LTM4623 datasheet(13/24 Pages) AD

13 / 24 page

LTM4663

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

PID COMPENSATION AMPLIFIER

Use the Chopper 2 amplifier as the PID compensation

amplifier. The voltage at TAMPOUT feeds into the PID

compensation amplifier. The frequency response is dic-

tated by the compensation network. Apply the temperature

set voltage at TSET. In Figure 7, the voltage at PAMPOUT

is calculated using the following equation:

VPAMPOUT = VTSET –

VTAMPOUT – VTSET

(

)

where:

VTSET is the control voltage input to the TSET pin. VTSET

sets the object target temperature. Take Figure 6 for an

example, 1.25V TSET voltage setting maintains a 25°C

target temperature for the object; 0.75V TSET voltage

setting cools down the object to 5°C; and 1.75V TSET

voltage setting heats up the object to 45°C.

Z1 is the impedance of RI, RD, and CD (see Figure 7).

Z2 is the impedance of RP, CI, and CF (see Figure 7).

The user sets the exact compensation network. This net-

work varies from a simple integrator to proportional-in-

tegral (PI), PID (proportional-integral-derivative), or any

other type of network. The user also determines the type

of compensation and component values because they are

dependent on the thermal response of the object and the

TEC. One method to empirically determine these values

is to input a step function to TSET; thus changing the tar-

get temperature, and adjust the compensation network to

minimize the settling time of the TEC temperature.

A typical compensation network for temperature control

of a laser module is a PID loop consisting of a very low

frequency pole and two separate zeros at higher frequen-

cies. Figure 7 shows a simple network for implementing

PID compensation. To reduce the noise sensitivity of the

control loop, an additional pole is added at a higher fre-

quency than that of the zeros. The bode plot of the mag-

nitude is shown in Figure 8.

Figure 7. Analog PID Compensation Network

PAMPN

TAMPOUT

PAMPOUT

PID COMPENSATOR

PID COMPENSATION

AMPLIFIER

TSET

LTM4663

VTSET

–

4663 F07

FREQUENCY

(Hz LOG SCALE)

2π • RDCD

2π • RPCI

2π • CD (RD + RI)

4663 F08

Figure 8. Bode Plot for PID Compensation

To ensure stability, the crossover frequency must be lower

than the thermal time constant of the TEC and thermistor.

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 for optimizing compensation networks.

TAMPOUT is a convenient measure to gauge the thermal

instability of the system. If the thermal loop is in steady

state, the TAMPOUT voltage equals the TSET voltage,

meaning that the temperature of the controlled object

equals the target temperature.

LTM4623 Datasheet(PDF) 13 Page - Analog Devices

LTM4623 Datasheet(HTML) 13 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL