AMC7820YSLASH2K datasheet(21/29 Pages) TI2

21 / 29 page

AMC7820

SBAS231B

www.ti.com

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.

FIGURE 10. TEC Loop Response for a –4

°C Change vs

Time (C1 = 0.47µF).

FIGURE 11. TEC Loop Response for a +4

°C Change vs

Time (C1 = 1.0µF).

FIGURE 12. TEC Loop Response for a +4

°C Change vs

Time (C1 = 0.47µF).

TEC LOOP RESPONSE FOR A –4

°C CHANGE vs TIME

1 = 0.47µF)

Time (2s/div)

DAC Step 2

°C/div

Pin 2 0.1

°C/div

2 = 0.1µF

1 = 2M

TEC LOOP RESPONSE FOR A +4

°C CHANGE vs TIME

1 = 1.0µF)

Time (2s/div)

DAC Step 2

°C/div

Pin 2 0.1

°C/div

2 = 0.1µF

1 = 2M

TEC LOOP RESPONSE FOR A +4

°C CHANGE vs TIME

1 = 0.47µF)

Time (2s/div)

DAC Step 2

°C/div

Pin 2 0.1

°C/div

2 = 0.1µF

1 = 2M

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

1 = 1.0µF)

Time (2s/div)

DAC Step 2

°C/div

Pin 2 0.1

°C/div

2 = 0.1µF

1 = 2M

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