This circuit provides voltage- controlled constant- current biasing of a grounded- cathode laser diode. As shown, the OPA569 op amp provides an output current of 500mA per volt input. The OPA569’s output is current- limited to 2A by R3 & R4. Bypass capacitors are not shown. This circuit takes advantage of the unique topology of the OPA569; it does not require a shunt resistor to measure its output current. This amplifier provides an output monitor current from pin 19 that is 1/475 th of its output current. The voltage developed across R1 & R2 are used as negative feedback to the amplifier’s inverting input (pin 5). A constant- current output is derived by this feedback. Since no shunt resistor is required to measure output current, there is no reduction in output voltage compliance due to shunt resistor voltage drop, this circuit can swing its output voltage very close to its supply rail. This increases efficiency and reduces heat sinking requirements. In fact, supply voltage can be reduced to only a few hundred mV above the load voltage.
The OPA569 features both a Current Limit (pin 4) and a Thermal Overtemp (pin 7) flag. These flags can be used to protect the amplifier and the Enable (pin 8) can be used to digitally control its status. (Circuit is created by Neil P. Albaugh, TI-Tucson)
Two OPA569 Current Sources in Parallel Circuit:
Online Simulation of the “Two OPA569 Current Sources in Parallel” Circuit:
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