AMC7820 Datasheet, PDF (20/28 Pages) Texas Instruments – ANALOG MONITORING AND CONTROL

inverting input of TEC integrator, OPA7. The noninverting
input of OPA7 connects to the thermostat DAC(DAC0),
which sets the desired temperature of the pump laser diode
module. The temperature that a certain voltage represents
depends upon the value of the thermistor used and the drive
current provided to that thermistor.
Pin 1 (SW2_OUT) controls the external TEC Power Current
Driver (DRV591, a PWM power driver) which can be re-
placed with a linear driver (OPA5xx) if configured properly. In
the application of Figure 7, 2.5V on pin 1 sets the TEC
current to zero.
In normal operation, SW2 is enabled and OPA7 connects to
the DRV591. The voltage across the thermistor controls the
TEC current. When SW2 is disabled, the DRV591 is con-
nected to SW3, which connects to 2.5V in the bipolar mode
(which is the default mode). This results in zero TEC current,
and shuts down the TEC. Meanwhile, TEC SOFT_START
CONTROLLER drives the output of OPA7 to become equal
to the reference voltage of 2.5V. This action provides a soft
start when SW2 is enabled.
The TEC current is sensed across an external sense resistor
(0.1Ω) by the amplifier OPA6, and is fed to analog channel
7 of the ADC. OPA6 is connected as a difference amplifier
and is offset (by 2.5V) to provide the measurement of the
bidirectional TEC current. The host processor monitors the
current and takes proper action when necessary.
TEC CONTROL LOOP COMPENSATION
The AMC7820 has a dedicated amplifier (OPA7) to control
the temperature of a Laser Diode Module. The amplifier can
be configured in several ways to implement the control loop
function, but it is commonly configured as a PID (Propor-
tional-Integral-Derivative) controller. In this mode, the control
loop consists of a low-frequency pole, a zero that cancels this
pole below the dominant pole frequency of the TEC and, at
higher frequencies, linear gain that promotes fast settling of
the TEC.
Usually, the component values of the control loop are se-
lected based upon the characteristics of the TEC (thermal
gain and time constant), the gain of the TEC power driver,
and the desired loop response. Frequently, the characteris-
tics of the TEC are not known, which can lead to difficulty in
designing the compensator analytically. An empirical design
procedure using the actual loop components (TEC, power
driver, and AMC7820) can be used to determine the com-
pensator component values.
With this method, the loop response is monitored while
making step changes to the control loop. A step change can
be provided by writing data values to DAC0. An oscilloscope
is used to monitor the temperature of the thermistor at the
T_Sensor _Voltage pin.
The suggested manual tuning procedure is based upon the
components shown in Figure 8.
1) Establish the setpoint of the control loop at the lowest
anticipated temperature setting of the laser module. At this
operating point, the gain of the temperature control loop is at
it’s highest. The loop is best compensated at this point.
2) Initially select R₁ = 2M, C₁ = 1uF, C₂ = 2uF. To improve
the response, decrease the value of C₂. This increases the
gain of the compensator near the dominant pole frequency
of the TEC, resulting in faster settling times of the loop.
However, at some value of C₂ the loop will start to oscillate.
At this point, fix C₂ at 3x to 4x this value.
3) Verify the settling behavior of the control loop at the
highest anticipated laser module setpoint temperature. The
loop response may be somewhat different at this operating
point. A compromise value of C₂ may be needed to balance
the loop response at these operating point extremes.
Typical values of the loop compensation components are:
R₁ = 2M, C₁ = 1uF, C₂ = 0.1uF. See Figures 9 through 12
for typical loop responses.
2.5V Ref
100µA
Monitor
C₁
Pin 2
R₁
OPA9
DAC0
Ref Out
+2.5V
61.9kΩ
C₂
143.3kΩ
61.9kΩ
OPA7
OPA548
143.3kΩ
TEC+
TEC–
R_T
10kΩ
FIGURE 8. Compensation Loop.
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AMC7820
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