Laser control circuit in recordable optical disk drive

Abstract: An improved laser control circuit in a recordable optical disk drive is disclosed. The laser control circuit includes a feeding circuit which supplies an auxiliary current into a bypass capacitor provided on a control line of a drive current switching circuit for a laser diode, so that the charge current for the bypass capacitor is increased. Therefore, the required time to start the emission of the laser beam for recording is reduced. The laser control circuit also includes a control circuit which controls the on/off state of the feeding circuit. The control circuit turns off upon detecting to the transition of the state of the drive current switching circuit to the on state, accordingly it ensure the normal operation of the laser control circuit.

Description: The present invention relates to laser control circuit in a recordable optical disk drive, and more particularly, it relates to a laser control circuit in a recordable optical disk drive in which a recording operation starts-quickly. Fig.2 shows an example of a conventional laser control circuit (applicant's unpublished prior art) in a recordable optical disk drive. The output of a laser diode LD is switched with a pnp transistor Q2, and the on/off operation of the transistor Q2 is controlled by a FET Q1. In reading operation, switches SW1 and SW2 are off, and a switch SW3 is on, and the transistor Q2 and the FET Q1 are insulated (off) by the switch SW1. Under this condition, only a low level drive current i BIAS flows into the laser diode LD, and a laser beam is emitted at the power level of the reading operation. In recording operation, in response to data pulses the switches SW1 and SW2 are turned on, while the switch SW3 is turned off, so that a current (i R - i PD ) obtained by subtracting a current i PD , which flows into a monitoring photo diode PD, from a control current i R charges a bypass capacitor C provided in a circuit at the side of the gate of the FET Q1. When a voltage across the capacitor C exceeds the threshold voltage V TH of the FET Q1, the FET Q1 and the trandistor Q2 are turned on, so that a high level drive current iw flows into the laser diode LD through the transistor Q2, then with a current (i BIAS + i W ), laser pulses for recording are emitted. The output level of the laser diode LD is monitored and controlled by the photo diode PD. In this connection, the control current i R and the capacitance of the bypass capacitor C determine the time T WR from the moment that the switches SW1 and SW2 are turned on untill the moment that the FET Q1 and the transistor Q2 are turned on. In the case of the CD-R drive device, 270 usec or less of the time T WR is required. The time T WR may be decreased by increasing the control current iR or decreasing the capacitance of the capacitor C, however, the upper limit of the control current i R is determined by the optical system of the pick-up or by the limitation of the recording output. If the value of the capacitance of the capacitor is decreased, noises are mixed with the drive current, which results in the deterioration of the recording laser pulses. EP 0 082 357 A, against which claim 1 is delimited, describes a semiconductor laser driving system which is especially adapted for use with an optical recording and reproducing device. A laser light power control loop is provided which is enabled or energized only in the readout or reproduction mode to maintain the laser light at a predetermined level. EP 0 597 644 A describes a laser driving circuit for performing high speed switching with a simple structure. Two current sources and a switch are used to set a peak value and a bottom value and to enable fast switching. The problem of decreasing the output time lag of the laser pulses as much as possible to thereby improve the recording performance of the optical disk drive arises. An object of the present invention is to solve this problem. The foregoing object of the invention is solved by the laser control circuit of claim 1. Preferred Embodiments are the subject matter of the dependent claims. According to the present invention, the laser control circuit in the recordable optical disk drive comprises a feeding circuit which supplies an auxiliary current into the bypass capacitor and a control circuit which turns on the current circuit in response to a recording-start signal, and then turns off the current supplying circuit upon detection of the transition of the state of the drive current switching circuit to the on state. Fig. 1 is a circuit diagram of a laser control circuit in a recordable optical disk drive according to the present invention. Fig. 2 is a circuit diagram of a conventional laser control circuit in a recordable optical disk drive. A Preferred embodiment of the present invention will be described with reference to the accompanying drawings. Fig. 1 shows a laser control circuit in a recordable optical disk drive. The laser control circuit is obtained by adding a feeding circuit comprising a pnp transistor Q4 and a control circuit comprising a npn transistor Q3 and a pnp transistor Q5 to the laser control circuit shown in Fig. 2. A recording signal line of a read/record processor in the optical disk drive is connected to the base of the transistor Q3, so that the on/off state of the transistor Q3 is controlled by the recording signal. The base of the transistor Q4 is connected between the collector of the transistor Q3 and a power source, and the collector of the transistor Q4 is connected to the circuit at the side of the gate of FET Q1. The base of the transistor Q5 is connected between the drain of the FET Q1 and the power source. In the circuit at the side of the collector of the transistor Q3, a collector resistor R1 is connected between the collector of the transistor Q3 and the base of the transistor Q4. Now, the operation of the laser control circuit will be described. At the time of reading, similarly as in the case of the laser control circuit shown in Fig. 2, the switches SW1 and SW2 are off, and the switch SW3 is on, accordingly the FET Q1 and the transistor Q2 are off. Then with the low level of the drive current i BIAS the laser diode LD emits a laser beam with the power of reading level. At the recording operation, in response to data pulses, the switches SW1 and SW2 are turned on and switch SW3 is turned off, while in response to the recording signal the transistor Q3 is turned on, and then the base current of the transistor Q4 is decreased, so that the transistor Q4 is turned-on. Accordingly a current (i R - i PD ) obtained by subtracting a current i PD , which flows into a monitoring photo diode PD, from a control current i R and the collector current i ADD of the transistor Q4 charges a bypass capacitor C provided in the circuit by the side of the gate of the FET Q1. That is, in this embodiment, the charge current for the capacitor C is larger than in the laser control circuit shown in Fig. 2, therefore the time required for the voltage across the capacitor C to reach the threshold voltage V TH of the FET Q1 turns on could be highly reduced. When a voltage across the capacitor C exceeds the threshold voltage V TH of the FET Q1, the FET Q1 and the transistor Q2 are turned on, so that a high level drive current i W flows into the laser diode LD through the transistor Q2, then with a current (i BIAS + i W ), laser pulses for recording are emitted. Simultaneously when the FET Q1 is turned on, a current in the base of the transistor Q5 which is connected in the circuit at the side of the drain of FET Q1 is decreased, so that the transistor Q5 is turned on. Hence, the collector current of the transistor Q5 flows into the base of the transistor Q4, so that the transistor Q4 is turned off, accordingly the collector current i ADD is no larger applied to the capacitor C. Therefore, normal operation is ensured, that is, the FET Q1 and the transistor Q2 are turned off when the switches SW1 and SW2 are turned off.

Claims: A laser control circuit for a recordable optical disk drive comprising a laser beam emitting device (LD) said laser control circuit comprising a drive current switching circuit (Q1, Q2) which switches a drive current for said laser beam emitting device (LD), characterized by: a bypass capacitor (C) connected to a control line of said drive current switching circuit (Q2, Q1); a power source which supplies a control current (i R - i PD ) to said bypass capacitor (C); a feeding circuit (Q4) which supplies an auxiliary current (i ADD ) to said bypass capacitor; and a control circuit (R1, Q3, Q5) which controls the on/off-state of said feeding circuit (Q4), wherein said control circuit (R1, Q3, Q5) turns on said feeding circuit in response to a recording signal, and turns off said feeding circuit upon the detection of a transition of the state of said drive current switching circuit (Q1, Q2) to the on-state. The circuit according to claim 1, wherein said control circuit (R1, Q3, Q5) includes a first control circuit (R1, Q3) and a second control circuit (Q5). The circuit according to claim 2, comprising means for changing said feeding circuit (Q4) to an on-state in response to said first control circuit with said recording signal as an input. The circuit according to one of claims 1 - 3, the on/off state of said drive current switching circuit (Q1, Q2) is controlled by the voltage across said bypass capacitor. The circuit according to one of claims 1 - 4, further comprising means for adding said auxiliary (i ADD ) current from said feeding circuit to said control current (i R -i PD ) from said power source to reduce the required charging time of said bypass capacitor.

Inventors: SAKANUSHI YOSHIHIRO, SAKANUSHI, YOSHIHIRO
Applicants: MITSUMI ELECTRIC CO, MITSUMI ELECTRIC CO., LTD.
Application year: 1997
Main classification: G11B7
IPC codes: H01S5, G11B7