IC-HK_1 Datasheet, PDF (4/13 Pages) IC-Haus GmbH – 155MHz LASER SWITCH

iC-HK
155MHz LASER SWITCH
Rev 21.11.01, Page 4/13
Controlling the laser power in conjunction with iC-WK
iC-HK operates as a voltage-controlled current source. Canges in temperature, ageing and reflections from
attached lenses will alter the power/current ratio of the laser diode in such a way that the emitted laser power
differs from the adjusted bias point. The laser power thus has to be monitored and the laser current readjusted
accordingly. This can be achieved by using laser driver iC-WK; this device has the added advantage of an
integrated soft start facility which protects the laser diode when the power supply is switched on.
When using laser diodes with integrated monitor diodes (all pin configurations are possible) iC-WK can monitor
the emitted laser power and control the voltage at pin CI in such a way that the mean value of the monitor current
Im_av - and thus the mean value of the emitted optical laser power - remains constant. For iC-WK to achieve a
proper control to the mean the pulse frequency has to be higher than 100kHz. Otherwise iC-WK will try to
readjust with every pulse.
It is imperative that the pulse signals are available at ENx when the power supply is switched on! Otherwise, as
a monitor signal is then lacking, iC-WK will set the voltage at CI to maximum which might damage the laser diode
with the first high pulse at ENx.
Example 4: Switching a current of 100mA with control to the mean value by iC-WK
VCC
LDA
TRANSIENT
PROTECTION
1
D
NQ
R
-
+
iC-WK
MDK
MDA
VREF
0.5V
CI
LDK
OVERCURRENT
FEEDBACK MON./
OVERTEMP.
GND
AGND
LDK
VDD
CI
EN1
EN2
AGND1
AGND2 GND
Fig. 9: Controlled laser power with iC-HK in conjunction with iC-WK
The typical monitor current (Im_hi) for the chosen laser
power is determined from the laser diode data sheet.
Since iC-WK controls the mean value of the monitor
current this has to be calculated from the duty cycle:
Im_av = Im_hi × t_hi / T
The value of RM is calculated from the internal
reference voltage of iC-WK (Item No. 101, iC-WK
data sheet: typically 500mV) as
RM = 500 mV / Im_av
Fig. 10: Pulse pattern for Example 4 -
laser power controlled by iC-WK
iC-WK sets voltage V(CI) so that the mean value of the monitor current matches the target current Im_av.