IC-NZN Datasheet, PDF (11/16 Pages) IC-Haus GmbH – N-TYPE LASER DIODE DRIVER

iC-NZN
N-TYPE LASER DIODE DRIVER
CURRENT CONTROL
preliminary
Rev A1, Page 11/16
The iC-NZN also supports laser current control, when
no monitor diode is present. For that purpose, a frac-
tion of the current flowing trough the laser diode is pro-
vided at IMON pin (ILDK / 240, cf. Electrical Charac-
teristics No. 407). Tables 7 and 8 show how to set the
inputs for laser control depending on the input interface
selected (TTL or LVDS).
Laser control in TTL mode (TTL = high/open)
EP
EN NSLP REGE SYN Mode
-
-
low/open -
- Power save mode
low/open open high
-
low LDA charged, laser off
high
open high
high/open high LDA charged, laser on, regulated
high
open high
low
high LDA charged, laser on, burst mode
Table 7: Laser control in TTL mode
Laser control in LVDS mode (TTL = Low)
EP EN NSLP REGE SYN Mode
-
-
low/open -
- Power save mode
< EN > EP high
-
low LDA charged, laser off
> EN < EP high
high/open high LDA charged, laser on, regulated
> EN < EP high
low
high LDA charged, laser on, burst mode
Table 8: Laser control in LVDS mode
The laser current is set by means of resistor RMON
(= RMD + PMD). Figure 7 shows the typical set-up for
current control.
Control modes
Averaging Operation mode
AVG = 0 Peak current control
AVG = 1 Averaging control
RMON calculation
RMON
=
V (RVDD)−V (MD)
IM
RMON
=
V (RVDD)−V (MD)
IM ×D
Table 9: Current control set-up
External capacitor mode
In applications where an external capacitor is required
(see best performance recommendations below), the
external capacitor mode must be enabled (pin CID =
high). This connects the capacitor to the control cir-
cuit and additionally enables a pull-down current at pin
CI to prevent this capacitor from being charged due
to residual currents (cf. Electrical Characteristics No.
406).
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 make the regulated system unstable under cer-
tain conditions such as inadequate PCB layout. In
these cases, an optional capacitor can be connected
as close as possible to the chip, across pins CI and
CIS. For averaging control a 10 nF capacitor at pin CI
is recommended. Special care must be taken in PCB
layout when laying out the path from the laser diode’s
cathode via pin LDK to AGND. 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.