AD9661A

REV. 0

–5–

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

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-

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

appropriate level to set the calibrated laser diode output

power level.

3. The feedback loop is closed for calibration (

CAL = logic

LOW, and PULSE = logic HIGH), and then opened (

CAL

= logic HIGH).

4. While the feedback loop is open, the laser is pulsed on and

off by PULSE.

5. The feedback loop is periodically recalibrated as needed.

6. The AD9661A is disabled when the laser will not be pulsed

for an indefinite period of time.

Control Loop Transfer Function

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

CAL = logic

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.

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

combination.

The user should define the maximum laser diode output power

chosen as:

1.6V

–50

Ω

4

3

0

2

1

0

120

20

40

60

80

100

CONSTANT WRITE POWER

0

25

CASE

50

FORWARD CURRENT – mA

Figure 2. Laser Diode Current-to-Optical Power Curve

POWER-UP

OR LASER

NOT IN USE

CAL

TIME

RECALIBRATE

LASER POWER

MODULATED

DISABLE

CAL

PULSE

LASER

OUTPUT POWER

CALIBRATED LEVEL

HOLD TIME

Figure 1. Normal Operating Mode