. The circuit operates as a DAC-con-
trolled voltage source. See section above on SY88912 for
R_FLTR, C_FLTR.
MIC3000
V_DD(1)
SY89307
MODDAC
V_DDA
V_CC
V_MOD+
R_FLTR
1k
CFLTR
100nF
V_CTRL
GNDA
V_EE
MODREF bit = 1
V_MOD–
Note:
1. Bypass capacitors not shown for clarity.
Figure 31. Controlling the SY89307
Modulation Current
Laser Drivers Programmed via a Sink Current
The modulation level of some laser diode drivers is controlled
by a current sourced out of the RSET pin (Type (b) above).
The circuit shown in Figure 32 allows the MIC3000’s V_MOD
outputs to control the set current, I_SET. The circuit operates as
a DAC-controlled current sink. The current sink is formed by
the V_MOD buffer amplifier, external 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 through R_SET is therefore regulated as
I_RSET = V_MOD+/R_SET. I_SET is given by the equation:
I_SET
=
⎛
⎝⎜
VMOD + ^{⎞ ⎛} β
R_SET ⎠⎟ ⎝⎜ 1+ β
⎞
⎠⎟
(8)
where β is the DC gain of Q1
The higher the gain of the transistor, the closer I_SET will be to
the current in R_SET. R_FLTR and C_FLTR act to eliminate any
noise that might be present on V_DDA or V_MOD. The values
shown give a 100µs time constant. See section above on
SY88912 for R_FLTR, C_FLTR and SHDN.
MODDAC
MIC3000
VDD(1)
Micrel
LD Driver
V_DDA
R_FLTR
1k
V_MOD+
C_FLTR
100nF
V_DD
I_SET
R_SET
Q1
2N3906
×23
I_MOD
R_SET
GNDA
GND
MODREF bit = 1
V_MOD–
SHDN
/EN
Optional - see text
Notes:
1. Bypass capacitors not shown for clarity
Figure 32. Controlling the Modulation Current
via a Sink Current
Drivers With Monitor Outputs
Laser diode driver ICs have been introduced with monitor
outputs. These outputs provide ground-referred signals that
mirror critical signals like laser bias current, modulation
current or monitor photodiode current, an analog of transmit-
ted power. Generally, these outputs source a current into an
external resistor to generate a ground referenced voltage.
Using these outputs with the MIC3000 is straightforward
since the MIC3000’s VILD+/– and VMPD inputs are polarity
programmable,
Shutdown Output
The shutdown output, SHDN, can be used in two ways: as an
enable or on/off control for the laser driver IC, and/or to control
a redundant switch in the laser current path. The redundant
switch provides a means for the MIC3000 to shut off the laser
current even if the bias transistor or modulator is damaged or
fails. SHDN is active any time the MIC3000 shuts down the
laser, i.e., if the TXDISABLE function is asserted in hardware
or software, or if the fault detection circuits trigger laser
shutdown. The shutdown output, SHDN, is essentially a logic
output with programmable polarity. The programmable polar-
ity allows SHDN to drive either high-side or low-side switches
or active-high or active-low enable inputs without the need for
external inversion circuits. If an active-low and an active-high
shutdown signal are required, an external inverter will be
necessary. Examples of redundant switch circuits are shown
in Figure 33.
M9999-101204
64
October 2004