ADN2830 데이터 시트보기 (PDF) - Analog Devices
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ADN2830 Datasheet PDF : 9 Pages
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PRELIMINARY TECHNICAL DATA
ADN2830
GENERAL
Laser diodes have current-in to light-out transfer functions
as shown in Figure 2. Two key characteristics of this transfer
function are the threshold current, Ith, and slope in the linear
region beyond the threshold current, referred to as slope
efficiency, LI.
Current
Required
Optical
Power
LI = ? P
?I
?P
?I
Ith
Current
Ibias
Figure 3. Laser Transfer Function
CONTROL
A monitor photo diode, MPD, is required to control the LD.
The MPD current is fed into the ADN2830 to control the
power, continuously adjusting the bias current in response
to the laser’s changing threshold current and light to current
(LI) slope (slope efficiency).
The ADN2830 uses automatic power control, APC, to
maintain a constant power over time and temperature.
Average Power is set using the PSET pin. Potentiometers are
connected between these pins and ground. The potentiom-
eter, RPSET , is used to change the average power. PSET is
kept 1.23V above GND.
The ratios of current-in-to-current-out are given by the
following formulae:
RPSET = 1.23V (Ω)
Iav
where Iav is average MPD current.
Note that IPSET will change from device to device. However
the control loop will determine actual values. It is not
required to know exact values for LI or MPD optical cou-
pling.
LOOP BANDWIDTH SELECTION
Capactitor values greater than 22nF are used to set the actual
loop bandwidth. This capacitor is placed between the
PAVCAP pin and ground. It is important that the capacitor is
a low leakage multilayer ceramic with an an insulation
resistance greater than 100GΩ or a time constant of 1000sec,
whichever is less.
ALARMS
The ADN2830 has two active high alarms, DEGRADE and
FAIL. A resistor between ground and the ASET pin is used
to set the current at which these alarms are raised. The
current through the ASET resistor is a ratio of 200:1 to the
FAIL alarm threshold. The DEGRADE alarm will be raised at
90% of this level.
Example: IFAIL = 50mA s o IDEGRADE = 45mA
IASET = Ibiastrip =
50mA = 250µA
200
**RASET = 1.23 V =
IASET
200
1.23 = 4.92 kΩ
250µA
The laser degrade alarm, DEGRADE, is provided to give a
warning of imminent laser failure if the laser diode degrades
further or environmental conditions continue to stress the
LD, eg. increasing temperature.
The laser fail alarm, FAIL, is activated when:
- The ASET threshold is reached.
- The ALS pin is set high. This shuts off the modulation
and bias currents to the LD, resulting in the MPD current
dropping to zero.
DEGRADE will only be raised when the bias current exceeds
90% of ASET current.
MONITOR CURRENTS
IBMON and IMPDMON are current controlled current
sources from Vcc. They mirror the bias and MPD current for
increased monitoring functionality. An external resistor to
GND gives a voltage proportional to the current monitored.
AUTOMATIC LASER SHUTDOWN
When ALS is logic high the bias current is turned off.
Correct operation of ALS can be confirmed by the fail alarm
being raised when ALS is asserted. Note this is the only time
that DEGRADE will be low while FAIL is high.
MODE
The MODE feature on the allows the user operate more than
one ADN2830 in parallel current boosting mode to achieve
up to 400mA of bias current. When using the parrallel
boosting mode one device is run as the master, the other the
slave. The MODE pin on the master is tied to ALS, the mode
pin on the slave is tied high (see Figure 8 for reference
circuit).
ALARM INTERFACES
A 30k internal pull up resistor is employed to pull the digital
high value of the alarm outputs to Vcc. However, the
ADN2830 has a feature which allows the user to externally
wire resistors in parallel with the 30k pull-up resistors thus
enabling the user to interface to non Vcc levels. Non Vcc
alarm output levels must be below the Vcc used for the
ADN2830.
**Note: The smallest valid value for RASET is 1.2KΩ, as this
corresponds to the IBIAS maximum of 200mA.
–4–
REV. PrE May 2002
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