High compliance laser driver circuit
View Patent ↗A laser driver circuit having a differential circuit and an output circuit includes a control circuit receiving a regulated supply voltage that also supplies the differential circuit as an input signal. The control circuit generates a feedback voltage across a first resistor to cause a first current to flow in the first resistor having a current value equal or proportional to the modulation current value. The laser driver circuit includes an operational amplifier receiving the feedback voltage and a reference voltage indicative of a desired modulation current value and to generate the regulated supply voltage. The control circuit and the operational amplifier form a feedback control loop to adjust the regulated supply voltage to regulate the feedback voltage to be equal to the reference voltage, thereby regulating the modulation current value to the desired modulation current value.
1. A laser driver circuit configured to receive a data input signal and to generate an output modulation current signal indicative of the data input signal, the laser driver circuit including a differential circuit configured to receive the data input signal and an output circuit configured to generate the output modulation current signal having a given modulation current value, the differential circuit being supplied by a regulated supply voltage, the laser driver circuit comprising:
a control circuit having an input node configured to receive the regulated supply voltage as an input signal and having an output node connected to a first resistor, the control circuit being configured to generate a feedback voltage across the first resistor to cause a first current to flow in the first resistor having a current value equal or proportional to the modulation current value; and
an operational amplifier configured to receive the feedback voltage and a reference voltage indicative of a desired modulation current value and to generate the regulated supply voltage supplying the differential circuit,
wherein the control circuit and the operational amplifier form a data-independent feedback control loop separate from the output circuit generating the output modulation current to adjust the regulated supply voltage so as to regulate the feedback voltage to be equal to the reference voltage, thereby regulating the modulation current value to the desired modulation current value independent of a data pattern of the data input signal.
2. The laser driver circuit of claim 1 , wherein the feedback voltage is isolated from noise associated with the switching action of the output modulation current signal.
3. The laser driver circuit of claim 1 , wherein the control circuit is configured to generate the feedback voltage across the first resistor to cause the first current to have a current value being the same as the modulation current value, the first current tracking changes in the modulation current value.
4. The laser driver circuit of claim 1 , wherein the control circuit is configured to generate the feedback voltage across the first resistor to cause the first current to have a current value being a fraction of the modulation current value, the first current tracking changes in the modulation current value.
5. The laser driver circuit of claim 1 , wherein the output circuit comprises a differential pair biased by a current source, the current source providing the modulation current value, a first voltage indicative of the modulation current value being developed across the current source, and wherein the control circuit is configured to generate the feedback voltage across the first resistor being proportional to the first voltage.
6. The laser driver circuit of claim 5 , wherein the control circuit is configured to generate the feedback voltage across the first resistor having the same voltage value as the first voltage, the feedback voltage tracking changes in the first voltage.
7. The laser driver circuit of claim 5 , wherein the control circuit is configured to generate the feedback voltage across the first resistor having a voltage value being a fraction of the first voltage, the feedback voltage tracking changes in the first voltage.
8. The laser driver circuit of claim 5 , wherein the current source comprises a second resistor, a current flowing through the second resistor having the modulation current value and a voltage across the second resistor being the first voltage.
9. The laser driver circuit of claim 1 , wherein the differential circuit comprises an output stage and the output circuit comprises a differential pair biased by a current source, a first voltage indicative of the modulation current value being developed across the current source, and wherein the control circuit comprises circuitry correlating to a part of the circuitry of the output stage of the differential circuit and a part of the circuitry of the differential pair of the output circuit, the control circuit being configured to generate the feedback voltage across the first resistor being proportional to the first voltage.
10. The laser driver circuit of claim 9 , wherein the control circuit comprises circuit elements having the same sizing and element values as the correlating circuitry of the differential circuit and the output circuit.
11. The laser driver circuit of claim 9 , wherein the control circuit comprises circuit elements having different sizing and element values as the correlating circuitry of the differential circuit and the output circuit.
12. The laser driver circuit of claim 11 , wherein the control circuit further comprises a compensation circuit configured to ensure that the first current flowing in the first resistor has a current value proportional to the modulation current value over process, supply voltage and temperature variations.
13. A method in a laser driver circuit, comprising:
receiving a data input signal and generating, using a differential circuit and an output circuit, an output modulation current indicative of the data input signal;
supplying the differential circuit using a regulated supply voltage;
generating, using a data-independent feedback control loop separate from the output circuit generating the output modulation current, a feedback voltage based on the regulated supply voltage;
generating a first current based on the feedback voltage and having a current value proportional to the modulation current value;
comparing the feedback voltage to a reference voltage indicative of a desired modulation current value to generate the regulated supply voltage supplying the differential circuit; and
adjusting the regulated supply voltage, using the data-independent feedback control loop, so as to regulate the feedback voltage to be equal to the reference voltage, thereby regulating the modulation current value to the desired modulation current value independent of a data pattern of the data input signal.
14. The method of claim 13 , wherein the feedback voltage is isolated from noise associated with the switching action of the output modulation current signal.
15. The method of claim 13 , wherein generating, using a data-independent feedback control loop separate from the output circuit generating the output modulation current, a feedback voltage based on the regulated supply voltage comprises:
generating the feedback voltage to cause the first current to have a current value being the same as the modulation current value, the first current tracking changes in the modulation current value.
16. The method of claim 13 , wherein generating, using a data-independent feedback control loop separate from the output circuit generating output modulation current, a feedback voltage based on the regulated supply voltage comprises:
generating the feedback voltage to cause the first current to have a current value being a fraction of the modulation current value, the first current tracking changes in the modulation current value.