iC-WJB
2.7 V LASER DIODE DRIVER
Rev E1, Page 7/12
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
RSET
=
CR1
∗
V
(ISET ) 3
∗
1.22
V
=
= 24.4
k
Ω
I
av
(AMD)
0.15
mA
with the Electrical Characteristics No. 301 for V(ISET)
and No. 108 for current ratio CR1.
Averaging control (APC)
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:
V(IN)
0V
3.120 V
V(CI)
3.118 V
600 uA
I(AMD)
0A
Time
CI
≥
440
∗
I(ISET
)
440
=
f
∗
V
(ISET )
f
∗
RSET
Figure 4: Steady-state APC, f(IN) = 100 kHz (1:4),
CI = 470 nF, RSET = 10 kΩ
3.0 V
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 average 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.
In steady-state condition and for a pulse duty factor of
50 % (pulse / pause = 1:1), wave forms as shown in
Figure 3.
5.0 V
2.0 V
V(CI)
1.0 V
0V
I(KLD)
0A
0s
2 ms
4 ms
6 ms
8 ms
10 ms
12 ms
Figure 5: Turn-on behaviour, f(IN) = 100 kHz (1:1),
CI = 470 nF, RSET = 10 kΩ
Turn-on and turn-off behaviour
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):
V(IN)
0V
2.552 V
V(CI)
2.550 V
250 uA
0A
I(AMD)
Time
T
on
≈
1.7
V
∗
CI
1.7
V
∗
CI
∗
RSET
=
I(ISET
)
1.22
V
Figure 3: Steady-state APC, f(IN) = 100 kHz (1:1),
CI = 470 nF, RSET = 10 kΩ
Figure 4 shows the corresponding signals for a pulse
duty factor of 20 %. The influence of the pulse duty
Example
CI = 470 nF, RSET = 10 kΩ: T
on
≈
6.5 ms
Figure 6 shows a detailed view of the start of laser
operation; Figure 7 shows the shut-down behaviour.
The decline in the voltage at CI and the absence of the
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