Laser drive device

Details Laser drive device Laser drive device

2000-11-22

2002-12-03

Chaudhuri, Olik (Department: 2813)

Coherent light generators

Particular operating compensation means

C372S034000, C372S031000

Reexamination Certificate

active

06490301

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a laser drive device.
With the recent trend toward larger-capacity and higher-speed optical disk devices, demands for laser drive devices with high speed and low power consumption have increased for data recording/reproduction for such optical disk devices.
As an example of conventional laser drive devices intending to increase the switching speed, Japanese Patent Publication No. 7-95610 discloses a laser drive device as shown in FIG.
9
.
In the conventional laser drive device, an inflow-current I
5
A flowing into an inflow-current source
5
and a set current I
5
B flowing from an outflow-current source
6
are set based on a set current I
4
flowing from a current setting circuit
4
. An outflow-current I
6
flowing from the outflow-current source
6
is set based on the set current I
5
B. Recording signals reverse to each other are applied to bases of transistors
3
A and
3
B of a differential current switch
3
. When the transistor
3
A is turned ON and the transistor
3
B is turned OFF, a current value of a current
13
A flowing through the transistor
3
A becomes equal to that of the inflow-current I
5
A and the outflow-current I
6
. As a result, a value of a laser current I
1
becomes zero, thereby turning OFF a laser
1
. When the transistor
3
A is turned OFF and the transistor
3
B is turned ON, a current value of a current I
3
B flowing through the transistor
3
B is made equal to that of the inflow-current I
5
A, while a current value of the current I
3
A flowing through the transistor
3
A becomes zero. As a result, a current value of the laser current I
1
becomes equal to that of the outflow-current I
6
, thereby turning ON the laser
1
.
The above mentioned conventional laser drive device satisfies desirable conditions for driving a laser, where the laser
1
is grounded on one side and is connected to the transistor
3
A as a switching element in a collector follower manner on the other side. Moreover, while satisfying the above conditions, the transistor
3
A as the switching element is made of an NPN transistor having a high switching speed. This enables easy attainment of a switching speed as high as several nanoseconds or less.
The conventional laser drive device shown in
FIG. 9
however has the following problem.
In general, in a laser drive device of data recording/reproduction for an optical disk device, the values of a laser current vary among the operations of reading, erasing, and writing. The current value is large during writing, while it is small during reading.
In the illustrated conventional laser drive device, the laser current I
1
itself is turned ON/OFF by the differential current switch
3
. Therefore, as the laser current I
1
is greater, power consumption of the laser drive device increases.
As the laser current I
1
is smaller, the currents flowing into the transistors
3
A and
3
B of the differential current switch
3
decrease, resulting in reducing the switching speed of the transistors
3
A and
3
B.
SUMMARY OF THE INVENTION
An object of the present invention is providing a laser drive device capable of suppressing increases in power consumption even on an increase in laser current.
Another object of the present invention provides a laser drive device suppressing decline in switching speed even on decrease in laser current.
The laser drive device of the present invention includes a laser, a first current source, a second current source, a current amplifier, a first transistor, and a second transistor. The first current source supplies a first current having a current value associated with a set current value. The second current source receives a second current having a current value associated with the set current value. The current amplifier amplifies a current from the first current source to generate a laser current and supplies the laser current to the laser. The first transistor is connected between the first current source and the second current source. The second transistor is connected between a power supply node receiving a power supply voltage and the second current source. The first and second transistors are turned ON/OFF complementarily.
In the above laser drive device, when the first transistor is OFF, a first current from the first current source is supplied to the current amplifier. The current amplifier amplifies the current supplied from the first current source to generate a laser current. The laser current is then supplied to the laser. Thus, the laser is turned ON. During this time, the second transistor is ON, allowing a second current to flow from a power supply node into the second current source through the second transistor. When the first transistor is ON, the entire or part of the first current flows into the second current source through the first transistor. This reduces the current supplied to the current amplifier and thus reduces the laser current, resulting in turning OFF the laser. During this time, the second transistor is OFF. The values of the first and second currents are determined by a set current value. The value of the laser current supplied to the laser during the ON-state of it is determined by the value of the first current. Therefore, by adjusting the set current value, a desired value of laser current can be supplied to the laser.
Since the laser drive device is provided with the current amplifier, the values of the first and second currents are smaller than the value of the laser current. This suppresses an increase in power consumed by the first and second power sources and the first and second transistors.
Preferably, the anode of the laser is connected to the power supply node, and the current amplifier includes first, second, and third NPN transistors. The first NPN transistor is connected between the first current source and a grounding node receiving a grounding potential with the emitter being grounded. The second NPN transistor has a collector connected to the power supply node, an emitter connected to the base of the first NPN transistor, and a base connected to the collector of the first NPN transistor. The third NPN transistor is connected between the cathode of the laser and the grounding node with the emitter being grounded. The base of it is connected to the base of the first NPN transistor.
Preferably, the current amplifier further includes a plurality of fourth NPN transistors connected between the cathode of the laser and the grounding node in parallel with the third NPN transistor with the emitters being grounded. The bases of the fourth NPN transistors are connected to the base of the first NPN transistor.
In the above laser drive device, the first, second, and third NPN transistors make a current mirror circuit. The current from the first current source flows through the first NPN transistor. A current of a value obtained by multiplying the current flowing through the first NPN transistor by the mirror ratio flows through the third NPN transistor, to be supplied to the laser as the laser current.
Further, the first and second NPN transistors and each of the plurality of fourth NPN transistors make a current mirror circuit. The sum of the currents flowing through the respective fourth NPN transistors and the current flowing through the third NPN transistor is supplied to the laser as the laser current.
Preferably, the anode of the laser is connected to the power supply node, and the current amplifier includes first, second, and third n-channel MOS transistors. The first n-channel MOS transistor is connected between the first current source and a grounding node receiving grounding potential. The second n-channel MOS transistor is connected between the power supply node and the gate of the first n-channel MOS transistor. The gate of the second n-channel MOS transistor is connected to the first current source. The third n-channel MOS transistor is connected between the cathode of the laser and the grounding node. The gate of the third n-channel MOS transistor is connected to the gate of the first n-channe

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