MIC3000BML datasheet(63/68 Pages) MICREL

63 / 68 page

October 2004

M9999-101204

MIC3000

Micrel

b) A current, ISET, is drawn out of a pin on the driver

IC. The modulation current delivered by the driver

is then some fixed multiple of ISET. 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.

c) A voltage, VSET, is applied to a pin on the driver IC.

This voltage may be referenced to GND or VDD.

The MIC3000’s VMOD+ output can supply this

voltage directly. If a voltage swing wider than VREF

is needed, gain can be applied with a pair of

external resistors. The SY88932, SY88982, and

SY89307 are examples of this type of driver.

SY88912 3.3V 3.2Gbps SONET/SDH Laser Driver

The modulation level of the SY88912 driver is controlled by

the current sourced into the RSET pin (Type (a) above). The

circuit shown in Figure 29 allows the MIC3000’s VMOD

outputs to control the SY88912’s modulation current from its

minimum value, 5mA, to its maximum value, 60mA. The

circuit operates as a DAC-controlled current source. The

current source is formed by the VMOD buffer amplifier, exter-

nal 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, ISET, through RSET is therefore regulated as

ISET = VMOD+/RSET (In this case, the DAC output and

therefore the op-amp output, are referenced to VDDA.) The

SY88912’s current gain, IMOD/ISET, is 23. A modulation

current level of 60mA requires ISET = 60mA/23 = 2.61mA; a

modulation current level of 5mA requires ISET = 5mA/23 =

0.217mA. RFLTR and CFLTR are optional and act to eliminate

any noise that might be present on VDDA or VMOD. The values

shown give a 100

µs time constant. Note that the time con-

stant is present whenever the laser is turned on or turned off.

This must be taken into account when designing to system

specifications such as the SFP MSA’s tON and tOFF require-

ments. The values of RFLTR and/or CFLTR may need to be

adjusted accordingly. The impact of the filter time constant on

the turn off time can be eliminated by using the MIC3000’s

SHDN signal to drive the SY88912’s enable input, /EN.

The use of the SHDN signal is completely optional. The main

benefit to using SHDN, however, is that it shuts down the

driver very quickly and irrespective of the values of RFLTR and

CFLTR. The values of RFLTR and CFLTR can therefore be

increased, enhancing their effect without incurring any turn-

off time penalty. Depending on the polarity chosen for SHDN

using the SPOL bit, an inversion may be required between

the MIC3000’s SHDN output and the driver’s /EN input. (The

SHDN output may also be used to drive a redundant safety

switch and the same polarity may not be appropriate for both

functions.)

MODDAC

MODREF bit = 1

MIC3000

SY88912

RSET

420

ISET

IMOD

×23

RFLTR

SHDN

/EN

GND

RSET

VMOD–

Optional - see text

VDDA

VMOD+

GNDA

CFLTR

100nF

2N3906

VDD

(1)

Notes:

Bypass capacitors not shown for clarity.

Figure 29. Controlling the SY88912

Modulation Current

For the circuit of Figure 29, the modulation current control

range and corresponding DAC values are shown in Table 21

below.

DAC VALUE

VDDA – VMOD

ISET

IMOD

0mA

0.091V

0.216mA

4.98mA

127

0.61V

1.45mA

33.4mA

255

1.22V

2.91mA

66.8mA

Table 21. Control Range of SY88912

Modulation Control Circuit

SY88932 3.3V 3.2Gbps SONET/SDH Laser Driver

The modulation level of the SY88932 driver is controlled by

the voltage applied to the VCTRL pin (Type (c) above). The

circuit shown in Figure 30 allows the MIC3000’s VMOD output

to control the SY88932’s modulation current. The circuit

operates as a DAC-controlled voltage source. VCTRL is

simply the DAC output voltage. See section above on SY88912

for RFLTR, CFLTR and SHDN.

MODDAC

MODREF bit = 0

MIC3000

SY88932

IMOD

RFLTR

SHDN

/EN

GND

VCTRL

VCC

VMOD–

Optional - see text

VDDA

VMOD+

GNDA

CFLTR

100nF

VDD

(1)

Note:

Bypass capacitors not shown for clarity.

Figure 30. Controlling the SY88932

Modulation Current

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