Datasheet ADN8835 (Analog Devices) - 19

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Data Sheet ADN8835
where: Z1 is the combination of RI, RD, and CD (see Figure 35). RTH is a resistance at T (K). Z2 is the combination of RP, CI, and CF (see Figure 35). RR is a resistance at TR (K). Calculate RX using the following equation:  R R + R R − 2R R  R =  LOW MID MID HIGH LOW HIGH  X    R + R − LOW HIGH 2RMID  The user sets the exact compensation network. This network
THERMISTOR AMPLIFIER (CHOPPER 1)
varies from a simple integrator to proportional integral (PI), PID, or any other type of network. The user also determines the type of The Chopper 1 amplifier can be used as a thermistor input compensation and component values because they are dependent amplifier. In Figure 33, the output voltage is a function of the on the thermal response of the object and the TEC. One method to thermistor temperature. The voltage at OUT1 is expressed as: empirical y determine these values is to input a step function to  R  IN2P (thus changing the target temperature), and adjust the FB RFB V = − + O V × UT1 1 REF    + compensation network to minimize the settling time of the TEC TH R RX R  2 temperature. where: R A typical compensation network for temperature control of a laser FB is the feedback resistor. R module is a PID loop consisting of a very low frequency pole and TH is a thermistor. R two separate zeros at higher frequencies. Figure 35 shows a simple X is a compensation resistor. network for implementing PID compensation. To reduce the noise Calculate R using the following equation: sensitivity of the control loop, an additional pole is added at a higher R = RX + RTH_AT_25°C frequency than that of the zeros. The bode plot of the magnitude is V shown in Figure 36. Use the fol owing equation to calculate the OUT1 is centered around VVREF/2 at 25°C. An average temperature to voltage coefficient is −25 mV/°C at a range of 5°C to 45°C. unity-gain crossover frequency of the feedforward amplifier:
2.5
1  R  FB R f FB = × − × 0dB TECGAIN 2πR   + ICI  RTH RX R 
2.0
where TECGAIN is the symbolic gain of the TEC module. TECGAIN is critical to the mathematical design of the PID
1.5 (V)
loop. However, the thermal time constant of the TEC module is
1 UT
usually unspecified, making it difficult to characterize
OV 1.0
TECGAIN as wel as the feedback transfer function. In this case, the PID loop can be determined empirical y by tuning the
0.5
components step by step. There are many documents written on loop stabilization, and it is beyond the scope of this data sheet to discuss al methods and trade-offs for optimizing compensation
0 –15 5 25 45 65
034 networks.
TEMPERATURE (°C)
14174- Figure 34. V VOUT1 is a convenient measure to gauge the thermal instability of OUT1 vs. Temperature the system, which is also known as TEMPOUT. If the thermal loop
PID COMPENSATION AMPLIFIER (CHOPPER 2)
is in steady state, the TEMPOUT voltage equals the TEMPSET Use the Chopper 2 amplifier as the PID compensation amplifier. voltage, meaning that the temperature of the control ed object The voltage at OUT1 feeds into the PID compensation amplifier. equals the target temperature. The frequency response of the PID compensation amplifier is dictated by the compensation network. Apply the temperature set voltage at IN2P. In Figure 39, the voltage at OUT2 is calculated using the fol owing equation: Z2 V = V − (V −V ) OUT2 TEMPSET OUT1 TEMPSET Z1 where: VTEMPSET is the temperature setpoint voltage to the IN2P pin. Rev. B | Page 19 of 27 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY DETAILED FUNCTIONAL BLOCK DIAGRAM SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE MAXIMUM POWER DISSIPATION ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION ANALOG PID CONTROL DIGITAL PID CONTROL POWERING THE CONTROLLER ENABLE AND SHUTDOWN OSCILLATOR CLOCK FREQUENCY External Clock Operation Connecting Multiple ADN8835 Devices TEMPERATURE LOCK INDICATOR SOFT START ON POWER-UP TEC VOLTAGE/CURRENT MONITOR Voltage Monitor Current Monitor MAXIMUM TEC VOLTAGE LIMIT Using a Resistor Divider to Set the TEC Voltage Limit MAXIMUM TEC CURRENT LIMIT Using a Resistor Divider to Set the TEC Current Limit APPLICATIONS INFORMATION SIGNAL FLOW THERMISTOR SETUP THERMISTOR AMPLIFIER (CHOPPER 1) PID COMPENSATION AMPLIFIER (CHOPPER 2) MOSFET DRIVER AMPLIFIERS PWM OUTPUT FILTER REQUIREMENTS Inductor Selection Capacitor Selection INPUT CAPACITOR SELECTION POWER DISSIPATION PWM Regulator Power Dissipation Conduction Loss (PCOND) Switching Losses (PSW) Transition Losses (PTRAN) Linear Regulator Power Dissipation THERMAL CONSIDERATION PCB LAYOUT GUIDELINES BLOCK DIAGRAMS AND SIGNAL FLOW GUIDELINES FOR REDUCING NOISE AND MINIMIZING POWER LOSS General PCB Layout Guidelines PWM Power Stage Layout Guidelines Linear Power Stage Layout Guidelines Placing the Thermistor Amplifier and PID Components EXAMPLE PCB LAYOUT USING TWO LAYERS OUTLINE DIMENSIONS ORDERING GUIDE