iC-WJB

2.7 V LASER DIODE DRIVER

Rev E1, Page 8/12

laser pulses indicate that the undervoltage detector is

active.

Time

0 A

V(CI)

2.55 V

2.45 V

I(KLD)

Figure 6: Turn-on behaviour, detailed view f(IN) =

100 kHz (1:1), CI = 470 nF, RSET = 10 kΩ

Time

I(KLD)

0 A

V(CI)

0 V

3.0 V

VCC

5.0 V

0 V

Figure 7: Turn-off behaviour, f(IN) = 100 kHz (1:1),

CI = 470 nF, RSET = 10 kΩ

Watchdog

The watchdog ensures that the capacitor CI is dis-

charged during protracted pulses at IN. During the

pulse pauses the voltage at CI increases by ∆V (Fig-

ure 3).

∆

V =

CI

The discharge of capacitor CI by the watchdog protects

the laser diode from being destroyed by an excessive

turn-on current during the next pulse.

The capacitor CWD should be dimensioned such that

sult, the watchdog is just short of being activated.

CWD =

No. 407, 408.

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.

Time

0 A

300 uA

V(CI)

3 V

2 V

V(IN)

I(AMD)

V(CWD)

0 V

5 V

0 V

5.0 V

Figure 8: Watchdog, CWD open, f(IN) = 100 kHz

(1:1), CI = 470 nF, RSET = 10 kΩ

Time

0 A

300 uA

V(CI)

V(CWD)

V(IN)

I(AMD)

5.0 V

0 V

5.0 V

0 V

3.0 V

2.0 V

Figure 9: Watchdog, CWD open, f(IN) = 100 kHz →

10 kHz (1:1), CI = 470 nF, RSET = 10 kΩ

Figure 9 shows the watchdog behaviour when the in-

put frequency is reduced from 100 kHz to 10 kHz. The

pulse pauses are longer than the watchdog’s response

time. The watchdog begins to discharge the capacitor

CI current limited. The remaining charge time during

the pulse pauses before further watchdog intervention

is not sufficient to maintain the initial potential at CI.

The potential is thus gradually reduced until it reaches

tics No. 405).

The watchdog therefore protects the laser diode from

destruction when the input signal change in such a

manner that the capacitor CI is not longer adequate

for averaging.