MIC3001BML datasheet(70/76 Pages) MICREL

70 / 76 page

Micrel, Inc.

MIC3001

August 2004

M9999-082404-A

hbwhelp@micrel.com or (408) 955-1690

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:

where:

ISLEW = 64µA,

GM = 125µMho

these relationships are shown graphically in Figure 28 and

Figure 29.

Figure 28. Slew Rate vs. CCOMP Value

Figure 29. 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 capacitors. 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 22.

Application

CCOMP (nF)

8b/10b encoding, ≥1Gbps, tON ≤ 1ms

SONET (62b/64b encoding), ≥1Gbps

≥155Mbps, tON ≤ 1ms

≥155Mbps

100

Table 22. 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 typical temperature compensation

update period is 1.6s. Therefore, a maximum size of 1mF

is recommended. If laser turn-on time is not a factor, a

value between 100nF and 1mF 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

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

Figure 26 and Figure 27 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 sensing

resistor could also be connected between VDD and the

emitter of Q1 on figure 26 or between the emitter of Q1

and GND on Figure 27.

Interfacing To Laser Drivers

In order for the MIC3001 to control the modulation current

of the laser diode, an interface circuit may be required

depending 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

MIC3001BML Datasheet(PDF) 70 Page - Micrel Semiconductor

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