IC-NZP_11 Datasheet, PDF (11/15 Pages) IC-Haus GmbH – P-TYPE LASER DIODE DRIVER

iC-NZP
P-TYPE LASER DIODE DRIVER
LASER CURRENT LIMITATION
preliminary
Rev B1, Page 11/15
CVDD
100 nF..
RSI
0.68..9 kΩ
VDD
RSI
RSI @ VDD
i(RSI)x540
0.5V
VDD-0.5V
VDDA
CVDDA
100 nF..
NSLP
NERR
Overcurrent
1
OverTemp.
Figure 6: iC-NZP VDDA current limitation
iC-NZP monitors the average laser current flowing
from pin VDDA (Figure 6). The DC current limit is set
by means of a resistor at pin RSI.
When dimensioning resistor RSI the following applies
(cf. Electrical Characteristics No. 107):
0.5 V
I_max (VDDA) = 540 × _RSI
If no current limitation is required, pin RSI must be con-
nected to VDD to disable this feature.
Short pulses at VDDA with higher currents are possi-
ble as only the DC current is monitored and capacitor
CVDDA supplies the current for short pulses.
BURST MODE
In controlled burst mode iC-NZP can pulse with up to
155 MHz. Controlled here means that a pre-set oper-
ating point is maintained during the burst phase.
Therefore an operating point is settled first, for which
pin REGE has to be high and the laser must be
switched on. Once the operating point has been
reached the laser can be switched off again. The oper-
ating point is stored in an on-chip capacitor and when
REGE is set to low, the burst mode is activated. The
pre-set operating point is maintained. To prevent the
laser current from rising due to residual currents, the
capacitor is discharged with a maximum of 150 nA (cf.
Electrical Characteristics No. 405). For a longer burst
mode, an external capacitor can be connected to pin
CI. As the capacitor is discharged gradually, the out-
put level must be re-settled again after a certain period,
depending on the admissible degradation of the laser
output power.
CURRENT CONTROL
The iC-NZP also supports laser current control, when
no monitor diode is present. For that purpose, a frac-
tion of the current flowing trough the laser is provided
at IMON pin (ILDA / 280, cf. Electrical Characteristics
No. 406). The laser current is set by means of resistor
RMON (= RMD + PMD).
V (MD)
RMON =
IMON
Figure 7 shows the typical set-up for current control.
External capacitor mode
In applications where an external capacitor is required
(see best performance recommendations below), the
external capacitor mode must be enabled (NCID pin
set to 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.
Setting AVG to hi, keeps the control loop always on
and settled, thus improving the dynamic performance.