AMC7820 Datasheet, PDF (17/28 Pages) Texas Instruments – ANALOG MONITORING AND CONTROL

Since the THERM_I_OUTPUT and DAC0 reference are
derived from the same reference, they track indepen-
dently of any reference voltage drift, as shown in Figure 4.
This can be viewed as a bridge arrangement common in
instrumentation. When the temperature measured by the
thermistor is equal to the temperature represented by the
DAC’s output voltage, the voltages at pin 2 and pin 6 are
equal regardless of the reference drift caused by the
change of the ambient temperature or other conditions.
Figure 5 shows the typical performance of pin 2 minus pin
6 voltage tracking over –40°C to +85°C. The error is
+0.75mV at –40°C and –0.4mV at +85°C, which, out of a
50mV/°C sensitivity for the thermistor, is equivalent to
0.015°C and 0.008°C, respectively.
26
THERM_I_OUTPUT
100µA
1.0V
2
44
Thermistor
10kΩ at 25°C
DAC0
R
7
R
1.0V
6
DAC Set to 1.0V
FIGURE 4. The Thermistor Feedback and Setpoint Viewed
as a Bridge.
CHANGE IN (Vpin2 – Vpin6) vs TEMPERATURE
(For Internal or External Reference)
2.0
1.5
1.0
0.5
0.0
–0.5
–1.0
–1.5
–2.0
–50
–25
0
25
50
Temperature (°C)
75
100
FIGURE 5. Change in (Vpin2 – Vpin6) vs Temperature.
SWITCHES
The AMC7820 has four internal switches (SW1, SW2, SW3,
and SW4) to shut down the TEC and laser diode (see the
application in Figure 7 for more detail).
SW1 and SW2 are controlled by the internal signals
SW1_DISABLE and SW2_DISABLE, respectively. The sta-
tus of these signals is determined by the four LSBs of the
SHUTDOWN register (bits 0 through 3) designated
SW1-1/SW1-0 and SW2-1/SW2-0. After power-on or reset,
SW1 connects to ground, and SW2 connects to SW3.
SW3 is controlled by bits 2 and 3 (POL1/POL0) of the
CONFIGURATION/STATUS register. After power-on or re-
set, SW3 connects to 2.5V (the internal reference voltage on
pin 26).
Closing switch SW4 activates the TEC soft-start function
implemented by the TEC SOFT_START CONTROLLER.
SW4 is controlled by the internal signals SW2_DISABLE and
SOFT_START_CTR. The LSBs (TS1/TS0) of the CONFIGU-
RATION/STATUS register determines the status of
SOFT_START_CTR. After power-on or reset, SW4 is closed
and soft-start is activated. The host can write “00” to TS1/TS0
to turn off SW4.
The host can change the status of any switch by writing a
proper pattern into the appropriate registers (refer to the
AMC7820 Registers section).
TEC SOFT_START_CONTROLLER
This controller provides a soft start for a bidirectional TEC
Driver when OPA7 is configured as an integrator (see Figure
6) and the TEC driver is referenced to 2.5V (see the Applica-
tion section). Pin 1 (SW2_OUT) drives the external TEC driver.
A value of 2.5V from this pin sets the TEC driver output current
to zero, implementing a safe (zero current) starting condition.
After power-on or reset, switch SW2 is disabled interrupting
control of the external TEC driver by the integrator OPA7. The
actions of switches SW2 and SW3 apply 2.5V to pin 1, setting
the external TEC driver output current to zero. Switch SW4 is
also closed and the TEC SOFT_START_CONTROLLER is
enabled. This controller drives the output of OPA7 to 2.5V
which corresponds to zero TEC current. When the loop control
of the external TEC driver is resumed, the TEC current is
safely ramped up from zero. If the soft start controller is
disabled and the control loop is also diabled with SW2, OPA7
will continue to integrate until its output reaches a supply rail.
This will result in max current drive to the TEC, when the loop
is enabled (a hard start).
AMC7820
17
SBAS231B
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