MIC3000 Datasheet, PDF (67/68 Pages) Micrel Semiconductor – SFP Management IC

MIC3000
Power Supplies
The MIC3000 has separate power supply and ground pins for
both the analog and digital supplies. This helps prevent digital
switching noise from corrupting the analog functions. The
individual supply and ground pins are not isolated from one
another inside the IC. Separate analog and digital power and
ground planes are NOT required on the PCB. Having one of
each plane (power and ground) is certainly good practice,
however. If dedicated power and ground layers are not
available, care should be taken to route the digital supply and
return currents back to the supply separate from the analog
supply connections. A schematic of this approach is shown in
Figure 36. Each supply should be bypassed as close to the
IC as possible with 0.01µF capacitor (Low ESR capacitors
such as ceramics are preferred.) as shown. This assumes
that bulk capacitance is already present upstream. If no other
filter capacitance is present nearby, a 1µF filter capacitor
should be added in parallel to the 0.01µF capacitor.
HOST P/S (+)
Power Plane
C1⁽¹⁾
1.0µF
C2
0.01µF
VDDD VDDA
MIC3000
GNDD GNDA
C3⁽¹⁾
0.01µF
C4
1.0µF
Micrel
Using The MIC3000 In a 5V System
It is fairly straightforward to use the MIC3000 in a system
powered from a 5V rail. In these systems, the laser diode
driver IC will usually be powered from the 5V rail. A small
linear regulator, such as Micrel’s MIC5213, can be used to
generate a 3.3V power supply rail if one does not otherwise
exist in the system. All of the MIC3000’s digital I/O’s except
for R_SOUT are 5V tolerant and may be pulled up to 5.5V
regardless of the MIC3000’s supply voltage. They can be
connected directly to a 5V host. The MIC5213 is ideal, as it is
capable of supplying up to 80mA, is in a tiny SC-70 package,
and is stable with small ceramic output capacitors.
The laser diode driver interface will be unchanged in most
cases. Ground referred voltages and currents can be gener-
ated the same way as with 3.3V-powerd drivers. The excep-
tion is drivers that are controlled by a voltage referenced to
V_DD such as the SY89307. The MIC3000’s V_BIAS or V_MOD
output will be referenced to its own 3.3V power supply
whereas the driver’s input will be referenced to its 5V power
supply. The solution is a simple level-shifting circuit that
converts the V_BIAS/V_MOD output into a current and then into
a V_DD-referenced voltage.
Ground Plane
HOST P/S (–)
Figure 36. Power Supply Routing and Bypassing
October 2004
67
M9999-101204