AD9660KR Analog Devices Inc, AD9660KR Datasheet - Page 7

• Manufacturer Part Number: AD9660KR
• Description: IC LASER DIODE DRIVER 28-SOIC
• Manufacturer: Analog Devices Inc
• Type: Laser Diode Driverr

Specifications of AD9660KR

• Number Of Channels: 1
• Rohs Status: RoHS non-compliant
• Voltage - Supply: 4.75 V ~ 5.25 V
• Current - Supply: 110mA
• Current - Modulation: 60mA
• Current - Bias: 90mA
• Operating Temperature: 0°C ~ 70°C
• Package / Case: 28-SOIC (0.300", 7.50mm Width)
• Mounting Type: Surface Mount
• Operating Temperature (max): 70C
• Operating Temperature (min): 0C
• Mounting: Surface Mount
• Operating Temperature Classification: Commercial
• Lead Free Status / RoHS Status: Compliant

REV. 0
Minimum specifications for I
choosing R
ode 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 AD9660 BIAS LEVEL and
WRITE LEVEL inputs should be limited to avoid damaging
laser diodes.
Although not recommended, another approach would be to
use a potentiometer for R
the value of R
tion’s monitor current. The drawback to this approach is that
potentiometer’s stray inductance and capacitance may cause the
transimpedance amplifier to overshoot and degrade its settling,
and the value of C
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.
The circuit in Figure 6 allows an adjustable gain with low vari-
ance in bandwidth, but requires several external components.
I
GAIN
MONITOR
MONITOR MAX
optimizes the response of the transimpedance amplifier
Figure 6. Adjustable Gain Configuration
GAIN
SENSE
AD9660
GAIN
1:2
IN
. Users are cautioned that laser diode/photo di-
V
2I PIN
GAIN
R
1.5 k
1 k
500
REF
for each laser diode/photo diode combina-
in the equation above will produce higher la-
2.5 k
GAIN
may not be optimized for the entire
GAIN
R
INTERNAL
Table I.
50
MONITOR MAX
MONITOR
Recommended
C
2 pF
3 pF
4 pF
8 pF
. This allows users to optimize
POWER
GAIN
EQUIVALENT
GAIN
V
CIRCUIT
REF
V
REF
R1
R2
GAIN
C1
R
should be used when
R3
1.7k
INTERNAL
50
is calculated using
R
R
EQ
F
+ 50 = (R
C
F
R
EQ
(1+
F
R2
R1
+ 50 )
GAIN
C
EQ
+
R1
R
F
)
–7–
Choosing C
Choosing values for the hold capacitors, C
tradeoff between output current droop when the control loops
are open, and the time it takes to calibrate and recalibrate the
laser power when the loops are closed.
The amount of output current droop is determined by the value
of the hold capacitor and the leakage current at that node.
When either of the two control loops are open (WRITE CAL or
BIAS CAL logic LOW), the pin connections for the hold
capacitors (WRITE HOLD and BIAS HOLD) are high imped-
ance inputs. Leakage currents will range from 200 nA; this
low current minimizes the droop in the output power level. As-
suming the worst case current of 200 nA, the output current
will change as follows:
To choose a value, the user will need to determine the amount
of time the loop will be in hold mode, t
the maximum 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 ex-
ample, if an application 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 in Figure 7.
Figure 7. Circuit Model for Determining Calibration Times
POWER MONITOR
OR BIAS LEVEL
WRITE LEVEL
I
MAX
C
HOLD
BIAS HOLD
I
I
BIAS
MODULATED
5 mW
18 10
and C
18 10
T/H
(5%) / 0.25
1.0 mA
C
AD9660
WRITE HOLD
9
TZA
BIAS HOLD
18 10
9
250 s
t
BIAS HOLD
R
C
WRITE HOLD
mW
mA
9
WRITE HOLD
WRITE HOLD
OR BIAS HOLD
t
WRITE HOLD
4.5 nF
WRITE
C
HOLD
1.0 mA
AD9660
and C
or t
EXTERNAL HOLD
CAPACITOR
BIAS HOLD
HOLD
, is a
,