IC-HK_1 Datasheet, PDF (8/13 Pages) IC-Haus GmbH – 155MHz LASER SWITCH

iC-HK
155MHz LASER SWITCH
Rev 21.11.01, Page 8/13
Overcurrent protection through permanent shutdown of iC-WK
To protect the laser diode the additional circuitry shown in Figure 17 can be applied. The permanent shutdown
of iC-WK prevents overcurrent.
VCC
LDA
TRANSIENT
PROTECTION
1
D
NQ
R
-
+
iC-WK
MDK
MDA
VREF
0.5V
CI
LDK
OVERCURRENT
FEEDBACKMON./
OVERTEMP.
GND
AGND
LDK
VDD
CI
EN1
EN2
AGND1
AGND2 GND
Fig. 17: Overcurrent protection by permanent shutdown of iC-WK
If for a given laser current Iop the overcurrent
threshold Iop_max is a certain percentage higher than
the desired high level Iop_hi, then this also applies to
each of the partial currents for the two channels. It is
thus sufficient when monitoring the laser current to
monitor a single channel only. To keep the
requirements on the specification of the switching
transistor Q1 relatively low it is advisable to monitor
the channel with the lower frequency. The
shutdown/limitation threshold is calculated as
Iop_lomax = Iop_lo × Iop_max / Iop_hi.
The required RKx for the bias channel is calculated as
RKx = U_BE / Iop_max
Fig. 18: Puls pattern for overcurrent shutdown
The value for the other RKx and voltage V(CI) can be determined using Figures 2..4 from the data sheet.
With excessive laser current the voltage drop across RKx exceeds U_BE(Q1). Transistor Q1 is switched on, thus
lowering the voltage at pin GND on iC-WK and therefore the voltage at CI. iC-WK tries to compensate for this,
resulting in a higher current in LDK which activates iC-WK‘s permanent overcurrent shutdown.