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參數資料
| 型號： | AD9660KR |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 其它接口 |
| 英文描述： | 200 MHz Laser Diode Driver with Light Power Control |
| 中文描述： | SPECIALTY INTERFACE CIRCUIT, PDSO28 |
| 封裝： | PLASTIC, SOIC-28 |
| 文件頁數： | 7/12頁 |
| 文件大小： | 256K |
| 代理商： | AD9660KR |
AD9660
REV. 0
–7–
Choosing C
BIAS HOLD
and C
WRITE HOLD
Choosing values for the hold capacitors, C
WRITE
and C
HOLD
, is a
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:
±
I
BIAS
=
18
×
10
9
t
BIAS HOLD
C
BIAS HOLD
±
I
MODULATED
=
18
×
10
9
t
WRITE HOLD
C
WRITE HOLD
To choose a value, the user will need to determine the amount
of time the loop will be in hold mode, t
WRITE HOLD
or t
BIAS HOLD
,
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
I
MAX
=
5
mW
×
(5%)/ 0.25
mW
mA
1.0
mA
If the same application had a hold time requirement of 250
μ
s,
then the minimum value of the hold capacitor would be:
C
HOLD
=
18
×
10
9
×
250
μ
s
1.0
mA
=
4.5
nF
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.
C
HOLD
R
AD9660
TZA
T/H
WRITE LEVEL
OR BIAS LEVEL
POWER MONITOR
WRITE HOLD
OR BIAS HOLD
EXTERNAL HOLD
CAPACITOR
Figure 7. Circuit Model for Determining Calibration Times
Minimum specifications for I
MONITOR MAX
should be used when
choosing R
GAIN
. Users are cautioned that laser diode/photo di-
ode combinations that produce monitor currents that are less
than I
MONITOR MAX
in the equation above will produce higher la-
ser output power than predicted, which may damage the laser
diode. Such a condition is possible if R
GAIN
is calculated using
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
GAIN
. This allows users to optimize
the value of R
GAIN
for each laser diode/photo diode combina-
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
GAIN
may not be optimized for the entire
potentiometer’s range.
C
GAIN
optimizes the response of the transimpedance amplifier
and should be chosen as from the table below. Choosing C
GAIN
larger than the recommended value will slow the response of the
amplifier. Lower values improve TZA bandwidth but may cause
the amplifier to oscillate.
Table I.
Recommended
C
GAIN
R
GAIN
≥
2.5 k
1.5 k
1 k
500
2 pF
3 pF
4 pF
8 pF
The circuit in Figure 6 allows an adjustable gain with low vari-
ance in bandwidth, but requires several external components.
V
REF
R
INTERNAL
GAIN
C1
R
F
C
F
POWER
MONITOR
V
REF
R1
R2
R3
1.7k
2I PIN
1:2
I
MONITOR
SENSE
IN
AD9660
EQUIVALENT
CIRCUIT
R
EQ
V
REF
R
INTERNAL
C
EQ
R
EQ
+ 50
= (R
F
+ 50
)
(1+R2
R1
R
F
Figure 6. Adjustable Gain Configuration
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