NZQFN28 datasheet(12/20 Pages) ROITHNER

12 / 20 page

iC-NZ

FAIL-SAFE LASER DIODE DRIVER

Rev B2, Page 12/20

Demo-Board

When using an N-type laser diode jumper J2 must be

placed between pins 2 and 3 on the demo board. If

a P-type laser diode is used the jumper must be con-

nected between pins 1 and 2.

Warning!

Following calibration pin EN has to be brieﬂy set to low

before actual operation may start with EN set to high

again. The following explains why.

When the low channel (LENL → hi) is switched on ca-

pacitor CIL is charged. When the middle channel is

switched in the internal control unit switches to CIM

and starts to charge it. The voltage at CIL is then no

longer regulated. As residual current could boost the

voltage at CIL, which in turn would increase the cur-

rent in the low channel, thus the capacitor at CIL is

actively discharged with a maximum of 100 nA. The

current in the low channel slowly decreases. As the

control unit endeavours to keep the optical power con-

stant, the capacitor at CIM is charged at the same rate

as CIL is gradually discharged. If the middle chan-

nel is now switched off, the control unit switches back

to the low channel. Capacitor CIL is regulated again;

CIM, however, remains charged. If the middle channel

were then again to be switched on, far too much cur-

rent would ﬂow; this could destroy the laser diode. The

same happens if the low and middle channels are reg-

ulated and the high channel switched on for a lengthy

period of time. In this instance CIL and CIM are slowly

discharged and the voltage at CIH rises in order to

maintain a constant optical power. The entire process

is illustrated in Figure 7.

LENL

CIL

CIM

LENM

Overcurent

I(Laser)

Figure 7: Behavior with very long power-on times

OPERATION WITH ONE OR TWO CHANNELS

It is of course also possible to operate iC-NZ with one

or two levels of light intensity. To make use of the safety

functions of the system in one- or two-channel mode,

pins LDKx (x: L, M) on the unused channels must be

kept unconnected. Nodes CIx should be directly short-

circuited with AGND. To use the safety functions of the

safety monitor diode the high channel must always be

in use in one- or two-channel mode. Figures 8 to 11

give the conﬁguration of iC-NZ for one- or two-channel

operation for both N- and P-type laser diodes.

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