MIC3000BML datasheet(67/68 Pages) MICREL

67 / 68 page

October 2004

M9999-101204

MIC3000

Micrel

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 0.01

µF 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 1

µF filter capacitor

should be added in parallel to the 0.01

µF capacitor.

VDDD

(1)

0.01

µF

1.0

µF

(1)

1.0

µF

0.01

µF

MIC3000

GNDD

HOST P/S (+)

HOST P/S (–)

VDDA

GNDA

Ground Plane

Power Plane

Figure 36. Power Supply Routing and Bypassing

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 RSOUT 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

VDD such as the SY89307. The MIC3000’s VBIAS or VMOD

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 VBIAS/VMOD output into a current and then into

a VDD-referenced voltage.

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