iC-WJB
2.7 V LASER DIODE DRIVER
Rev E1, Page 8/12
laser pulses indicate that the undervoltage detector is
active.
2.55 V
V(CI)
2.45 V
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.
V(IN)
I(KLD)
5.0 V
0V
5V
0A
Time
V(CWD)
0V
3V
Figure 6: Turn-on behaviour, detailed view f(IN) =
100 kHz (1:1), CI = 470 nF, RSET = 10 kΩ
5.0 V
V(CI)
I(AMD)
2V
300 uA
VCC
0V
3.0 V
0A
Time
Figure 8: Watchdog, CWD open, f(IN) = 100 kHz
(1:1), CI = 470 nF, RSET = 10 kΩ
V(CI)
0V
5.0 V
V(IN)
I(KLD)
0A
Time
0V
5.0 V
V(CWD)
0V
3.0 V
Figure 7: Turn-off behaviour, f(IN) = 100 kHz (1:1),
CI = 470 nF, RSET = 10 kΩ
Watchdog
The watchdog ensures that the capacitor CI is dis-
charged during protracted pulses at IN. During the
pulse pauses the voltage at CI increases by
∆V
(Fig-
ure 3).
I(ISET
)
∗
t
wlo
∆V
=
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.
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
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 9 shows the watchdog behaviour when the in-
put frequency is reduced from 100 kHz to 10 kHz. The
pulse pauses are longer than the watchdog’s response
time. The watchdog begins to discharge the capacitor
CI current limited. The remaining charge time during
the pulse pauses before further watchdog intervention
is not sufficient to maintain the initial potential at CI.
The potential is thus gradually reduced until it reaches
the saturation voltage Vs(CI)
wd
(Electrical Characteris-
tics No. 405).
The watchdog therefore protects the laser diode from
destruction when the input signal change in such a
manner that the capacitor CI is not longer adequate
for averaging.
CWD
=
with t
pmin
and K
wd
from Electrical Characteristics
No. 407, 408.
ICHAUS [ IC-HAUS GMBH ]
