iC-WJB

2.7 V LASER DIODE DRIVER

Rev E1, Page 7/12

RSET =

CR1 ∗ V (ISET )

=

3 ∗ 1.22 V

0.15 mA

= 24.4 k Ω

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:

CI ≥

440 ∗ I(ISET )

f ∗ V (ISET )

=

440

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 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.

I(AMD)

V(CI)

V(IN)

Time

2.552 V

2.550 V

0 A

250 uA

0 V

5.0 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

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.

Time

I(AMD)

600 uA

0 A

3.120 V

3.118 V

V(IN)

5.0 V

0 V

V(CI)

Figure 4: Steady-state APC, f(IN) = 100 kHz (1:4),

CI = 470 nF, RSET = 10 kΩ

0 s

2 ms

4 ms

6 ms

8 ms

10 ms

12 ms

I(KLD)

0 A

V(CI)

3.0 V

2.0 V

1.0 V

0 V

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):

1.7 V ∗ CI

I(ISET )

=

1.7 V ∗ CI ∗ RSET

1.22 V

Example

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