AD9660KR View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AD9660KR

200 MHz Laser Diode Driver with Light Power Control

Analog Devices 

AD9660

POWER-UP

DISABLE OR LASER

NOT IN USE

BIAS CAL

BIAS

CAL

TIME

RECALIBRATE THE BIAS LOOP

WRITE CAL

WRITE

LOOP

CAL

TIME

WRITE LOOP

HOLD TIME

RECALIBRATE WRITE LOOP

WRITE PULSE

LASER

OUTPUT POWER

CALIBRATED WRITE

LASER POWER

MODULATED

BETWEEN

BIAS AND WRITE LEVELS

CALIBRATED BIAS

Figure 4. Normal Operating Mode

The disable circuit turns off IOUT and returns the hold capacitor

voltages to their minimum levels (minimum output current)

when DISABLE = logic HIGH. It is used during initial power-

up of the AD9660 or during time periods when the laser is inac-

tive. When the AD9660 is re-enabled the control loops must be

recalibrated.

Normal operation of the AD9660 involves (in order, see figure):

1. The AD9660 is enabled (DISABLE = logic LOW).

2. The input voltages (BIAS LEVEL and WRITE LEVEL) are

driven to the appropriate levels to set the calibrated laser di-

ode output power levels.

3. The bias loop is closed for calibration (BIAS CAL = logic

HIGH), and then opened (BIAS CAL = logic LOW).

4. The write loop is closed for calibration (WRITE PULSE and

WRITE CAL = logic HIGH) and then opened.

5. While both loops are open, the laser is pulsed between the

two calibrated levels by WRITE PULSE.

6. The bias and write loops are periodically recalibrated as

needed.

7. The AD9660 is disabled when the laser will not be pulsed for

an indefinite period of time.

Control Loop Transfer Functions

The relationship between IMONITOR and VBIAS LEVEL is

I MONITOR

=

VBIAS LEVEL − VREF

1.85 × (RGAIN + 50 Ω)

once the bias loop is calibrated. When the bias loop is open

(BIAS CAL = logic LOW), its output current, IBIAS, is propor-

tional to the held voltage at BIAS HOLD; the external hold ca-

pacitor on this pin determines the droop error in the output bias

current between calibrations.

The relationship between IMONITOR and VWRITE LEVEL is

I MONITOR

=

VWRITE LEVEL

1.85 × ( RGAIN

− VREF

+ 50 Ω)

once the write loop is calibrated. The current supplied by the write

loop output is referred to as the modulation current, IMODULATION.

When the write loop is open (WRITE CAL logic LOW),

IMODULATION is proportional to the held voltage at WRITE HOLD.

The external hold capacitor (WRITE HOLD) determines the

droop error between calibrations. IMODULATION may be switched

on and off by WRITE PULSE when the write loop is open.

4

0°C CASE

3

CONSTANT WRITE POWER

2

BIAS CALIBRATION POWER

1

25°C

CASE

50°C CASE

0

0

20

40

60

80

100

120

FORWARD CURRENT – mA

BIAS

MOD

Figure 5. Typical Laser Diode Current-to-Optical

Power Curve

The sections below discuss choosing the external components in

the feedback loops for a particular application.

Choosing RGAIN

The gain resistor, RGAIN, allows the user to match the feedback

loop’s transfer function to the laser diode/photo diode

combination.

The user should define the maximum laser diode output power

for the intended application, PLD MAX, and the corresponding

photo diode monitor current, IMONITOR MAX. A typical laser di-

ode transfer function is illustrated in Figure 5. RGAIN should be

chosen as:

RGAIN

=

1.6 V

1.85 × IMONITOR MAX

− 50 Ω.

The laser diode’s output power will then vary from 0 to PLD MAX

for an input range of VREF to VREF +1.6 V @ the BIAS LEVEL

and WRITE LEVEL inputs.

–6–

REV. 0

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