GO POUND GROUND ...

ABOUT TEST LOADS IN CONSTANT POWER

MODE

To test or understand the photodiode feedback and current

monitor circuits, you can replace the constant current

test load with one specific to constant power mode. The

circuits in Figure 2 and Figure 3 vary simulated photodiode

feedback current as laser diode output current changes.

The values in Table 1 are based on the following:

•

The voltage at P

MON will be approximately 1 V if the

setpoint is 1 V in CP mode.

•

A 5 V power supply is used

•

The laser diode current (I

LD) will be approximately one

half the maximum capacity of the laser diode driver

model.

NOTE:

If the current limit is set below half of the maximum

capacity of the laser diode driver, the P

MON voltage and

feedback will be lower.

To decrease I

LD, but keep PMON at approximately 1 V, reduce

R

E.

To decrease P

MON for a given laser diode current, increase

R

E. The new PMON will be proportional to the change in RE.

R

L

D1

D2

LD ANODE

0.33 μF

R

E

PD ANODE

LD CATHODE

Figure 2. Test Load for a Laser Diode (Type A/B) in

Constant Power Mode

R

E

D1

D2

2N3904

R

L

0.33 μF

PD CATHODE

LD ANODE

LD CATHODE

Figure 3. Test Load for a Laser Diode (Type C) in Constant

Power Mode

SET UP THE LASER DIODE TEST LOAD

USING CONSTANT POWER MODE

1. With the settings maintained from testing in Constant

Current mode, attach a test load to the LDA, LDC, and

the Photodiode Anode (PDA) (see Figure 2 and Figure

3

).

NOTE:

For any laser package, confirm the laser type

before testing.

2. Set the driver in Constant Power mode and in the correct

PD range to support the photodiode current (PDI) in the

table below.

3. Connect the multimeter, in voltage mode, between the

P

MON and common GND pins.

4. Attach a 5 V power supply. A higher voltage power supply

can overheat the controller when using test loads.

5. Enable the unit.

6. When the Constant Power mode testing is complete,

return the Current Setpoint and Current Limit trimpots to

their respective OFF positions (usually 12 turns CCW).

Table 1. Load Parameters

MODEL

R

L (Ω)VRL (V)RE (Ω)

D1, D2

PDI

FL591FL

8.8

1.1

1000

1N4001

500 μA

FL593FL

8.8

1.6

1000

1N4001

500 μA

LDD200P -1 & 2P

16

1.6

1000

1N4001

1.0 mA

LDD200P-3P

16

1.6

2000

1N4001

50 μA

LDD400P -1&2P

8

1.6

1000

1N4001

1.0 mA

LDD400P-3P

8

1.6

2000

1N4001

50 μA

LDTC0520

4.4

1.1

1000

1N4001

500 μA

LDTC1020

2.2

1.1

1000

80SQ035

500 μA

LDTC2/2 (E or O)

1.1

1.1

4990

80SQ035

100 μA

PLD200

16

1.6

4990

1N4001

200 μA

PLD500

6.4

1.6

4990

1N4001

200 μA

PLD1250

2.56

1.6

4990

80SQ035

200 μA

PLD5000

0.64

1.6

4990

80SQ035

200 μA

PLD6500

0.49

1.6

4990

80SQ035

200 μA

PLD10000

0.32

1.6

4990

STPS20M100ST

200 μA

PLD12500

0.256

1.6

4990

STPS20M100ST

200 μA

PLD5K-CH

0.64

1.6

4990

80SQ035

200 μA

PLD10K-CH

0.32

1.6

4990

STPS20M100ST

200 μA

PLD12.5K-CH

0.256

1.6

4990

STPS20M100ST

200 μA

WLD3343 or HB

See NOTE below

WLD3343-3A

See NOTE below

NOTE:

Settings for the WLD laser diode driver family are

not given. This is because the photodiode gain resistor is

external to the WLD. Contact the factory for assistance in

setting up a constant power test load for the WLD.

© 2014 • Sales & Technical Support: (406) 587-4910 • email: Sales@teamWavelength.com • web: www.teamWavelength.com

Application Note AN-LDTC04 Rev. A

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