iC-WJ, iC-WJZ
LASER DIODE DRIVER
Rev C1, Page 7/12
For the monitor diode current of 0.25 mA the resistor
RSET is calculated as:
RSET
=
CR1
∗
V
(ISET ) 3
∗
1.22
V
=
= 14.6
k
Ω
I
av
(AMD)
0.25
mA
Figure 4 shows the corresponding signals for a pulse
duty factor of 20 %. The influence of the pulse duty
factor on the peak value of the monitor current pro-
portional to the laser current is apparent. The average
kept constant by the control (RSET unchanged) means
a peak value increased by the factor 2.5. The pulse
duty factor for which RSET was dimensioned should
therefore be kept constant if possible.
5.0 V
with Electrical Characteristics No. 301 for V(ISET) and
No. 108 (iC-WJZ) for current ratio CR1.
Averaging control
The control of the average optical laser power requires
a capacitor at pin CI. This capacitor is used for aver-
aging and must be adjusted to the selected pulse rep-
etition frequency and the charging current preset with
RSET. The ratios are linear in both cases, i.e. the ca-
pacitor CI must be increased in size proportionally as
the pulse repetition frequency slows or the current from
ISET increases:
440
∗
I(ISET
)
440
CI
≥
=
f
∗
V
(ISET )
f
∗
RSET
Example
Pulse repetition frequency 100 kHz, RSET = 10 kΩ:
CI = 440 nF, chosen 470 nF.
Otherwise the charging of the capacitor CI during the
pulse pauses (with I(ISET) = 1.22 V / RSET) will create
an excessive mean value potential and may destroy
the laser diode during the next pulse. The capacitor CI
is correctly dimensioned when the current through the
laser diode and the optical output signal do not show
any overshots following the rising edge.
5.0 V
V(IN)
0V
3.120 V
V(CI)
3.118 V
600 uA
I(AMD)
0A
Time
Figure 4: Steady-state averaging, f(IN) = 100 kHz
(1:4), CI = 470 nF, RSET = 10 kΩ
3.0 V
2.0 V
V(CI)
1.0 V
0V
I(KLD)
V(IN)
0V
2.552 V
0A
0s
2 ms
4 ms
6 ms
8 ms
10 ms
12 ms
V(CI)
Figure 5: Turn-on behavior, f(IN) = 100 kHz (1:1), CI
= 470 nF, RSET = 10 kΩ
Turn-on and turn-off behavior
Capacitor CI also determines the starting time from
switching on the supply voltage VCC to steady-state
laser pulse operation or after a discharge of CI by
the watchdog. The following applies to estimating the
starting time (Figure 5):
2.550 V
250 uA
0A
I(AMD)
Time
Figure 3: Steady-state averaging control, f(IN) =
100 kHz (1:1), CI = 470 nF, RSET = 10 kΩ
In steady-state condition and for a pulse duty factor of
50 % (pulse/pause 1:1), signals as shown in Figure 3
are present at the IC pins.
T
on
=
2.5
V
∗
CI
2.5
V
∗
CI
∗
RSET
=
I(ISET
)
1.22
V
ICHAUS [ IC-HAUS GMBH ]
