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ADN8831ACPZ-R2 Datasheet(PDF) 15 Page - Analog Devices |
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ADN8831ACPZ-R2 Datasheet(HTML) 15 Page - Analog Devices |
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15 / 20 page  Data Sheet ADN8831 Rev. A | Page 15 of 20 APPLICATIONS INFORMATION Chop1 Chop2 IN1P IN2P IN2N IN1N OUT1 OUT2 17.68kΩ 7.68kΩ RX RFB RTH (10kΩ @ 25°C) VREF VREF/2 R 7 4 3 2 VTEMPSET 5 6 VOUT1 VOUT2 Z1 Z2 TEC LPF SFB SPGATE SNGATE LPGATE LNGATE LFB PWM LINEAR THERMISTOR INPUT AMPLIFIER AV = RFB/(RTH + RX) – RFB/R PID COMPENSATOR AMPLIFIER AV = Z2/Z1 MOSFET DRIVER AV = 5 CONTROL Figure 17. Signal Flow Block Diagram SIGNAL FLOW The ADN8831 integrates two auto-zero amplifiers defined as the Chop1 amplifier and the Chop2 amplifier. Both of the amplifiers can be used as standalone amplifiers, therefore, the implementation of temperature control can vary. Figure 17 shows the signal flow through the ADN8831, and a typical implementation of the temperature control loop using the Chop1 amplifier and the Chop2 amplifier. In Figure 17, the Chop1 amplifier and the Chop2 amplifier are configured as the thermistor input amplifier and the PID compensation amplifier, respectively. The thermistor input amplifier gains the thermistor voltage then outputs to the PID compensation amplifier. The PID compensation amplifier then compensates a loop response over the frequency domain. The output from the compensation loop at OUT2 is fed to the linear MOSFET gate driver. The voltage at LFB is fed with OUT2 into the PWM MOSFET gate driver. Including the external transistors, the gain of the differential output section is fixed at 5. For details on the output drivers, see the MOSFET Driver Amplifier section. THERMISTOR SETUP The thermistor has a nonlinear relationship to temperature; near optimal linearity over a specified temperature range can be achieved with the proper value of RX placed in series with the thermistor. First, the resistance of the thermistor must be known, where HIGH TH HIGH MID TH MID LOW TH LOW T R R T R R T R R @ @ @ = = = TLOW and THIGH are the endpoints of the temperature range and TMID is the average. In some cases, with only B constant available , RTH is calculated using the following equation: − = R R TH T T B R R 1 1 exp where: RTH is a resistance at T[K]. RR is a resistance at TR[K]. RX is calculated using the following equation: − + − + = MID HIGH LOW HIGH LOW HIGH MID MID LOW X R R R R R R R R R R 2 2 THERMISTOR AMPLIFIER (Chop1) The Chop1 amplifier can be used as a thermistor input amplifier. In Figure 17, the output voltage is a function of the thermistor temperature. The voltage at OUT1 is expressed as 2 1 REF FB X TH FB OUT1 V R R R R R V × + − + = where: RTH is a thermistor. RX is a compensation resistor. R is calculated using the following equation: C TH X R R R ° + = 25 @ VOUT1 is centered around VREF/2 at 25°C. With the typical values shown in Figure 17, an average temperature-to-voltage coefficient is −25 mV/°C at a range of +5°C to +45°C. |
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