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© 2001-2013
WLD3343
PAGE 12
WLD3343-00400-O
4. SELECTING THE MONITOR
PHOTODIODE CURRENT RANGE
The monitor photodiode current range
determines the selection of the resistor R
PD
.
Equation 4 calculates R
PD
for a maximum
desired photodiode current, I
PDMAX
. Refer to
Table 3 to select R
PD
for common photodiode
current ranges.
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 Figure 5 for photodiode
resistor connections.
Figure 5
Connecting R
PD
Equation 4
Calculating R
PD
Table 3
Monitor Photodiode Sense Resistor R
PD
vs
Maximum Photodiode Current I
PDMAX
20
μ
A 50
k
200
μ
A 5
k
2 mA
500
20 mA
50
Maximum Photodiode
Current,
IPDMAX
Monitor Photodiode
Current Sense Resistor,
R
PD
PD
R
=
1
I
PDMAX
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
WLD3343
WLD3343
OR
PIN 10
R
PD
R
PD
CONSTANT POWER
OPERATION
5. HELPFUL HINTS FOR
CHOOSING
RPD
• Never use a carbon
fi
lm resistor for R
PD
.
• Select a resistor with a low temperature
coef
fi
cient (ppm/
˚
C).
• Place
R
PD
as close to Pins 12 (PD+) and 13
(PD-) on the WLD3343 as possible to avoid
parasitic resistance effects from the PCB
layout when in constant power mode.
An online tool is available
to determine values for external components:
http://www.teamwavelength.com/support/tools.asp
.
