AMC7820_15 Datasheet, PDF (21/32 Pages) Texas Instruments – ANALOG MONITORING AND CONTROL

TEC LOOP RESPONSE FOR A –4°C CHANGE vs TIME
(C₁ = 1.0µF)
C₂ = 0.1µF
R₁ = 2M
DAC Step 2°C/div
Pin 2 0.1°C/div
TEC LOOP RESPONSE FOR A +4°C CHANGE vs TIME
(C₁ = 0.47µF)
DAC Step 2°C/div
C₂ = 0.1µF
R₁ = 2M
Pin 2 0.1°C/div
Time (2s/div)
FIGURE 9. TEC Loop Response for a –4°C Change vs Time
(C₁ = 1.0µF).
TEC LOOP RESPONSE FOR A –4°C CHANGE vs TIME
(C₁ = 0.47µF)
C₂ = 0.1µF
R₁ = 2M
DAC Step 2°C/div
Pin 2 0.1°C/div
Time (2s/div)
FIGURE 10. TEC Loop Response for a –4°C Change vs
Time (C₁ = 0.47µF).
TEC LOOP RESPONSE FOR A +4°C CHANGE vs TIME
(C₁ = 1.0µF)
DAC Step 2°C/div
C₂ = 0.1µF
R₁ = 2M
Pin 2 0.1°C/div
Time (2s/div)
FIGURE 11. TEC Loop Response for a +4°C Change vs
Time (C₁ = 1.0µF).
Time (2s/div)
FIGURE 12. TEC Loop Response for a +4°C Change vs
Time (C₁ = 0.47µF).
LASER DIODE CONTROL
The laser diode control loop, see Figure 7, maintains a
constant diode current. The loop consists of an integrator
(OPA1), a DAC (DAC2) to set the desired laser diode current,
a transimpedance amplifier (OPA4) to monitor the optical
power, an external current sense resistor, an external instru-
mentation amplifier (or difference amplifier) to sense the laser
diode current, and an external laser diode current driver.
The current through the laser diode is sensed by the external
sense resistor. The voltage across this resistor is fed to the
instrumentation amplifier (gain of 10) which can have its
inputs driven below ground. The output from this amplifier
represents the laser diode current, and is fed back into the
inverting input of the integrator (OPA1), closing the loop. The
output of OPA1 drives the external power current driver. In
normal operation, SW1 is enabled connecting the output of
DAC2 (which represents the set point of the desired laser
diode current) to the noninverting input of OPA1. After
power-on or reset, SW1 is disabled and the noninverting
input of OPA1 connects to ground. This forces laser current
to zero, thereby shutting it down.
For applications where the laser diode cathode is grounded,
all biasing voltages will be positive relative to ground. In this
case, the external INA143 instrumentation amplifier can be
eliminated, and an internal op amp can be substituted and
configured as a 4-resistor difference amplifier.
The output optical power of the laser diode is monitored by the
ADC (analog channel 1) through the back facet PIN diode and
the transimpedance amplifier OPA4. The host processor moni-
tors this power and takes proper action when necessary. Pin
15 is the output of OPA4 and represents the output optical
power of the laser diode. The AMC7820 can put the laser
diode in “constant power mode” as well. When the inverting
input of the integrator OPA1 is connected to pin 15, instead of
the output of INA143 that was shown in Figure 7, the laser
control loop forces the output optical power of the laser diode
to a constant level determined by the output of DAC2.
AMC7820
21
SBAS231B
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