iC-WJ, iC-WJZ
LASER DIODE DRIVER
Rev C1, Page 8/12
Example
The capacitor CWD should be dimensioned such that
the response time tp of the watchdog is slightly longer
than the pulse pause twlo of the input signal. As a re-
sult, the watchdog is just short of being activated.
CI = 470 nF, RSET = 10 kΩ: Ton = 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.
For response times tp longer than tpmin applies:
tp −tpmin
CWD =
2.55 V
Kwd
V(CI)
2.45 V
with tpmin and Kwd from Electrical Characteristics
No. 407, 408.
I(KLD)
V(IN)
5.0 V
0 A
0 V
Time
5 V
V(CWD)
Figure 6: Turn-on behavior, detailed view f(IN) =
100 kHz (1:1), CI = 470 nF, RSET = 10 kΩ
0 V
3 V
V(CI)
5.0 V
2 V
VCC
300 uA
I(AMD)
0 V
3.0 V
0 A
Time
V(CI)
Figure 8: Watchdog, CWD open, f(IN) = 100 kHz
0 V
(1:1), CI = 470 nF, RSET = 10 kΩ
V(IN)
5.0 V
I(KLD)
0 A
Time
0 V
5.0 V
Figure 7: Turn-off behavior, f(IN) = 100 kHz (1:1), CI
V(CWD)
= 470 nF, RSET = 10 kΩ
0 V
3.0 V
Watchdog
V(CI)
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).
2.0 V
300 uA
I(AMD)
0 A
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):
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.
I(ISET)∗twlo
∆V =
CI
The discharge of capacitor CI by the watchdog protects Figure 9 shows the watchdog behavior when the in-
the laser diode from being destroyed by an excessive put frequency is reduced from 100 kHz to 10 kHz. The
turn-on current during the next pulse.
pulse pauses are longer than the watchdog’s response
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