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AD96606 Datasheet(PDF) 6 Page - Analog Devices

Part No. AD96606
Description  200 MHz Laser Diode Driver with Light Power Control
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD96606 Datasheet(HTML) 6 Page - Analog Devices

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REV. 0
–6–
AD9660
RECALIBRATE THE BIAS LOOP
POWER-UP
OR LASER
NOT IN USE
BIAS
CAL
TIME
WRITE
LOOP
CAL
TIME
RECALIBRATE WRITE LOOP
LASER POWER
MODULATED
BETWEEN
BIAS AND WRITE LEVELS
DISABLE
BIAS CAL
WRITE CAL
WRITE PULSE
LASER
OUTPUT POWER
CALIBRATED WRITE
CALIBRATED BIAS
WRITE LOOP
HOLD TIME
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 =
V
BIAS LEVEL − VREF
1.85
× (R
GAIN + 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 =
V
WRITE LEVEL − VREF
1.85
× (R
GAIN + 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
3
0
2
1
0
120
20
40
60
80
100
CONSTANT WRITE POWER
BIAS CALIBRATION POWER
0
°C CASE
25
°C
CASE
50
°C CASE
BIAS
MOD
FORWARD CURRENT – mA
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:
R
GAIN =
1.6 V
1.85
× I
MONITOR 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.


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