ADN8835ACPZ-R7 datasheet(20/27 Pages) AD

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ADN8835

Data Sheet

Rev. B | Page 20 of 27

Figure 35. Implementing a PID Compensation Loop

Figure 36. Bode Plot for PID Compensation

MOSFET DRIVER AMPLIFIERS

The ADN8835 has two separate MOSFET drivers: a switched

output or PWM amplifier, and a high gain linear amplifier. Each

amplifier has a pair of outputs that drive the gates of the internal

MOSFETs, which, in turn, drive the TEC as shown in Figure 33. A

voltage across the TEC is monitored via the SFB and LDR pins.

Although both MOSFET drivers achieve the same result, to

provide constant voltage and high current, their operation is

different. The exact equations for the two outputs are

VLDR = VB − 80(VOUT2 − 1.25 V)

VSFB = VLDR + 5(VOUT2 − 1.25 V)

where:

VOUT2 is the voltage at OUT2.

VB is determined by VVDD as

VB = 1.5 V for VVDD < 4.0 V

VB = 2.5 V for VVDD > 4.0 V

The compensation network that receives the temperature set voltage

and the thermistor voltage fed by the input amplifier determines

the voltage at OUT2. VLDR and VSFB have a low limit of 0 V and

an upper limit of VVDD. Figure 37, Figure 38, and Figure 39

show the graphs of these equations.

Figure 37. LDR Voltage vs. OUT2 Voltage

Figure 38. SFB Voltage vs. OUT2 Voltage

Figure 39. TEC Voltage (LDR – SFB) vs. OUT2 Voltage

PWM OUTPUT FILTER REQUIREMENTS

A Type III compensator internally compensates the PWM

amplifier. Because the poles and zeros of the compensator are

designed and fixed by assuming the resonance frequency of the

output LC tank is 50 kHz, the selection of the inductor and the

capacitor must follow this guideline to ensure system stability.

OUT1

IN2N

OUT2

PID COMPENSATOR

CHOPPER 2

IN2P

ADN8835

VTEMPSET

FREQUENCY

(Hz Log Scale)

0dB

2π × RICI

2π × RICD

2π × RPCI

2π × CD (RD + RI)

RD || RI

OUT2 (V)

1.25

0.75

0.25

1.75

2.25

2.75

–2.5

2.5

7.5

5.0

VVDD = 3.3V

VVDD = 5.0V

OUT2 (V)

1.25

0.75

0.25

1.75

2.25

2.75

–2.5

2.5

7.5

5.0

VVDD = 3.3V

VVDD = 5.0V

–2.5

–5.0

2.5

5.0

OUT2 (V)

1.25

0.75

0.25

1.75

2.25

2.75

VVDD = 3.3V

VVDD = 5.0V

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