MIC3000BML datasheet(62/68 Pages) MICREL

62 / 68 page

MIC3000

Micrel

M9999-101204

October 2004

Choosing CCOMP

The APC loop is compensated by a capacitor, CCOMP,

connected from COMP to either VDDA or GNDA. This

capacitor adjusts the slew rate and bandwidth of the loop as

follows:

SlewRate

dV dt

SLEW

COMP

where:

ISLEW = 64µA,

GM = 125µMho

these relationships are shown graphically in Figure 27 and

Figure 28.

6 11 16 21 26 31 36 41 46 51

COMP (nF)

Figure 27. Slew Rate vs. CCOMP Value

0.50

1.00

1.50

2.00

2.50

10 20 30 40 50 60 70 80 90 100

COMP (nF)

Figure 28. Open Loop Unity-Gain Bandwidth

vs. CCOMP

The loop response should be tailored to the data rate,

encoding format and maximum run-lengths, and required

laser turn-on time. Higher data rates and/or shorter maximum

run lengths and/or faster turn-on times call for smaller capaci-

tors. Lower data rates and/or longer maximum run lengths

and/or slower turn-on times call for larger capacitors. In order

to meet the SFP/GBIC turn-on requirement of 1ms, for

example, do not employ a capacitor larger than 20nF. Low

ESR capacitors such as ceramics will give the best results.

Excessive ESR will reduce the effectiveness of CCOMP. The

capacitor’s voltage rating must exceed VDDA. Some typical

values are shown in Table 20.

Application

CCOMP (nF)

8b/10b encoding, •1Gbps, tON - 1ms

SONET (62b/64b encoding), •1Gbps

•155Mbps, tON - 1ms

•155Mbps

100

Table 20. Typical Values for CCOMP

While there is no theoretical upper limit on the size of CCOMP,

it is desirable for the loop to be able to track the changes

resulting from periodic temperature compensation. The typi-

cal temperature compensation update period is 1.6s. There-

fore, a maximum size of 1

µF is recommended. If laser turn-

on time is not a factor, a value between 100nF and 1

µF can

be used for virtually any typical application. The tradeoff is

that higher value capacitors have a larger physical size and

cost.

In order to maximize the power supply rejection ratio (PSRR),

CCOMP should be returned to GNDA when the VBIAS output

is sourcing current, e.g., driving an NPN transistor

(SRCE bit = 1). CCOMP should be returned to VDDA when the

VBIAS output is sinking current, e.g., driving a PNP transistor

(SRCE bit =0).

Measuring Laser Bias Current

VILD+ and VILD– form a pair of pseudo-differential A/D

inputs for measuring laser diode bias current via a sense

resistor. The signal applied to these inputs is converted to a

single-ended, ground-referenced signal for input into the

ADC and bias current fault comparator. These inputs have

limited common-mode voltage range. The full-scale differen-

tial input range is VREF/4 or about 300mV.

Figure 25 and Figure 26 illustrate the typical implementation

of this function. Note that VILD– is always connected to the

circuit’s reference potential: VDD in the case of a common-

anode transmitter optical sub-assembly (TOSA) and GND in

the case of a common-cathode TOSA. Note that the monitor

photodiode current will also flow in the sense resistor. This

will result in a small offset in the measured bias current. The

APC function will hold this term constant, so it can be

corrected for in the external calibration constants. The sens-

ing resistor could also be connected between VDD and the

emitter of Q1 on Figure 25 or between the emitter of Q1 an

GND on Figure 26.

Interfacing To Laser Drivers

In order for the MIC3000 to control the modulation current of

the laser diode, an interface circuit may be required depend-

ing on the method used by the driver to set its modulation

current level. Generally, most laser diode driver ICs use one

of three methods:

a) A current, ISET, is sourced into a pin on the driver

IC. The modulation current delivered by the driver

is then some fixed multiple of ISET. The SY88912

is an example of this type of driver. 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.

MIC3000BML Datasheet(PDF) 62 Page - Micrel Semiconductor

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