AMC7820YSLASH2K datasheet(17/29 Pages) TI2

17 / 29 page

AMC7820

SBAS231B

www.ti.com

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).

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.

FIGURE 4. The Thermistor Feedback and Setpoint Viewed

as a Bridge.

Thermistor

10k

Ω at 25°C

1.0V

THERM_I_OUTPUT

100

µA

DAC0

1.0V

DAC Set to 1.0V

FIGURE 5. Change in (Vpin2 – Vpin6) vs Temperature.

CHANGE IN (Vpin2 – Vpin6) vs TEMPERATURE

(For Internal or External Reference)

Temperature (

°C)

2.0

1.5

1.0

0.5

0.0

–0.5

–1.0

–1.5

–2.0

–50

–25

100

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