iC-WJ, iC-WJZ
LASER DIODE DRIVER
Rev C1, Page 8/12
Example
CI = 470 nF, RSET = 10 kΩ:
T
on
= 9.6 ms
Figure 6 shows a detailed view of the start of laser op-
eration; Figure 7 shows the shut-down behavior. The
decline in the voltage at CI and the absence of the
laser pulses are signs that the undervoltage detector
is active.
2.55 V
The capacitor CWD should be dimensioned such that
the response time
t
p
of the watchdog is slightly longer
than the pulse pause
t
wlo
of the input signal. As a re-
sult, the watchdog is just short of being activated.
For response times
t
p
longer than
t
pmin
applies:
t
p
−
t
pmin
K
wd
CWD
=
V(CI)
2.45 V
I(KLD)
with
t
pmin
and
K
wd
from Electrical Characteristics
No. 407, 408.
5.0 V
V(IN)
0A
Time
0V
5V
Figure 6: Turn-on behavior, detailed view f(IN) =
100 kHz (1:1), CI = 470 nF, RSET = 10 kΩ
5.0 V
V(CWD)
0V
3V
V(CI)
I(AMD)
VCC
0V
3.0 V
2V
300 uA
0A
Time
V(CI)
0V
Figure 8: Watchdog, CWD open, f(IN) = 100 kHz
(1:1), CI = 470 nF, RSET = 10 kΩ
I(KLD)
5.0 V
V(IN)
0A
Time
0V
5.0 V
Figure 7: Turn-off behavior, f(IN) = 100 kHz (1:1), CI
= 470 nF, RSET = 10 kΩ
Watchdog
In order for the watchdog to function correctly, the input
IN must be activated with a CMOS output (e.g. with an
HCMOS gate: see Figure 1).
The watchdog ensures that the capacitor CI is dis-
charged during protracted pulse pulses at IN. During
the pulse pauses the potential at CI increases by
∆V
(Figure 3):
V(CWD)
0V
3.0 V
V(CI)
2.0 V
300 uA
I(AMD)
0A
Time
Figure 9: Watchdog, CWD open, f(IN) = 100 kHz
→
10 kHz (1:1), CI = 470 nF, RSET = 10 kΩ
Figure 8 shows the signals during normal operation,
without the watchdog being activated. The potential at
CWD rises during pulse pauses but does not reach the
watchdog activation threshold.
Figure 9 shows the watchdog behavior when the in-
put frequency is reduced from 100 kHz to 10 kHz. The
pulse pauses are longer than the watchdog’s response
∆V
=
I(ISET
)
∗
t
wlo
CI
The discharge of capacitor CI by the watchdog protects
the laser diode from being destroyed by an excessive
turn-on current during the next pulse.
ICHAUS [ IC-HAUS GMBH ]
