AD9661AKR Analog Devices Inc, AD9661AKR Datasheet - Page 5

• Manufacturer Part Number: AD9661AKR
• Description: IC LASER DIODE DRIVER 28-SOIC
• Manufacturer: Analog Devices Inc
• Type: Laser Diode Driverr

Specifications of AD9661AKR

• Rohs Status: RoHS non-compliant
• Number Of Channels: 1
• Voltage - Supply: 4.75 V ~ 5.25 V
• Current - Supply: 75mA
• Operating Temperature: 0°C ~ 70°C
• Package / Case: 28-SOIC (0.300", 7.50mm Width)
• Mounting Type: Surface Mount

REV. 0
THEORY OF OPERATION
The AD9661A combines a very fast output current switch with
an onboard analog light power control loop to provide the user
with a complete laser diode driver solution. The block diagram
illustrates the key internal functions. The control loop of the
AD9661A adjusts the output current level, I
photo diode feedback current, I
portional to the analog input voltage at POWER LEVEL. Since
the monitor current is proportional to the laser diode light
power, the loop effectively controls laser power to a level pro-
portional to the analog input. The control loop should be peri-
odically calibrated (see Choosing C
The disable circuit turns off I
voltages to their minimum levels (minimum output current)
when DISABLE = logic HIGH. It is used during initial power
up of the AD9661A or during time periods where the laser is
inactive. When the AD9661A is re-enabled the control loop
must be recalibrated.
Normal operation of the AD9661A involves the following (in
order, see Figure 1):
1. The AD9661A is enabled (DISABLE = logic LOW).
2. The input voltage (POWER LEVEL) is driven to the
3. The feedback loop is closed for calibration (CAL = logic
4. While the feedback loop is open, the laser is pulsed on and
5. The feedback loop is periodically recalibrated as needed.
6. The AD9661A is disabled when the laser will not be pulsed
appropriate level to set the calibrated laser diode output
power level.
LOW, and PULSE = logic HIGH), and then opened (CAL
= logic HIGH).
off by PULSE.
for an indefinite period of time.
OUT
OUTPUT POWER
MONITOR
and returns the hold capacitor
HOLD
DISABLE
PULSE
LASER
CAL
CALIBRATED LEVEL
, into SENSE IN is pro-
).
NOT IN USE
POWER-UP
OR LASER
OUT
, so that the
Figure 1. Normal Operating Mode
TIME
CAL
–5–
Control Loop Transfer Function
The relationship between I
once the loop is calibrated. When the loop is open (CAL = logic
HIGH), the output current, I
voltage at HOLD; the external hold capacitor on this pin
determines the droop error in the output current between
calibrations.
The sections below discuss choosing the external components in
the feedback loop for a particular application.
Choosing R
The gain resistor, R
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, P
photo diode monitor current, I
diode transfer function is illustrated below. R
chosen as:
RECALIBRATE
LASER POWER
Figure 2. Laser Diode Current-to-Optical Power Curve
HOLD TIME
MODULATED
4
3
2
1
0
GAIN
0
I
MONITOR
R
GAIN
CONSTANT WRITE POWER
20
GAIN
, allows the user to match the feedback
FORWARD CURRENT – mA
I
40
MONITOR
MONITOR MAX
V
POWER LEVEL
I
OUT
OUT
LD MAX
1.6V
(R
MONITOR MAX
, is proportional to the held
60
GAIN
and V
, and the corresponding
80
50 )
– 50
POWER LEVEL
–V
0 C CASE
. A typical laser
REF
AD9661A
GAIN
100
50 C CASE
CASE
should be
25 C
120
is