LED driving circuit and optical transmitting module
First Claim
1. An LED driving circuit comprising:
- a pulse generation circuit for shaping a waveform of an externally input signal and generating first and second pulses having a complementary relationship;
a first voltage transfer circuit for applying to an LED a first output voltage of a low level on the basis of the first pulse, and a second voltage transfer circuit for applying to the LED a second output voltage of a high level on the basis of the second pulse, wherein the high level is substantially equal to a froward voltage for generating a forward peak current of the LED necessary for outputting light of a predetermined intensity from the LED, the low level is substantially equal to a voltage for changing an emission intensity of the LED to zero or a negligible value, and a real part of an output impedance of said pulse generation circuit is smaller than an internal series resistance of the LED.
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Accused Products
Abstract
An LED is an element in which when a voltage pulse applied to the intrinsic diode of an electrical equivalent model reaches a peak value, a current suddenly flows to obtain an optical output proportional to the forward current. By utilizing this property, the LED receives a rectangular voltage pulse having a large-current driving ability at a low output impedance, or a voltage pulse having two high levels. The low level of the voltage pulse is set within a voltage range where the extinction ratio of an output signal from the LED can be maintained. Even in an LED having a large internal capacitance, an increase in power consumption can be minimized, the transient response time can be shortened, high-speed modulation can be performed, and output light almost free from pulse waveform distortion can be obtained.
111 Citations
28 Claims
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1. An LED driving circuit comprising:
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a pulse generation circuit for shaping a waveform of an externally input signal and generating first and second pulses having a complementary relationship;
a first voltage transfer circuit for applying to an LED a first output voltage of a low level on the basis of the first pulse, and a second voltage transfer circuit for applying to the LED a second output voltage of a high level on the basis of the second pulse, wherein the high level is substantially equal to a froward voltage for generating a forward peak current of the LED necessary for outputting light of a predetermined intensity from the LED, the low level is substantially equal to a voltage for changing an emission intensity of the LED to zero or a negligible value, and a real part of an output impedance of said pulse generation circuit is smaller than an internal series resistance of the LED. - View Dependent Claims (2, 3, 4, 5, 6, 7, 28)
the low level is not more than a voltage necessary for generating a forward diode current having a value obtained by dividing the forward peak current of the LED by an extinction ratio of the LED, and is not less than a forward voltage at which low level of an optical signal generated in transferring a maximum repetitive signal is not more than twice a low level of an optical signal generated in low-speed transmission, and the LED always receives a predetermined pre-bias voltage higher than 0V. -
3. A circuit according to claim 1, wherein the low and high levels change following an operation temperature of the LED so as to always keep a forward current flowing through the LED constant even when the operation temperature of the LED changes.
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4. A circuit according to claim 1, further comprising a third voltage transfer circuit for applying to the LED a third output voltage on the basis of the third pulse,
wherein said pulse generation circuit generates the third pulse, a high level of the third output voltage is higher than the high level of the second voltage, and a difference between the high levels of the second and third output voltages is not more than 1V, and a period of the high level of the third output voltage is not more than 5 us and is not more than a pulse width of the third pulse. -
5. A circuit according to claim 1, wherein a pulse width of an output pulse for driving the LED is shaped to be equal to a pulse width of the first pulse or the second pulse, and
a period of the high level of the output pulse is not more than 2 ns. -
6. A circuit according to claim 1, wherein
an output pulse for driving the LED has an amplitude overshoot of not less than 20% a complete rectangular shape in transition, and a pulse width of the output pulse is equal to a pulse width of the first pulse or the second pulse, not more than 3 ns. -
7. A circuit according to claim 1, further comprising:
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a photodetector for monitoring an emission intensity of the LED; and
a circuit for adjusting an amplitude of the output pulse for driving the LED so as to keep the average emission intensity of the LED constant.
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28. An optical transmitting module comprising:
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an IC having the LED driving circuit defined in any one of claim 1, 8, or 17;
an LED connected to an output terminal of the LED driving circuit;
a submodule on which said IC and said LED are mounted;
an optical connector which is optically coupled to said LED;
a lead which is electrically coupled to said IC and said LED; and
a package for housing said IC, said LED, said submodule, said optical connector, and said lead.
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8. An LED driving circuit comprising:
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a voltage generation circuit for generating a first output voltage of a low level and a second output voltage of a high level;
a first MOS switch for transferring the first output voltage to an output terminal;
a second MOS switch for transferring the second output voltage to the output terminal; and
a pulse generation circuit for shaping a waveform of an externally input signal and generating first and second rectangular pulses having a complementary relationship, wherein an LED is electrically connected to the output terminal, the first rectangular pulse is input to a gate of said first MOS switch, the second rectangular pulse is input to a gate of said second MOS switch, the high level is determined by a forward peak current or forward voltage of the LED which is necessary for outputting light of a predetermined intensity from the LED, and the low level is set to a voltage value for changing an emission intensity of the LED to zero or a negligible value. - View Dependent Claims (9, 10, 11, 12, 13, 14, 15, 16)
the first output voltage is input to said first MOS switch via a first buffer, and the second output voltage is input to said second MOS switch via a second buffer. -
11. A circuit according to claim 8, wherein the low level is set to a voltage value not more than a voltage necessary for generating a forward diode current having a value obtained by dividing the forward peak current of the LED by an extinction ratio of the LED.
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12. A circuit according to claim 8, wherein an
intensity of light generated by the LED in operation at a maximum frequency is set to be not more than twice an intensity of light generated by the LED in operation at a normal frequency. -
13. A circuit according to claim 8, wherein the LED always receives a predetermined voltage higher than 0V as a DC pre-bias.
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14. A circuit according to claim 8, further comprising a control circuit for changing a forward bias voltage of the LED following variations in operation temperature so as to maintain the forward current flowing through the LED at a constant value when the operation temperature of the LED changes.
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15. A circuit according to claim 8, wherein
a rectangular pulse output from said pulse generation circuit has an amplitude overshoot characteristic of not less than 20% in transition, and a pulse width of the rectangular pulse is equal to a pulse width of the externally input signal or not more than 3 ns. -
16. A circuit according to claim 8, further comprising:
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a photodetector arranged near the LED to monitor an emission intensity; and
an adjustment circuit for adjusting amplitudes of the first and second rectangular pulses output from said pulse generation circuit so as to keep an average of the emission intensity constant.
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17. An LED driving circuit comprising:
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a voltage generation circuit for generating a first output voltage of low level, a second output voltage of first high level, and a third output voltage of second high level which is higher than the first high level;
a first MOS switch for transferring the first output voltage to an output terminal;
a second MOS switch for transferring the second output voltage to the output terminal;
a third MOS switch for transferring the third output voltage to the output terminal; and
a pulse generation circuit for shaping a waveform of an externally input signal and generating first, second, and third rectangular pulses, wherein an LED is electrically connected to the output terminal, the first rectangular pulse is input to a gate of said first MOS switch, the second rectangular pulse is input to a gate of said second MOS switch, the third rectangular pulse is input to a gate of said third MOS switch, after the third rectangular pulse changes to high level, the second rectangular pulse successively changes to a high level, a level of the first rectangular pulse has a complementary relationship with a level (ORed level) obtained by ORing the second and third rectangular pulses, and a high-level voltage applied to the LED is binary. - View Dependent Claims (18, 19, 20, 21, 22, 23, 24, 25, 26, 27)
the first output voltage is input to said first MOS switch via a first buffer, the second output voltage is input to said second MOS switch via a second buffer, and the third output voltage is input to said third MOS switch via a third buffer. -
20. A circuit according to claim 17, wherein the low level is set to a voltage value not more than a voltage necessary for generating a forward diode current having a value obtained by dividing the forward peak current of the LED by an extinction ratio of the LED.
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21. A circuit according to claim 17, wherein an intensity of light generated by the LED in operation at a maximum frequency is set to be not more than twice an intensity of light generated by the LED in operation at a normal frequency.
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22. A circuit according to claim 17, wherein the LED always receives a predetermined voltage higher than 0V as a DC pre-bias.
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23. A circuit according to claim 17, further comprising a control circuit for changing a forward bias voltage of the LED following variations in operation temperature so as to maintain the forward current flowing through the LED at a constant value when the operation temperature of the LED changes.
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24. A circuit according to claim 17, wherein
a high-level voltage pulse for driving the LED is set to the second high level during a predetermined period immediately after rise of the voltage pulse, and to the first high level during a remaining period, and high levels of the first to third rectangular pulses are set to the same value as the second high level. -
25. A circuit according to claim 17, wherein
a voltage difference between the first and second high levels is not more than 1V, the third output voltage of the second high level is kept output for a period of not more than 5 ns, and the third output voltage is smaller in amplitude and width than the third rectangular pulse. -
26. A circuit according to claim 17, wherein
a rectangular pulse output from said pulse generation circuit has an amplitude overshoot characteristic of not less than 20% in transition, and a pulse width of the rectangular pulse is equal to a pulse width of the externally input signal or not more than 3 ns. -
27. A circuit according to claim 17, further comprising:
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a photodetector arranged near the LED to monitor an emission intensity; and
an adjustment circuit for adjusting amplitudes of the first and second rectangular pulses output from said pulse generation circuit so as to keep an average of the emission intensity constant.
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Specification