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LDTC0520 / LDTC1020
PAGE 17
LDTC1020-00400-A Rev B
FL OPERATION, continued
3. SET THE LASER DIODE
CURRENT LIMIT
The maximum laser diode current that can be
output to the laser diode is confi gured using the
ILIM potentiometer. The ILIM circuit provides
current limiting without overshooting or ringing
at the limit point, and without saturating control
elements, providing very fast recovery from limit
conditions, without phase shifts or inversions.
See Figure 8 for location of current limit trimpot
and test point.
The limit current can be set while the output
is disabled (recommended), by monitoring the
voltage at the test point while adjusting the ILIM
trimpot. The voltage range is 0-2 VDC, matching
the voltage range and scale of the laser diode
setpoint voltage.
The transfer function for the limit current voltage,
measured at the limit set point is 250 mA / V
for the LDTC0520. The transfer function for the
LDTC1020 is 500 mA / V.
4. CONNECT THE LASER DIODE AND
MONITOR PHOTODIODE
With power removed from the LDTC connect the
output connector (J3) to your laser diode. See
fi gure 1 for proper pin connections. The FL500
is compatible with Type A and B laser package
confi gurations.
Note: We recommend using a simulated laser
diode load for initial operation (See Figure 9).
Simulation of the laser allows the user to
become familiar with the confi guration and
operation of the laser driver without risking
damage to an expensive laser diode. Always
check the Laser Diode Driver, Safe Operating
Area analysis tool to avoid excessive power
dissipation in the laser driver
(
h t t p : / / w w w. t e a m w a ve l e n g t h . c o m / t o o l s /
calculator/soa/defaultld.htm
).
Constant Current Mode:
Laser Diode Anode
Laser Diode Cathode
The 1N4001 general purpose rectifi er has a
forward voltage of 1.1V per diode and is suitable
for simulation of drive currents less than 1 Amp.
National Semiconductor P/N 1N4001
(optional)
Constant Power Mode:
This circuit provides a fi xed photodiode current.
The 24
Ω
resistor typically produces 30 mA of
laser diode current. Vary the resistor size to
change the current output.
ILI
M
Figure 8
Limit Current Trimpot and Testpoint
Figure 9
Simulated Laser Diode Loads
