© 2022
12
WLD SERIES LASER DIODE DRIVERS
SELECTING THE MONITOR PHOTODIODE
CURRENT RANGE
The monitor photodiode current range determines the
selection of the resistor R
PD
.
Equation
PD
for a maximum desired
photodiode current, I
PDMAX
. Refer to
PD
for
common photodiode current ranges.
R
PD
=
1
(8)
I
PDMAX
I
PDMAX
R
PD
20 μA
50 kΩ
200 μA
5 kΩ
2 mA
500 Ω
20 mA
50 Ω
Table 6. Monitor photodiode sense resistor R
PD
versus
maximum photodiode current I
PDMAX
In Constant Power mode, the photodiode is used to control
laser output and R
PD
is connected across Pin 12 (PD+) and
Pin 13 (PD-). In Constant Current mode, the photodiode is
excluded from the control circuit and Pin 12 (PD+) and Pin
13 (PD-) are shorted together. The photodiode current can
still be monitored in constant current mode with R
PD
. Refer
to
for photodiode resistor connections.
TYPE A / B
TYPE C
GND
GND
R
PD
R
PD
V
DD
OR
OR
CONSTANT CURRENT
TYPE A / B
TYPE C
PD-
PD+
GND
GND
V
DD
V
DD
PD-
PD+
7
7
13
13
12
12
14
14
OR
PIN 11
WLD33ND
WLD33ND
OR
PIN 10
R
PD
R
PD
CONSTANT POWER
Figure 6. Connecting R
PD
HELPFUL HINTS FOR CHOOSING R
PD
•
Never use a carbon film resistor for R
PD
.
•
Select a resistor with a low temperature coefficient
(ppm/°C).
• Place R
PD
as close to Pins 12 (PD+) and 13 (PD-) on
the WLD33ND as possible to avoid parasitic resistance
effects from the PCB layout when in Constant Power
mode.
DISABLING OUTPUT CURRENT
The output current can be enabled and disabled as shown
using a Single-Pole Single-Throw (SPST) switch
or a TTL signal.
!
Do not insert or remove the laser diode
from the WLD33ND circuit with power
applied to the unit.
14
1
VDD
WLD33ND
ENABLE
DISABLE
SHUTDOWN
SWITCH
SHD
14
1
VDD
WLD33ND
SHD
TTL INPUT
ENABLE
DISABLE
OR
Figure 7. Enable or disable output current.
Use the online WLD Circuit Calculator to automatically determine
values for external components.
