NZQFN28 datasheet(10/20 Pages) ROITHNER

10 / 20 page

iC-NZ

FAIL-SAFE LASER DIODE DRIVER

Rev B2, Page 10/20

1. Enabling the system

Before iC-NZ is put into operation for the ﬁrst time the

external circuitry should be checked for the following

(see block diagram):

• Resistor RSMD should be of low impedance. It may

not, however, be lower than 250 Ω, otherwise the in-

ternal iC check will generate an error message.

• The resistors at pins RMDL, RMDM and RMDH

should be of high impedance (low output).

• Inputs LENL, LENM and LENH should be switched

to low (the driver stages switched off).

So that iC-NZ can be switched on pin EN must be

set to high and pin ENCAL to low. A current in the

milliampere range (Electrical Characteristics No. 501)

now ﬂows into pin NERR. This indicates that the de-

vice is now in its start-up phase. The voltage at pin

RSI is set to 0.5 V and capacitor CLDA charged at pin

LDA. Once voltage V(LDA) has reached its ﬁnal value

the control unit and safety monitor diode are checked,

if the latter is in use (see also Operation without a

second monitor diode). Following this check the cur-

rent into pin NERR is lowered (Electrical Characteris-

tics No. 502). The residual current still remaining (ca.

500 µA) indicates that the control unit is not yet in oper-

ation (N-type: V(MDL) < 0.5 V, P-type: VDD – V(MDL)

0.5 V). The entire process is described in Figure 6.

V(EN)

V(SMD)

VCHK

V(MDL)

VREF

V(LDA)

V(NERR)

Figure 6: Signal pattern when the system is enabled

Demo board

Before the demo board is put into operation trimmers

PSMDL, PML, PMM and PMH should be set to the left

and trimmer PSMDH to its centre setting.

Possible sources of error

• V(NERR) remains at low:

– Check the current in NERR. Is the impedance

of the pull-up resistor too high?

• Too high a current is ﬂowing through pin NERR:

An error has occurred during the device self-test.

Check the following:

– Is ENCAL lo?

– Is RSMD of too low an impedance? RSMD

may not be lower than 250 Ω.

– Is pin SMD open or has it directly short-

circuited with VCC?

– Is there a direct connection between pin LDK

and ground?

– Is there a low-impedance connection between

pin LDA and ground?

– Have all CIx capacitors (x: L, M, H) been prop-

erly soldered and placed close to the iC?

– Is VCC properly back upped?

2. Calibrating the low and middle channels

So that the optical power can be regulated pin REGEN

must be set to high. The level at pin ENCAL is of no

consequence when setting the low and middle chan-

nels.

Setting the low output level

Pin LENL is ﬁrst switched from low to high. The in-

tegration capacitor at CIL is then charged. Until the

control unit has settled a small amount of current ﬂows

in pin NERR. Following this the voltage at pin MDL is

0.5 V for an N-type diode and VDD – 0.5 V for a P-type

diode. The required optical power can now be set at

resistor RML.

Demo board

By way of example, three output levels of 1 mW, 3 mW

and 5 mW are to be set. With an optical power of 1 mW

laser diode HL6339G has a typical monitor current (IM)

of 15 µA. The following value is then obtained for the

resistor at pin MDL (RMLn = PML + RML):

RMLn =

V (MDL)

0.5 V

15 µA

= 33.34 k Ω

Setting the middle channel

Pin LENL must remain at high and LENM be switched

from low to high. Following the settling of the control

unit at CIM (N-type: V(MDL) < 0.5 V; P-type: VDD –

V(MDL) < 0.5 V) the required optical power can be set

at resistor RMM.

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