iC-VJ, iC-VJZ

LASER DIODE CONTROLLER

Rev A1, Page 7/10

Oscillator

The internal oscillator operates in the range approx.

10 kHz to 4 MHz. This enables laser pulse repetition

frequencies from 1 to 200 kHz. Fig. 2 shows the pulse

repetition frequency as a function of the oscillator cir-

cuit.

Example

R1 = 620 Ω, C1 = 82 pF: f ≈ 200 kHz

200

100

f [kHz]

R1 [k

Ω]

1

2

3

4

5

6

7

8

C1= 82pF

C1= 1nF

C1= 220pF

Figure 2: Pulse repetition frequency

Averaging Control

The control of the average optical laser power requires

the external capacitor C2 at pin CI. This capacitor is

used for averaging and must be adjusted to the se-

lected pulse repetition frequency and the charging cur-

rent preset with RSET. The ratios are linear in both

cases, i.e. C2 must be increased in size proportionally

as the pulse repetition frequency slows or resistance

RSET decreases.

C2 ≥

440 ∗ I(ISET )

f ∗ V (ISET )

=

440

f ∗ RSET

Example

Frequency 10 kHz, RSET = 10 kΩ: C2 ≈ 4.7 µF

Otherwise the charging of C2 during the pulse pauses

(with I(ISET) = 1.22 V / RSET) will result in excessive

mean value potential at pin CI and the laser diode may

be destroyed with the next pulse. C2 is correctly di-

mensioned when the current through the laser diode

and the optical output signal do not show any over-

shooting on the rising edge.

In steady-state condition, signals will then appear at

the IC pins as shown in Fig. 3. In this case the laser

pulse exhibits a minimal overshoot on the rising edge,

but this can be tolerated. The increase in the current in

KLD and the laser pulse follow directly after the signal

at the divider output PRF. The outputs PRF and NPRF

are used for receiver synchronisation.

LASER Output

PRF [1V/div]

I(KDL) [50mA/div]

KDL [1V/div]

Timebase = 1:s/div

[2mW/div]

Figure 3: Settled control with 200 kHz pulse repeti-

tion frequency

Turn-on and Turn-off Behavior

Capacitor C2 also determines the starting time from

switching on the supply voltage VCC to steady-state

laser pulse operation. The values of C2 which are

necessary higher for low pulse repetition frequencies

increase this starting time to several milliseconds (Fig.

4). The following applies for estimating the starting

time:

2.5 V ∗ C2

I(ISET )

=

2.5 V ∗ C2 ∗ RSET

1.22 V

Example

Timebase = 20 ms/div

LASER Output [2mW/div]

VCC [2V/div]

PRF [2V/div]

C [1V/div]

Figure 4: Turn-on behavior f = 10 kHz, RSET =

10 kΩ, C2 = 4.7 µF

LASER Output

LASER Output [1mW/div]

C [200mV/div]

Figure 5: Setteling of the averaging control

For high pulse repetition frequencies (200 kHz) and low

C2 values (220 nF) and for RSET = 10 kΩ the aver-

aging control achieves its operating point after 3.5 ms.

Fig. 5 shows the turn-on, Fig. 6 the turn-off behavior,

here in case of undervoltage.