AD9661AKR Analog Devices Inc, AD9661AKR Datasheet - Page 6

• 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

AD9661A
The laser diode’s output power will then vary from 0 to P
for an input range of V
LEVEL input.
Minimum specifications for I
choosing R
diode combinations that produce monitor currents that are less
than I
ser output power than predicted, which may damage the laser
diode. Such a condition is possible if R
typical instead of minimum monitor current specifications. In
that case the input range to the AD9661A POWER LEVEL
input should be limited to avoid damaging laser diodes.
Another approach would be to use a potentiometer for R
This allows users to optimize the value of R
diode/photo diode combination’s monitor current. The draw-
back to this approach is that potentiometers’ stray inductance
and capacitance may cause the transimpedance amplifier to
overshoot and degrade its settling, and the value of C
not be optimized for the entire potentiometer’s range.
C
and should be chosen as from the table below. Choosing C
larger than the recommended value will slow the response of the
amplifier. Lower values improve TZA bandwidth but may cause
the amplifier to oscillate.
Choosing C
Choosing values for the hold capacitor, C
between output current droop when the control loop is open,
and the time it takes to calibrate and recalibrate the laser power
when the loop is closed.
The amount of output current droop is determined by the value
of the hold capacitor and the leakage current at that node.
When the control loop is open (CAL logic HIGH), the pin con-
nection for the hold capacitor (HOLD) is a high impedance in-
put. Leakage current will range from 200; this low current
minimizes the droop in the output power level. Assuming the
worst case current of 200 nA, the output current will change
as follows:
GAIN
MONITOR MAX
optimizes the response of the transimpedance amplifier
GAIN
HOLD
. Users are cautioned that laser diode/photo
R
2.5 k
1.5 k
1 k
500
GAIN
in the equation above will produce higher la-
REF
I
Table I.
OUT
to V
MONITOR MAX
REF
Recommended
C
2 pF
3 pF
4 pF
8 pF
18 10
C
GAIN
+1.6 V @ the POWER
HOLD
GAIN
–9
HOLD
should be used when
GAIN
is calculated using
, is a tradeoff
for each laser
GAIN
GAIN
LD MAX
may
GAIN
.
–6–
To choose a value, the user will need to determine the amount
of time the loop will be in hold mode, t
change in laser output power the application can tolerate, and
the laser efficiency (defined as the change in laser output power
to the change in laser diode current). As an example, if an ap-
plication requires 5 mW of laser power 5%, and the laser diode
efficiency is 0.25 mW/mA, then
If the same application had a hold time requirement of 250 s,
then the minimum value of the hold capacitor would be:
When determining the calibration time, the T/H and the exter-
nal hold capacitor can be modeled using the simple RC circuit
illustrated below.
Using this model, the voltage at the hold capacitor is
where t = 0 is when the calibration begins (CAL goes logic
LOW), V
steady state voltage at the hold cap with the loop closed, and
V
be estimated from the following table and chart:
Figure 3. Circuitry Model for Determining Calibration Times
= R
C HOLD
POWER MONITOR
POWER LEVEL
C HOLD
for a finite calibration time, as compared to V
t
7
6
5
4
3
2
t = 0
CALIBRATION
I
V
is the time constant. With this model the error in
MAX
C HOLD
is the voltage on the hold cap at t = 0, V
C
HOLD
5 mW (5%)/ 0.25
T/H
V
18 10
t 0
AD9661A
TZA
% Final Value
99.9
99.7
99.2
98.1
95.0
86.5
63.2
Table II.
1.0 mA
(V
–9
t
250 s
–V
R
HOLD
t
mW
mA
0
HOLD
) 1– e
4.5 nF
, the maximum
Error %
0.09
0.25
0.79
1.83
4.97
13.5
36.8
C
HOLD
1.0 mA
–t
EXTERNAL HOLD
CAPACITOR
t =
t =
REV. 0
is the
, can