IRF7233 datasheet(12/24 Pages) AD

12 / 24 page

REV. C

–12–

ADN8830

The unity-gain crossover frequency of the feedforward amplifier

is given as

TEC GAIN

23 1

=×

(15)

To ensure stability, the unity-gain crossover frequency should be

lower than the thermal time constant of the TEC and thermistor.

However, this thermal time constant may not be specified and

can be difficult to characterize.

There are many texts written on loop stabilization, and it is beyond

the scope of this data sheet to discuss all methods and trade-offs

in optimizing compensation networks. A simple method that

can be used to empirically determine a PID compensation loop

as shown in Figure 9 involves the following procedure:

1. Connect thermistor and TEC to the ADN8830 application

circuit. Power does not need to be applied to the laser diode

for this procedure. Monitor output voltage across the TEC

with an oscilloscope.

2. Short C1 and open C2, leaving just R1 and R3 as a simple

proportional-only compensation loop.

3. While maintaining a constant TEMPSET voltage, increase

the ratio of R1/R3, thus increasing the gain until loop oscilla-

tion starts to occur. Decrease this ratio by a factor of 2 from

the point of oscillation. The R1/R3 ratio will likely be less

than unity for most laser modules.

4. Add C1 capacitor and decrease value until oscillation starts,

then increase by a factor of 2. A good initial starting value for

C1 is to create a unity-gain crossover of 0.1 Hz based on

Equation 15.

5. Short R2 and increase C2 until oscillation starts. At this point,

either C2 can be decreased or R2 can be added to regain

stability. Generally speaking, R2 will be greater than R3 and

C2 will be one or more orders of magnitude less than C1.

6. TEMPSET should be adjusted with a step change while

observing the output voltage settling time. A step change of

100 mV should suffice. From here, C2, R2, and even C1 can

be decreased to minimize settling time at the expense of

additional output voltage overshoot.

7. An additional feedback capacitor, CF, in parallel with R1

and C1, can be added to add another high frequency pole. In

many cases, this improves the stability of the system without

increasing the settling time as out-of-band noise is filtered

out of the control signal. A 330 pF to 1 nF capacitor should

suffice, if required.

The typical values shown in the typical application circuit in

Figure 1 have R1 = 100 k

Ω, R2 = 1 MΩ, R3 = 205 kΩ, C1 = 10 µF,

C2 = 1

µF, and an additional feedback capacitor of 330 pF. For

most pump laser modules, this results in a 10

°C TEMPSET step

settling time to within 0.1

°C in less than 5 seconds.

ADN8830

COMPOUT

TEMPCTL

COMPFB

REFERENCE

VOLTAGE

Figure 9. Implementing a PID Compensation Loop

FREQUENCY (Hz LOG SCALE)

R2||R3

2 R1C1

2 C2(R2+R3)

2 R2C2

0dB

2 R3C1

Figure 10. Bode Plot for PID Compensation

Using the TEC Controller ADN8830 with a Wave Locker

Many optical applications require precision control of laser

wavelength. The wavelength of the laser diode can be adjusted

by changing its temperature, which is done through temperature

control of the TEC. Wavelength control can be done by feeding

a wave locker or etalon output back to the microprocessor and

using the microprocessor to calculate and reinstruct the TEC

controller with a new target temperature. However, this method

is computationally expensive and has time delays before the

adjustment is done. A faster responding and simpler method is

to feed the wave locker signal back to the TEC controller for

direct temperature control.

The ADN8830 is designed to be compatible with a wave locker

controller. Figure 11 shows the basic schematic. The TEMPCTL

output from ADN8830 is proportional to the object’s actual

temperature. This voltage is fed to the wave locker controller.

Also fed to the wave locker controller are the photodiode out-

puts from the wave locker, as well as the laser diode power and

a digital signal indicating a functional laser diode, both of which

come from the CW controller. The output of the wave locker

controller is then connected to the input of the compensation

network. This allows the wave locker controller to adjust the

TEC temperature based on the current temperature of the

object, the current wavelength of the laser diode, and the target

wavelength. Once the target wavelength is reached, the wave

locker controller sends a signal to the microcontroller indicating

that the laser signal is good.

ADN8830

COMPOUT

COMPFB

COMPENSATION

NETWORK

TEMPCTL

TEMP IN

LOCKER

PD1

LOCKER

PD2

LASER DIODE

POWER

LASER DIODE

GOOD

FROM

LOCKER

FROM CW

CONTROLLER

WAVE LOCKER

GOOD

MICRO-

PROCESSOR

TEC

CONTROL

Figure 11. Using the ADN8830 with a Wave Locker

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