AD9660 200MHz Laser Diode Driver with Light

AD9660 200MHz Laser Diode Driver with Light
a
FEATURES
1.5 ns Rise/2.0 ns Fall Times
Output Current: 180 mA @ 3 V, 200 mA @ 2.5 V
Bias Current: 90 mA @ 3 V
Modulation Current: 60 mA @ 3 V
Offset Current: 30 mA @ 3 V
Single +5 V Power Supply
Switching Rate: 200 MHz
Onboard Light Power Control Loops
APPLICATIONS
Laser Printers and Copiers
Optical Disk Drives
FO Datacomm
GENERAL DESCRIPTION
The AD9660 is a highly integrated driver for laser diode applications such as optical disk drives, printers, and copiers. The
AD9660 gets feedback from an external photo detector and includes two analog feedback loops to allow users to set “bias”
and “write” (for optical disk drives) power levels of the laser,
and switch between the two power levels at up to 200 MHz.
Output rise and fall times are typically 1.5 ns and 2.0 ns to
complement printer applications that use image enhancing techniques such as pulse width modulation to achieve gray scale,
and allow disk drive applications to improve density and take
advantage of pulsed write formats. Control signals are TTL/
CMOS compatible.
200 MHz Laser Diode Driver
with Light Power Control
AD9660
FUNCTIONAL BLOCK DIAGRAM
WRITE
PULSE
WRITE
CALIBRATE
WRITE
LEVEL
T/H
OUTPUT
DRIVER
V:1
TRANSIMPEDANCE
AMPLIFIER
BIAS
LEVEL
BIAS
CAL
BIAS
T/H
V:1
OUTPUT
DRIVER
OUTPUT
SENSE
INPUT
LASER
DIODE
PHOTO
DETECTOR
DIODE
AD9660
The driver output provides up to 180 mA of current @ 3 V,
90 mA of BIAS current, 60 mA of modulation current, and
30 mA of offset current. The onboard disable circuit turns off
the output drivers and returns the light power control loops to a
safe state.
The AD9660 can also be used in closed loop applications in
which the output power level follows an analog WRITE LEVEL
voltage input. By optimizing the external hold capacitor, and
the photo detector, the write loop can achieve bandwidths as
high as 25 MHz.
The AD9660 is offered in a 28-pin plastic SOIC for operation
over the commercial temperature range (0°C to +70°C).
REV. 0
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
© Analog Devices, Inc., 1995
One Technology Way, P.O. Box 9106, Norwood. MA 02062-9106, U.S.A.
Tel: 617/329-4700
Fax: 617/326-8703
(+VS = +5 V, Temperature = +25°C unless otherwise noted. Sourced currents defined
AD9660–SPECIFICATIONS as positive.)
Parameter
Test
Level
Temp
Min
AD9660KR
Typ
Max
ANALOG INPUTS
(WRITE LEVEL, BIAS LEVEL)
Input Voltage Range
Input Bias Current
Analog Bandwidth
IV
I
V
Full
+25°C
Full
VREF
–50
OUTPUTS
Maximum Output Current, IOUT
IOUT
Bias Current, IBIAS
Modulation Current, IMODULATION
Offset Current, IOFFSET
Output Compliance Range
Idle Current
I
I
I
I
I
I
I
+25°C
+25°C
+25°C
+25°C
+25°C
+25°C
+25°C
200
180
90
60
30
0
3
SWITCHING PERFORMANCE
Maximum Pulse Rate
Output Propagation Delay (tPD), Rising1
Output Propagation Delay (tPD), Falling1
Output Current Rise Time2
Output Current Fall Time3
WRITE CAL Aperture Delay4
Disable Time5
IV
IV
IV
IV
IV
V
V
+25°C
Full
Full
Full
Full
+25°C
+25°C
200
1.6
1.6
1.1
1.4
HOLD NODES
(WRITE HOLD, BIAS HOLD)
Input Bias Current
Input Voltage Range
Minimum External Hold Cap
I
IV
V
+25°C
Full
Full
–200
VREF
TTL INPUTS6
Logic “1” Voltage
Logic “1” Voltage
Logic “0” Voltage
Logic “0” Voltage
Logic “1” Current
Logic “0” Current
I
IV
I
IV
I
I
+25°C
Full
+25°C
Full
+25°C
+25°C
2.0
2.0
–10
–1.5
20
BANDGAP REFERENCE
Output Voltage VREF
Temperature Coefficient
Output Current
I
V
V
+25°C
1.55
1.75
–0.2
+25°C
–0.5
SENSE IN
Current Gain
Voltage
Input Resistance
V
I
V
+25°C
+25°C
+25°C
3.7
POWER SUPPLY (DISABLE = HIGH)
+VS Voltage
+VS Current
Power Dissipation
I
I
I
+25°C
+25°C
+25°C
4.75
75
5.00
110
550
5.25
150
V
mA
mW
DISABLE = HIGH
OFFSET CURRENT
OFFSET SET Voltage
I
+25°C
1.1
1.4
1.7
V
IMONITOR = 4.0 mA
VREF + 1.6
+50
25
3.0
13
250
1.5
2.0
13
5
20
3.0
2.5
1.7
2.8
Conditions
V
µA
MHz
External Hold Cap = 20 pF
mA
mA
mA
mA
mA
V
mA
VOUT = 2.5 V
VOUT = 3.0 V
VOUT = 3.0 V
VOUT = 3.0 V
VOUT = 3.0 V
MHz
ns
ns
ns
ns
ns
ns
3 dB Reduction in IOUT
200
nA
VREF + 1.6 V
pF
0.8
0.8
10
1.90
1.0
1.85
4.0
<150
Units
4.3
V
V
V
V
µA
mA
WRITE PULSE = LOW,
DISABLE = HIGH
VHOLD = 2.5 V
Open Loop Application Only
DISABLE = LOW
While Other
TTL Inputs Are
Tested
V
mV/°C
mA
mA/mA
V
Ω
IMONITOR = 2 mA
NOTES
1
Propagation delay measured from the 50% of the rising/falling transition of WRITE PULSE to 50% point of the rising/falling edge of the output modulation current.
2
Rise time measured between the 10% and 90% points of the rising transition of the modulation current.
3
Fall time measured between the 10% and 90% points of the falling transition of the modulation current.
4
Aperture Delay is measured from the 50% point of the rising edge of WRITE PULSE to the time when the output modulation begins to recalibrate, WRITE CAL is
held during this test.
5
Disable Time is measured from the 50% point of the rising edge of DISABLE to the 50% point of the falling transition of the output current. Fall time during disable
is similar to fall time during normal operation.
6
WRITE PULSE, WRITE CAL, BIAS CAL, OFFSET PULSE are TTL compatible inputs.
Specifications subject to change without notice.
–2–
REV. 0
AD9660
ABSOLUTE MAXIMUM RATINGS 1
EXPLANATION OF TEST LEVELS
Test Level
+VS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +6 V
VREF Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 mA
WRITE LEVEL, BIAS LEVEL . . . . . . . . . . . . . –0.5 V to +VS
TTL INPUTS . . . . . . . . . . . . . . . . . . . . . . . . . . –0.5 V to +VS
Output Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300 mA
Operating Temperature
AD9660KR . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C
Storage Temperature . . . . . . . . . . . . . . . . . . –65°C to +150°C
Maximum Junction Temperature2 . . . . . . . . . . . . . . . . +150°C
Lead Soldering Temp (10 sec) . . . . . . . . . . . . . . . . . . . +300°C
I. 100% Production Tested.
II. 100% production tested at +25°C, and sample tested at
specified temperatures. AC testing done on sample basis.
III. Sample Tested Only.
IV. Parameter is guaranteed by design and characterization
testing.
V. Parameter is a typical value only.
VI. All devices are 100% production tested at +25°C, sample
tested at temperature extremes.
1
Absolute maximum ratings are limiting values, to be applied individually, and
beyond which the serviceability of the circuit may be impaired. Functional
operability under any of these conditions is not necessarily implied. Exposure of
absolute maximum rating conditions for extended periods of time may affect
device reliability.
2
Typical thermal impedance is θJA = 45°C/W, θJC = 41°C/W.
+VS
50Ω
50Ω
WRITE
HOLD
ORDERING GUIDE
Model
Temperature Range
AD9660KR
0°C to +70°C
AD9660KR-REEL 0°C to +70°C
BIAS
HOLD
Package Option
SENSE
IN
R-28
R-28 (1000/reel)
T/H
PIN ASSIGNMENTS
1mA
OFFSET
SET
WRITE CAL
1
28 OFFSET PULSE
WRITE PULSE
2
27 OFFSET SET
WRITE LEVEL
3
26 GROUND
VREF
4
25 +VS
WRITE HOLD 5
AD9660KR
TOP VIEW
23 +VS
(Not to Scale)
6
+VS
7
22 OUTPUT
SENSE IN
8
21 +VS
GAIN
9
20 OUTPUT
+VS 11
VBANDGAP
24 OUTPUT
GROUND
POWER MONITOR 10
+VS
100Ω
VREF
+VS
450Ω
19 +VS
1250Ω
18 OUTPUT
GROUND 12
17 GROUND
BIAS HOLD 13
16 DISABLE
BIAS LEVEL 14
15 BIAS CAL
TTL
INPUT
+VS
OUTPUT
Equivalent Circuits
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the AD9660 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
REV. 0
–3–
WARNING!
ESD SENSITIVE DEVICE
AD9660
PIN DESCRIPTIONS
Pin
Function
OUTPUT
Analog laser diode current output. Connect to anode of laser diode, cathode connected to GROUND externally.
BIAS LEVEL
Analog voltage input, VREF to VREF + 1.6 V. Bias current is set proportional to the BIAS LEVEL during calibration as follows:
−V
V
I MONITOR =
BIAS LEVEL
REF
1.85 × ( RGAIN + 50 Ω )
BIAS CAL
TTL/CMOS compatible, Bias loop T/H control signal. Logic HIGH enables calibration mode, and the bias loop
T/H immediately goes into track mode. Logic LOW disables the bias loop T/H and immediately places it in hold
mode. WRITE PULSE should be held logic LOW while calibrating. Floats logic HIGH.
BIAS HOLD
External hold capacitor for the bias loop T/H. Approximate droop in the bias current while BIAS CAL is logic
LOW is: ±∆I BIAS =
18 × 10
–9
t BIAS HOLD
CBIAS HOLD
1
. Bandwidth of the loop is: BW = 2π (550 Ω) C
BIAS HOLD
WRITE PULSE
TTL/CMOS compatible, current control signal. Logic HIGH supplies IMODULATION to the laser diode. Logic
LOW turns IMODULATION off. Floats logic HIGH.
WRITE CAL
TTL/CMOS compatible, write loop T/H control signal. Logic HIGH enables calibration mode; before enabling
calibration the bias loop should be calibrated and OFFSET PULSE driven to an appropriate state. In calibration
mode, 13 ns after the WRITE PULSE goes logic HIGH, the write loop T/H goes into track mode (there is no delay if WRITE PULSE is HIGH before WRITE CAL transitions to a HIGH level). The write loop T/H immediately goes into hold mode when the WRITE PULSE goes Logic LOW. WRITE CAL LOW disables the write
loop T/H and places it in hold mode. Floats logic HIGH.
WRITE LEVEL
Analog voltage input, VREF to VREF +1.6 V. Write current is set proportional to the input voltage during calibration as follows: I MONITOR =
WRITE HOLD
VWRITE LEVEL − V REF
1.85 × (RGAIN + 50 Ω)
External hold capacitor for the write loop T/H. Approximate droop in IMODULATION current while WRITE CAL is
−9
logic LOW is: ±∆I MODULATED =
BW =
18 × 10 tWRITE HOLD
CWRITE HOLD
. Bandwidth of the loop is:
1
2π (550 Ω) CBIAS HOLD
SENSE IN
Analog current input, IMONITOR, from PIN photo detector diode. SENSE IN should be connected to the cathode
of the PIN diode, with the PIN anode connected to GROUND or a negative voltage. Voltage at SENSE IN varies slightly with temperature and current, but is typically 4.0 V.
GAIN
External connection for the feedback network of the transimpedance amplifier. External feedback network, RGAIN
and CGAIN, should be connected between GAIN and POWER MONITOR. See text for choosing values.
POWER
MONITOR
Output voltage monitor of the internal feedback loop. Voltage is proportional to feedback current from photo
diode.
OFFSET
Set resistor connection for the offset current source. Resistor between OFFSET CURRENT SET and +VS
CURRENT SET determines offset current level. The input voltage at this node varies slightly with temperature and current, but is
typically 1.4 V. See curves. Can also be driven with a current out DAC.
OFFSET
PULSE
TTL/CMOS compatible, OFFSET current control signal. Logic HIGH adds IOFFSET to IOUT. Logic LOW
turns off IOFFSET. Floats logic HIGH.
DISABLE
TTL/CMOS compatible, current output disable circuit. Logic LOW for normal operation; logic HIGH disables
the current outputs to the laser diode, and drives the voltage on the hold capacitors close to VREF (minimizes the
output current when the device is re-enabled). DISABLE floats logic HIGH.
VREF
Analog Voltage Output, internal bandgap voltage reference, ~1.75 V, provided to user for power level offset.
+VS
Positive Power Supply. Nominally +5 V, pin connections should be tied together externally.
GROUND
Ground Reference. All grounds should be tied together externally.
–4–
REV. 0
Typical Performance Characteristics–AD9660
6.25mV
35
30
IOFFSET
25
20mV
–193.8mV
2ns
20
15
10
ML64116R
OUTPUT
TEK 11802
O'SCOPE
50Ω
5
0
AD9660
50Ω
0
5
10
15
ROFFSET – kΩ
20
25
–120V
Figure 2. IOFFSET vs. ROFFSET
ANTEL
ARS1
PHOTO DETECTOR
Figure 1. Driving ML64116R Laser @ 30 mW
current is proportional to the laser diode light power, the loops
effectively control laser power to a level proportional to the analog inputs. The control loops should be periodically calibrated
independently (see Choosing CBIAS HOLD and CWRITE HOLD).
THEORY OF OPERATION
The AD9660 combines a very fast output current switch with
onboard analog light power control loops to provide the user
with a complete laser diode driver solution. The block diagram
illustrates the key internal functions. The control loops of the
AD9660, the bias loop and the write loop, adjust the output
current level, IOUT, so that the photo diode feedback current,
IMONITOR, out of SENSE IN is proportional to the analog input
voltage at BIAS LEVEL or WRITE LEVEL. Since the monitor
The offset current generator produces an open loop output current, IOFFSET. Its level is controlled by an external set resistor or
a current out DAC (see Figure 2). While IOFFSET is not calibrated as the currents from the bias and write loops are, it can
be very versatile (see Offset Current below).
WRITE HOLD
TTL
WRITE PULSE
WRITE CAL
WRITE LEVEL
AD9660
DISABLE
CIRCUIT
DISABLE
VOLT
REF
VREFOUT
TTL
*
TTL
DELAY
WRITE LOOP
ANALOG
IMODULATION
V:1
5pF
IOUT
OUTPUT
1:10
LASER
DIODE
VREF
GAIN
50
2* IMONITOR
CGAIN
RGAIN
TZA
POWER
MONITOR
VREF
BIAS LEVEL
BIAS CAL
4.0V
1.85:1
ANALOG
IMONITOR
VREF
OFFSET PULSE
PHOTO
DETECTOR
IBIAS
V:1
1:10
ANALOG
5pF
BIAS LOOP
TTL
1:10
IOFFSET
BIAS HOLD
SENSE IN
1.4V
TTL
OFFSET CURRENT
SET
+VS
* 10ns DELAY ON RISING
EDGE; 0ns ON FALLING
Figure 3. Functional Block Diagram
REV. 0
–5–
AD9660
DISABLE
POWER-UP
OR LASER
NOT IN USE
RECALIBRATE THE BIAS LOOP
BIAS
CAL
TIME
BIAS CAL
WRITE
LOOP
CAL
TIME
WRITE CAL
WRITE LOOP
HOLD TIME
RECALIBRATE WRITE LOOP
WRITE PULSE
CALIBRATED WRITE
LASER
OUTPUT POWER
LASER POWER
MODULATED
BETWEEN
BIAS AND WRITE LEVELS
CALIBRATED BIAS
Figure 4. Normal Operating Mode
When the write loop is open (WRITE CAL logic LOW),
IMODULATION is proportional to the held voltage at WRITE HOLD.
The external hold capacitor (WRITE HOLD) determines the
droop error between calibrations. IMODULATION may be switched
on and off by WRITE PULSE when the write loop is open.
The disable circuit turns off IOUT and returns the hold capacitor
voltages to their minimum levels (minimum output current)
when DISABLE = logic HIGH. It is used during initial powerup of the AD9660 or during time periods when the laser is inactive. When the AD9660 is re-enabled the control loops must be
recalibrated.
4
Normal operation of the AD9660 involves (in order, see figure):
0°C CASE
OPTICAL OUTPUT – mW
1. The AD9660 is enabled (DISABLE = logic LOW).
2. The input voltages (BIAS LEVEL and WRITE LEVEL) are
driven to the appropriate levels to set the calibrated laser diode output power levels.
3. The bias loop is closed for calibration (BIAS CAL = logic
HIGH), and then opened (BIAS CAL = logic LOW).
4. The write loop is closed for calibration (WRITE PULSE and
WRITE CAL = logic HIGH) and then opened.
5. While both loops are open, the laser is pulsed between the
two calibrated levels by WRITE PULSE.
3
50°C CASE
BIAS CALIBRATION POWER
1
0
0
20
6. The bias and write loops are periodically recalibrated as
needed.
7. The AD9660 is disabled when the laser will not be pulsed for
an indefinite period of time.
120
The sections below discuss choosing the external components in
the feedback loops for a particular application.
The relationship between IMONITOR and VBIAS LEVEL is
VBIAS LEVEL − VREF
Choosing RGAIN
The gain resistor, RGAIN, allows the user to match the feedback
loop’s transfer function to the laser diode/photo diode
combination.
1.85 × (RGAIN + 50 Ω)
once the bias loop is calibrated. When the bias loop is open
(BIAS CAL = logic LOW), its output current, IBIAS, is proportional to the held voltage at BIAS HOLD; the external hold capacitor on this pin determines the droop error in the output bias
current between calibrations.
The user should define the maximum laser diode output power
for the intended application, PLD MAX, and the corresponding
photo diode monitor current, IMONITOR MAX. A typical laser diode transfer function is illustrated in Figure 5. RGAIN should be
The relationship between IMONITOR and VWRITE LEVEL is
I MONITOR =
40
60
80
100
FORWARD CURRENT – mA
MOD
BIAS
Figure 5. Typical Laser Diode Current-to-Optical
Power Curve
Control Loop Transfer Functions
I MONITOR =
25°C
CASE
CONSTANT WRITE POWER
2
VWRITE LEVEL − VREF
chosen as:
1.85 × (RGAIN + 50 Ω)
RGAIN =
1.6 V
1.85 × I MONITOR MAX
− 50 Ω
.
The laser diode’s output power will then vary from 0 to PLD MAX
for an input range of VREF to VREF +1.6 V @ the BIAS LEVEL
and WRITE LEVEL inputs.
once the write loop is calibrated. The current supplied by the write
loop output is referred to as the modulation current, IMODULATION.
–6–
REV. 0
AD9660
Minimum specifications for IMONITOR MAX should be used when
choosing RGAIN. Users are cautioned that laser diode/photo diode combinations that produce monitor currents that are less
than IMONITOR MAX in the equation above will produce higher laser output power than predicted, which may damage the laser
diode. Such a condition is possible if RGAIN 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.
Choosing CBIAS HOLD and CWRITE HOLD
Choosing values for the hold capacitors, CWRITE and CHOLD, 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 impedance inputs. Leakage currents will range from ± 200 nA; 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:
Although not recommended, another approach would be to
use a potentiometer for RGAIN. This allows users to optimize
the value of RGAIN for each laser diode/photo diode combination’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 CGAIN may not be optimized for the entire
potentiometer’s range.
±∆I BIAS =
CGAIN optimizes the response of the transimpedance amplifier
and should be chosen as from the table below. Choosing CGAIN
larger than the recommended value will slow the response of the
amplifier. Lower values improve TZA bandwidth but may cause
the amplifier to oscillate.
Recommended
CGAIN
≥2.5 kΩ
1.5 kΩ
1 kΩ
500 Ω
2 pF
3 pF
4 pF
8 pF
−9
tWRITE HOLD
CWRITE HOLD
If the same application had a hold time requirement of 250 µs,
then the minimum value of the hold capacitor would be:
CHOLD =
REQ
CEQ
REQ + 50Ω = (RF + 50Ω)
(1+
18 × 10
−9
× 250 µs
1.0 mA
= 4.5 nF
When determining the calibration time, the T/H and the external hold capacitor can be modeled using the simple RC circuit
illustrated in Figure 7.
VREF
R2 R1
+ )
R1 RF
GAIN
IMONITOR
18 × 10

mW 
∆I MAX = 5 mW × (5%) /  0.25
 = 1.0 mA

mA 
RINTERNAL
50Ω
SENSE
IN 2I PIN
t BIAS HOLD
To choose a value, the user will need to determine the amount
of time the loop will be in hold mode, tWRITE HOLD or tBIAS 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 example, if an application requires 5 mW of laser power ± 5%, and
the laser diode efficiency is 0.25 mW/mA, then
The circuit in Figure 6 allows an adjustable gain with low variance in bandwidth, but requires several external components.
EQUIVALENT
CIRCUIT
−9
CBIAS HOLD
±∆I MODULATED =
Table I.
RGAIN
18 × 10
AD9660
RINTERNAL
50Ω
C1
RF
WRITE LEVEL
OR BIAS LEVEL
CF
1:2
R
WRITE HOLD
OR BIAS HOLD
T/H
CHOLD
POWER
MONITOR
VREF
VREF
R1
POWER MONITOR
TZA
R2
Figure 7. Circuit Model for Determining Calibration Times
R3
1.7kΩ
AD9660
Figure 6. Adjustable Gain Configuration
REV. 0
EXTERNAL HOLD
CAPACITOR
–7–
AD9660
Using this model, the voltage at the hold capacitor is
Initial calibration is required after power-up or any other time
the laser has been disabled. Disabling the AD9660 drives the
hold capacitors back down to VREF. In this case, or in any case
where the output current is more than 10% out of calibration, R
will range from 300 Ω to 550 Ω for the model above; the higher
value should be used for calculating the worst case calibration
time. Following the example above, if CHOLD were chosen as
4.5 nF, then τ = RC = 550 Ω × 4.5 nF would be 2.5 µs. For an
initial calibration error <1%, the initial calibration time should
be >5τ = 12.4 µs.
−t 

VCHOLD = Vt = 0 + (V t = ∞ − Vt = 0 ) 1 − e τ 




where t0 is when the calibration begins (WRITE CAL or BIAS
CAL goes logic HIGH), Vt = 0 is the voltage on the hold cap at
t = 0, Vt = ∞ is the steady state voltage at the hold cap with the
loop closed, and τ = RCHOLD is the time constant. With this
model the error in VCHOLD for a finite calibration time, as compared to Vt = ∞ , can be estimated from the following table and
chart:
Initial calibration time will actually be better than this calculation indicates, as a significant portion of the calibration time will
be within 10% of the final value, and the output resistance in
the AD9660’s T/H decreases as the hold voltage approaches its
final value.
Table II.
tCALIBRATION
% Final Value
Error %
7τ
6τ
5τ
4τ
3τ
2τ
τ
99.9
99.7
99.2
98.1
95.0
86.5
63.2
0.09
0.25
0.79
1.83
4.97
13.5
36.8
Recalibration is functionally identical to initial calibration, but
the loop need only correct for droop. Because droop is assumed
to be a small percentage of the initial calibration (<10%), the
resistance for the model above will be in the range of 75 Ω to
140 Ω. Again, the higher value should be used to estimate the
worst case time needed for recalibration.
Continuing with the example above, since the error during hold
time was chosen as 5%, we meet the criteria for recalibration
and τ = RC = 140 Ω × 4.5 nF = 0.63 µs. To get a final error of
1% after recalibration, the 5% droop must be corrected to
within a 20% error (20% × 5% = 1%). A 2τ recalibration time
of 1.26 µs is sufficient.
100
% FINAL VALUE – % of Volts
90
80
Continuous Recalibration
70
In applications where the hold capacitor is small (<500 pF) and
the WRITE PULSE signals always have a pulse width >25 ns,
the user may continuously calibrate the write loop. In such an
application, the WRITE CAL signal should be held logic
HIGH, and the WRITE PULSE signal will control write loop
calibration via the internal AND gate.
60
50
40
30
20
The bias loop may be continuously recalibrated whenever
WRITE PULSE is logic LOW.
10
0
0
1
2
3
4
CALIBRATION TIME – Time Constants = ␶
5
Figure 8. Calibration Time Curve
–8–
REV. 0
AD9660
• From the monitor current specification and the max power
specified:
Example Calculations
The example below (in addition to the one included in the sections above) should guide users in choosing RGAIN, CGAIN, the
hold capacitor values, and worst case calibration times.
I MONITOR MAX = 25 mW
System Requirements:
5 µA
= 125 µA
mW
and
• Bias laser power: 4 mW Bias ± 5%
RGAIN =
• Write laser power: 25 mW ± 0.5%
• Bias Hold Time: 1 ms
1.6 V
1.85 × I MONITOR MAX
• CGAIN would be chosen as 2 pF (see Table I).
• Write Hold Time: 1 µs
Driving the Analog Inputs
Laser diode/photo diode characteristics:
The BIAS LEVEL and WRITE LEVEL inputs of the AD9660
drive the track and hold amplifiers and allow the user to adjust
the amount of output current as described above. The input
voltage range on both inputs is VREF to VREF + 1.6 V, requiring
the user to create an offset of VREF for a ground based signal.
The circuit in Figure 9 will perform the level shift and scale the
output of a DAC whose output is from ground to a positive
voltage. This solution is attractive because both the DAC and
the op amp can run off a single +5 V supply, and the op amp
doesn’t have to swing rail to rail.
• Laser efficiency 0.5 mA/mA
• Monitor current: 5 µA/mA
• From the laser power requirements and efficiency we can
estimate:

mW 
∆I BIAS MAX = 4 mW × (5%) /  0.5
 = 400.0 µA

mA 
and

mW 
∆IWRITE MAX = 25 mW × (0.5% ) /  0.5
 = 250 µA

mA 
VREF + VDAC1
• Choosing hold caps based on these:
CBIAS HOLD =
18 × 10
−9
× 1 ms
400 µA
R1
VDAC1
= 0.045 µF
+5V
OP291
R1
AD9660
BIAS LEVEL
DAC1
R2
18 × 10
−9
× 1 µs
250 µA
VDAC2
= 72 pF
VREF
R3
R4
DAC2
R3
• The bias loop initial calibration time for a <1% error:
5τ = 5 × RC = 5 × 550 Ω × 0.045 µF = 123.75 µs
OP291
WRITE LEVEL
R4
• Bias loop recalibration for a 1% error after 5% droop (need to
correct within 20%):
2τ = 2 × RC = 2 × 140 Ω × 0.045 µF = 12.6 µs
VREF + VDAC2 R4 = VWRITE LEVEL
R3
Figure 9. Driving the Analog Inputs
• The write loop initial calibration time for <0.1% error:
7τ = 7 × RC = 7 × 550 Ω × 72 pF = 277.2 ns
• Write loop re-calibration for a 0.1% error after 0.5% droop
(need to correct within 20%):
2τ = RC = 2 × 140 Ω × 72 pF = 20.2 ns
REV. 0
R2
= VBIAS LEVEL
R1
R2
and
CWRITE HOLD =
− 50 Ω = 6.9 kΩ
–9–
AD9660
Offset Current Generator
AD9660 Layout Considerations
The offset current source allows the user to inject a fixed,
uncalibrated current into the laser diode. The offset current
source is set by an external resistor connected between OFFSET CURRENT SET and +VS, and is controlled by OFFSET
PULSE. See Figure 2 for a transfer function of the offset current source.
As in all high speed applications, proper layout is critical; it is
particularly important when both analog and digital signals are
involved. Analog signal paths should be kept as short as possible, and isolated from digital signals to avoid coupling in noise.
In particular, digital lines should be isolated from OUTPUT,
PIN SENSE, WRITE LEVEL, and BIAS LEVEL traces. Digital signal paths should also be kept short, and run lengths
matched to avoid propagation delay mismatch.
The offset current may be used to increase the output current
provided by the bias and/or write loops after calibration. Alternatively, the offset current may be added during the calibration
of the bias loop and switched off after calibration to drop the
bias current below the knee of the laser diode power curve.
This is illustrated in Figure 10.
4
OPTICAL OUTPUT – mW
0°C CASE
3
Layout of the ground and power supply circuits is also critical.
A single, low impedance ground plane will reduce noise on the
circuit ground. Power supplies should be capacitively coupled
to the ground plane to reduce noise in the circuit. 0.1 µF surface mount capacitors, placed as close as possible to the
AD9660 +VS connections meet this requirement. Multilayer
circuit boards allow designers to lay out signal traces without
interrupting the ground plane, and provide low impedance
power planes to further reduce noise.
Minimizing the Impedance of the Output Current Path
25°C
CASE
CONSTANT WRITE POWER
2
50°C CASE
BIAS CALIBRATION POWER
1
OPERATING
BIAS LEVEL
0
0
20
40
60
80
FORWARD CURRENT – mA
100
120
BIAS
OFFSET2
1
BIAS
MOD
1
WITH OFFSET CURRENT TURNED OFF,
BIAS CURRENT IS BELOW THE KNEE OF THE LASER DIODE
2
OFFSET CURRENT TURNED ON DURING BIAS-CAL
Figure 10. Laser Diode Current-to-Optical Power Curve
Illustrating Bias Below Diode Knee
Because of the very high current slew that the AD9660 is
capable of producing (70+ mA in 1.5 ns), the inductance of the
output current path to and from the laser diode is critical. A
good layout of the output current path will yield high quality
light pulses with rise times of about 1.5 ns and less than 5%
overshoot. A poor layout can result in significant overshoot and
ringing. The most important guideline for the layout is to minimize the impedance (mostly inductance) of the output current
path to the laser. It is important to recognize that the laser current path is a closed loop. The figure illustrates the path that
current travels: (1) from the output pins of the AD9660 to the
anode of the laser, (2) through the laser to the cathode
(ground), (3) through the return path, (4) through the 0.1 µF
bypass capacitors back to the +VS pins of the AD9660 where (5)
the current travels through the output driver circuitry of the
AD9660, and back to the output pins. The inductance of this
loop can be minimized by placing the laser as close to the
AD9660 as possible to keep the loop short, and by placing the
send and return paths on adjacent layers of the PC board to take
advantage of mutual coupling of the path inductances. This
mutual coupling effect is the most important factor in reducing
inductance in the current path.
–10–
REV. 0
AD9660
The best possible TZA settling will be achieved by using a
single carbon surface mount resistor (usually 5% tolerance) for
RGAIN and small surface mount capacitor for CGAIN. Because the
GAIN pin (Pin 9) is essentially connected to the inverting input
of the TZA, it is very sensitive to stray capacitance. RGAIN
should be placed between Pin 9 and Pin 10, as close as possible
to Pin 9. Small traces should be used, and the ground and +VS
planes adjacent to the trace should be removed to further minimize stray capacitance.
The trace from the output pins of the AD9660 to the anode of
the laser (send trace) should be several millimeters wide and
should be as direct as possible. The return current will choose
the path of least resistance. If the return path is the ground
plane, it should have an unbroken path, under the output trace,
from the laser cathode back to the AD9660. If the return path is
not the ground plane (such as on a two layer board, or on the
+VS plane), it should still be on the board plane adjacent to the
plane of the output trace. If the current cannot return along a
path that follows the output trace, the inductance will be drastically increased and performance will be degraded.
The trace from SENSE IN to the cathode of the PIN photodetector should be thin and routed away from the laser anode
trace.
Optimizing the Feedback Layout
In applications where the dynamic performance of the analog
feedback loop is important, it is necessary to optimize the layout
of the gain resistor, RGAIN, as well as the monitor current path to
SENSE IN. Such applications include MOD systems which
recalibrate the write loop on pulses as short as 25 ns, and closed
loop applications.
PIN ASSIGNMENTS
MUTUAL COUPLING
REDUCES INDUCTANCE
+VS PIN CONNECTIONS
25
LASER DIODE CURRENT
PATH SEGMENTS
(SEE TEXT)
24
23
1
22
21
5
20
3
2
19
AD9660
4
OUTPUT PIN CONNECTIONS
BYPASS CAPS
GROUND PIN
CONNECTIONS
GROUND PLANE
Figure 11. Laser Diode Current Loop
REV. 0
–11–
AD9660
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
C2040–6–7/95
28-Pin Plastic SOIC
(R-28)
0.712 (18.08)
0.700 (17.78)
28
15
0.419 (10.64)
0.393 (9.98)
0.300 (7.60)
0.292 (7.40)
1
14
PIN 1
0.104 (2.64)
0.093 (2.36)
0.012 (0.30)
0.004 (0.10)
0.019 (0.48)
0.014 (0.36)
SEATING
PLANE
0.013 (0.33)
0.009 (0.23)
0.04 (1.02)
0.024 (0.61)
PRINTED IN U.S.A.
0.0500
(1.27)
BSC
–12–
REV. 0
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