MIC3000BML datasheet(64/68 Pages) MICREL

64 / 68 page

MIC3000

Micrel

M9999-101204

October 2004

SY89307 5.0V/ 3.3V 2.5Gbps VCSEL Driver

The modulation level of the SY89307 driver is controlled by

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

circuit shown in Figure 31 allows the MIC3000’s VMOD output

to control the SY89307’s output swing. VCTRL is simply the

DAC output voltage. The circuit operates as a DAC-con-

trolled voltage source. See section above on SY88912 for

RFLTR, CFLTR.

MODDAC

MODREF bit = 1

MIC3000

SY89307

RFLTR

VEE

VCTRL

VCC

VMOD–

VDDA

VMOD+

GNDA

CFLTR

100nF

VDD

(1)

Note:

Bypass capacitors not shown for clarity.

Figure 31. Controlling the SY89307

Modulation Current

Laser Drivers Programmed via a Sink Current

The modulation level of some laser diode drivers is controlled

by a current sourced out of the RSET pin (Type (b) above).

The circuit shown in Figure 32 allows the MIC3000’s VMOD

outputs to control the set current, ISET. The circuit operates as

a DAC-controlled current sink. The current sink is formed by

the VMOD buffer amplifier, external 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 through RSET is therefore regulated as

IRSET = VMOD+/RSET. ISET is given by the equation:

VMOD

SET

⎛

⎝

⎜

⎞

⎠

⎟ +

⎛

⎝⎜

⎞

⎠⎟

(8)

where

β is the DC gain of Q1

The higher the gain of the transistor, the closer ISET will be to

the current in RSET. RFLTR and CFLTR act to eliminate any

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

shown give a 100

µs time constant. See section above on

SY88912 for RFLTR, CFLTR and SHDN.

MODDAC

MODREF bit = 1

MIC3000

LD Driver

RSET

ISET

IMOD

×23

RFLTR

SHDN

/EN

GND

VDD

RSET

VMOD–

Optional - see text

VDDA

VMOD+

GNDA

2N3906

VDD(1)

CFLTR

100nF

Notes:

Bypass capacitors not shown for clarity

Figure 32. Controlling the Modulation Current

via a Sink Current

Drivers With Monitor Outputs

Laser diode driver ICs have been introduced with monitor

outputs. These outputs provide ground-referred signals that

mirror critical signals like laser bias current, modulation

current or monitor photodiode current, an analog of transmit-

ted power. Generally, these outputs source a current into an

external resistor to generate a ground referenced voltage.

Using these outputs with the MIC3000 is straightforward

since the MIC3000’s VILD+/– and VMPD inputs are polarity

programmable,

Shutdown Output

The shutdown output, SHDN, can be used in two ways: as an

enable or on/off control for the laser driver IC, and/or to control

a redundant switch in the laser current path. The redundant

switch provides a means for the MIC3000 to shut off the laser

current even if the bias transistor or modulator is damaged or

fails. SHDN is active any time the MIC3000 shuts down the

laser, i.e., if the TXDISABLE function is asserted in hardware

or software, or if the fault detection circuits trigger laser

shutdown. The shutdown output, SHDN, is essentially a logic

output with programmable polarity. The programmable polar-

ity allows SHDN to drive either high-side or low-side switches

or active-high or active-low enable inputs without the need for

external inversion circuits. If an active-low and an active-high

shutdown signal are required, an external inverter will be

necessary. Examples of redundant switch circuits are shown

in Figure 33.

MIC3000BML Datasheet(PDF) 64 Page - Micrel Semiconductor

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