prel

imin

ary

prel

imin

ary

iC-NZP

P-TYPE LASER DIODE DRIVER

Rev A1, Page 8/13

Control modes

Averaging Operation mode RMON calculation

AVG = 0

Peak control

RMON =

V (MD)

IM

AVG = 1

Averaging control RMON =

V (MD)

IM×D

Table 6: RMON dimensioning

Example

By way of example, an output level of 1 mW is to be

set. With an optical power of 1 mW e.g. laser diode

HL6342G has a typical monitor current (IM) of 15 µA.

The following value is then obtained for the resistor at

pin MD (RMON = PMD + RMD, where RMD is a fixed

resistor and PMD a potentiometer.):

RMON =

V (MD)

IM

=

0.25 V

15 µA

= 16.67 k Ω

External capacitor mode

In applications where an external capacitor is required

(see best performance recommendations below), the

external capacitor mode must be enabled (pin NCID =

low).

Best performance recommendations

The operating point for the laser diode is stored in an

on-chip capacitor. This permits a fast start-up but can

lead to an unstable control circuit under certain condi-

tions such as inadequate PCB layout or laser with very

low monitor current. In that cases, an external capaci-

tor is to be connected as close as possible to the chip,

across pin CI and CIS. This will prevent instability of

the control circuit. For averaging control a 10 nF ca-

pacitor at CI is recommended. Special care must be

taken in PCB layout when laying out the path from the

laser diode’s cathode to GND. This path must be kept

as short as possible to avoid parasitic inductances. A

snubber network across the laser diode also helps to

compensate for these parasitic inductances.

Figures 3, 4 and 5 show the typical set-up for the dif-

ferent P, N and M-type diode configurations.

P-Type diodes

T.PAD

VDD

RVDD

CIS

1

&

SYN

VSY

VDDL

OUTPUT MONITOR

GND

EP

-

LDA MONITOR

OverCurrent

-

VDD

100 nF..

VDD

LDA

IMON

VDD

MD

Low V(LDA)

INPUT INTERFACE

100 nF..

LVDS/TTL

TTL

..300mA

&

+

EN

CI

CVDD

CI

RSI

OUTPUT DRIVER

ECI

RMD

RSI

NSLP

PMD

..300mA

AVG

1

1

i(RSI)x540

x280

REGE

GND

ECI

RGND

&

P

+

CVDDA

NERR

NCID

VDDA

9kΩ

REF

+3..+5.5V

OverTemp.

1

RSI

&

0.68..

iC-NZP

RMD

PMD

CI

Bandgap, Reference, Overtemp

..10 nF..

P

Figure 3: Circuit example for P-type laser diodes (case grounded)