INTEGRATED CIRCUIT FOR DRIVING A LIGHT SOURCE

Abstract

An integrated circuit is provided for driving a light source. The light source outputs light. A light receiver receives part of the light output from the light source. In response to a detection current from the light receiver, an automatic power controller adjusts an operating current of the light source. A power converter efficiently supplies power from a power supply to the light source. A modulation signal input unit inputs a modulation signal for modulation of light output power. This circuit structure performs a control operation for constantly maintaining the intensity of light even in a variation in an ambient temperature or deterioration in the light source and also reduces power consumption in the integrated circuit for driving the light source.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a circuit diagram of a conventional laser diode (LD) drive circuit;

FIG. 2 illustrates a schematic diagram of an integrated circuit for driving a light source in accordance with an exemplary embodiment of the present invention;

FIG. 3 illustrates an internal circuit diagram of an integrated circuit for driving a green LD in accordance with an exemplary embodiment of the present invention;

FIG. 4 illustrates an internal circuit diagram of an integrated circuit for driving a blue LD in accordance with an exemplary embodiment of the present invention; and

FIG. 5 illustrates a waveform diagram of a pulse width modulation operation in accordance with an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention are now described in detail herein below with reference to the accompanying drawings.

FIG. 2 illustrates a schematic diagram of an integrated circuit (IC) for driving a light source in accordance with an exemplary embodiment of the present invention.

As illustrated in FIG. 2, a light source drive IC 200 in accordance with the exemplary embodiment of the present invention receives part of light output from the light source and performs a control operation for constantly maintaining a light output of the light source in response to an electric current signal detected by a light receiver and a modulation signal input provided by an external image controller (not illustrated). At this time, the external modulation signal is used to improve color reproduction and image brightness and is an analog modulated input signal rather than a simple pulse ON/OFF modulation signal.

The light source is a device for outputting light in the front and rear directions and is arranged with a plurality of light source units. For example, for improved color reproduction of image signals, red, blue and green laser diodes (LDs) are arranged. Each LD emits laser light in proportion to the magnitude of an operating current applied thereto.

In terms of the green laser diode, a semiconductor laser implemented in one chip has not been proposed. A pumped solid-state laser serving as a semiconductor laser conventionally uses a laser for second-harmonic conversion. For example, when an electric current is applied to a semiconductor laser based on gallium arsenide (GaAs), laser light with a wavelength of 808 nm is generated. The neodymium doped yttrium orthovanadate (Nd:YVO4) solid-state laser is pumped with the generated laser light, such that laser light with a wavelength of 1064 nm is generated. Green laser light with a wavelength of 532 nm can be obtained by passing the laser light with the wavelength of 1064 nm to a single crystal for second-harmonic generation (such as potassium titanyl phosphate (KTP), periodically poled lithium niobate (PPLN), or so on).

A light receiver is located in the front direction of an LD chip and is configured with a monitor photodiode (MPD) for detecting part of light. Conventionally, the MPD is driven by reverse bias. Alternatively, the MPD may be driven by forward bias, if needed. In this case, a forward bias voltage is set at a low level of about 0V, since the diode turned on in the forward direction cannot operate as the photodiode if the forward bias voltage is set at a high level of more than 0.5˜0.6V. The forward bias is widely used in a common cathode connection structure in which cathodes of the LD and the MPD are connected. If the MPD is driven by the forward bias in the common cathode connection structure, an additional negative power voltage is not needed.

A green laser module driven by the light source drive IC 200 of the present invention has a structure in which an anode of the LD and a cathode of the MPD are connected. An anode of the MPD is connected to a resistor 240. In this connection structure, the MPD is operated by the reverse bias. According to a detection current supplied from the anode of the MPD, a voltage dropped by the resistor 240 or a feedback voltage is provided to an automatic power controller 210.

The light source drive IC 200 includes the automatic power controller 210 for controlling an operating current such that a light output of the LD is constantly maintained in response to a modulation signal provided from an external image controller (not illustrated) to a modulation signal input unit. In the light source drive IC 200, a type of output light varies with an RGB modulation signal input to the modulation signal input unit. The operating current of each light source differs according to the resistance value of the resistor 240 and the magnitude of the modulation signal.

On the other hand, the light source drive IC 200 includes a power converter 220 for performing conversion to a desired output voltage by controlling a supply of an external power supply voltage Vcc in response to a modulation signal output through the automatic power controller 210. That is, the power is applied to the light source by the power converter 220. To increase conversion efficiency, the power converter 220 operates according to a pulse width modulation control scheme. As the power converter 220 is provided, the power conversion efficiency can be improved up to 80˜90%.

FIG. 3 illustrates an internal circuit diagram of an IC for driving a green LD in accordance with an exemplary embodiment of the present invention. As illustrated in FIG. 3, an automatic power controller 210 of a green LD, that drives IC 205 in accordance with the exemplary embodiment of the present invention, includes a current mirror 230 for outputting a modulation current in response to a green modulation signal input. In the current mirror 230, a modulation signal input unit 235 is connected to a resistor R_MOD. The current mirror 230 is provided with P-channel metal oxide semiconductor (PMOS) field effect input and output transistors Q3 and Q4 in which a common gate is connected to the resistor R_MOD and a drain. Sources of the transistors Q3 and Q4 are connected to a power supply voltage Vcc.

When a modulation signal is input to the modulation signal input unit 235 in the green LD drive IC 205, the current mirror 230 outputs the modulation current from a drain of the output transistor Q4 in response to the input modulation signal as shown in Equation (1).

I _MOD=(V_m−V_a)/R_MOD  Equation (1)

A largest value of the output current I_MOD can increase until it is equal to an output current I_MPD of an MPD. In this case, the generated light is not output. Next, an operation of the current mirror 230 is briefly described when the external modulation signal is an ON/OFF pulse signal.

When V_a=0, the input transistor Q3 is turned on and an electric current of (V_m−V_a)/R_MOD flows to the resistor 240 through the output transistor Q4. The current I_MOD is conventionally set to be more than the detection current I_MPD When V_a=0, the feedback current is supplied only from the current I_MOD and I_MPD=0. In this case, a light output is absent. When V_a=Vcc, the input transistor Q3 is turned off. In this case, because I_MOD=0, a detection voltage due to only I_MPD occurs in the resistor 240. At this time, the light output has a largest value. When V_a=Vcc/2, the magnitude of I_MPD is similar to that of I_MOD. At this time, the light output is reduced to a half of the largest value. In this principle, analog light can be output in response to an analog modulation input.

On the other hand, the detection current I_MPD of the MPD for receiving part of light output from the green laser and performing conversion to a current signal is determined by current characteristics of the green laser. That is, an operating current of the laser for obtaining a desired light output is defined by the direct current (DC) characteristics of the LD. When the operating current flows to the laser, the output current I_MPD of the MPD corresponding to part of the generated light output is defined.

The current I_MPD output from the MPD generates a feedback voltage through the resistor 240. The feedback voltage is compared with a reference voltage V_ref preset in a differential amplifier 260 of the power converter 220. When the feedback operation and the automatic power control operation are performed normally, the feedback voltage is equal to the reference voltage of the differential amplifier 260. The resistance value R_MPD of the resistor 240 for the feedback circuit operation is computed by Equation (2). The light output of the green laser can be adjusted according to the resistance value R_MPD of the resistor 240.

R _MPD =V _ref /I _MPD  Equation (2)

In the present invention, the light output is set to a largest value according to the resistance value R_MPD of the resistor 240. As an input modulation voltage is varied to a largest value or less, the light output is adjusted.

The automatic power controller 210 is provided with the resistor 240 for generating the feedback voltage V_b according to the detection current I_MPD output from the MPD for receiving part of light output from the green laser and performing conversion to a current signal. The modulation current I_MOD output from the current mirror 230 is added to the detection current I_MPD output from the MPD. The resistor 240 drops a voltage. The feedback voltage can be obtained from the dropped voltage. A current source 250 adjusts an operating current to be supplied to the green laser such that the light intensity can be constantly maintained in the green laser according to the feedback voltage.

The automatic power controller 210 performs a control operation for constantly maintaining a laser light output of the green laser by varying an operating current to be applied to the green laser according to the magnitude of a feedback current provided from the laser as in an automatic power control (APC) scheme. In the present invention, the input modulation signal can be used to adjust the light output of the green laser. A modulation current is output in response to an input modulation voltage of the current mirror 230. Feedback is formed such that a sum of the modulation current and the detection current is constantly maintained. As a result, an amount of current flowing to the light source is reduced and an amount of output light is reduced, such that the light output can be adjusted.

On the other hand, the power converter 220 is provided with the differential amplifier 260 for outputting an error signal by comparing the feedback voltage V_b output from the automatic power controller 210 with the preset reference voltage V_ref, a pulse width comparator 280 for comparing a sawtooth wave signal generated from a sawtooth wave signal generator 270 with the error signal and outputting a pulse width modulation signal in proportion to the magnitude of the error signal, and a buck converter 290 for performing conversion to a desired output voltage by controlling the external power supply voltage Vcc in response to a pulse signal.

The feedback voltage V_b generated by the current flowing to the resistor 240 of the automatic power controller 210 is input to an inversion input terminal of the differential amplifier 260, and the preset reference voltage V_ref is input to a non-inversion input terminal of the differential amplifier 260. The differential amplifier 260 outputs the error signal corresponding to a positive voltage difference to an inversion input terminal of the pulse width modulation comparator 280. The differential amplifier 260 includes a time constant control capacitor C2 for adjusting a response characteristic of a feedback circuit in response to an output voltage signal of the resistor 240.

The pulse width modulation comparator 280 receives the error signal through its inversion input terminal and receives a sawtooth wave signal generated from the sawtooth wave signal generator 270 through its non-inversion input terminal. As seen from a waveform diagram of a pulse width modulation operation as illustrated in FIG. 5, the pulse width modulation comparator 280 compares the error signal with the sawtooth wave signal and outputs a pulse width modulation (PWM) signal in inverse proportion to the voltage of the error signal. That is, the width of the pulse signal becomes narrow when the voltage of the error signal increases to drive a switch 300, and a square wave pulse signal with a wide width is output when the voltage of the error signal decreases.

On the other hand, the buck converter 290 includes the switch 300 configured with a PMOS field effect transistor (FET) Q5 corresponding to a switching device for controlling the external power supply voltage Vcc in response to a pulse signal output from the pulse width modulation comparator 280, a diode D1 for allowing the current output from a drain terminal of the PMOS FET Q5 to flow only in one direction, an inductor L1 for generating magnetic induction flux in response to a variation in the current output from the drain terminal of the FET Q5, a capacitor C1 for charging and discharging an electric charge according to a current flow passing through the inductor, and a feed-forward capacitor 310 for preventing oscillation of the feedback circuit.

The switch 300 of the buck converter 290 controls a supply of the external power supply voltage Vcc in response to the pulse signal output from the pulse width modulation comparator 280. The buck converter 290 performs conversion to a preset output voltage required by the light source by controlling the power supply voltage Vcc output from the switch 300. Basically, an output voltage Vout is lower than the power supply voltage Vcc.

The output voltage Vout and the inversion input terminal of the differential amplifier 260 are coupled by the feed-forward capacitor 310, such that an oscillation capable of being generated by a feedback voltage signal of the automatic power controller 210 is prevented. That is, the feed-forward capacitor 310 bypasses a high frequency component of the output voltage and the automatic power controller 210 eliminates the high frequency component, such that the oscillation is prevented.

Next, an operation of the buck converter 290 is described with reference to the automatic power controller 210. When an electric current flows to the green laser light source, laser light is output. Part of the light is detected by the MPD. According to the detection current, a feedback voltage V_b is output by the resistor 240. The feedback voltage V_b is supplied to the inversion input terminal of the differential amplifier 260. The differential amplifier 260 compares the feedback voltage V_b with the reference voltage V_ref of its non-inversion input terminal. If the detection current is less than a pre-set value (V_ref/R_MPD), an output voltage of the differential amplifier 260 is increased and is provided to the inversion input terminal of the pulse width modulation comparator 280. This voltage is compared with the sawtooth wave signal input to the non-inversion input terminal of the pulse width modulation comparator 280. When the output voltage of the differential amplifier 260 increases, an output pulse width of the pulse width modulation comparator 280 decreases. Thus, an OFF time of the switch 300 decreases and an ON time thereof increases, such that the output voltage Vout increases.

When the output voltage increases, an amount of current supplied to the green laser increases and light power of the green laser increases according to the operation of the current source 250. Thus, the detection current of the MPD increases and a feedback operation is performed until the reference voltage V_ref is equal to the detection voltage V_b. The present invention increases an efficiency of supplying power from the power supply to the light source by combining the automatic power controller 210 with the buck converter 290 operating in response to the pulse width modulation.

To improve the power efficiency, the current source 250 uses an N-channel power MOSFET Q6. When the power MOSFET Q6 is used, an on-resistance value between a drain and a source is only in a range of several ten milliohms to several hundred milliohms. Thus, the voltage drop between the drain and the source can decrease and therefore the power efficiency can increase. The internal power consumption of the power MOSFET Q6 is less than 1/10 of the power consumption of a bipolar power transistor.

FIG. 4 is an internal circuit diagram illustrating an IC for driving a blue LD in accordance with an exemplary embodiment of the present invention. Because the IC for driving the blue LD is provided with a power converter 220 and an automatic power controller 210, it is similar to the green LD drive IC 205 of FIG. 3. A structure and operation of the blue LD drive IC is now described on the basis of several differences.

An operating voltage of the blue LD is about 5V and uses a lithium-ion battery of a power supply voltage of 3.7V. When a blue LD is used for a portable light source, an additional boost converter 490 is required. This circuit can be implemented by replacing the buck converter 290 of the green LD drive IC 205 with the boost converter 490. A connection structure of the blue LD and an MPD is a common cathode structure. As described above, the MPD is driven by forward bias. Thus, a reference voltage to be supplied to a non-inversion input terminal of a differential amplifier 460 is set to be less than 0.25V. A PMOS transistor used in a current source 450 for supplying an electric current to the light source is different from the NMOS transistor used in the current source 250 for driving the green laser.

Part of laser light output from the blue LD is detected by the MPD. When the detection current flows to a resistor 440, a feedback voltage is generated. When the modulation current generated by an operation of a current mirror 430 receiving an external modulation signal flows to the resistor 440, a feedback voltage is generated. Because a feedback operation is performed such that a sum of the modulation current and the detection current is constantly maintained, the detection current can be adjusted when the modulation current is varied. Thus, the light output of the blue LD can be adjusted.

A feedback voltage generated by the sum of the detection current and the modulation current is provided to the inversion input terminal of the differential amplifier 460. The feedback voltage is compared with the reference voltage of the non-inversion input terminal. If the feedback voltage is less than the reference voltage because the light output of the blue LD is insufficient, an output of the differential amplifier 460 increases. The output of the differential amplifier 460 is provided to the inversion input terminal of a pulse width modulation comparator 480. The pulse width modulation comparator 480 compares the output of the differential amplifier 460 with a sawtooth wave signal of its non-inversion input terminal. As seen from a waveform of a pulse width modulation operation, as illustrated in FIG. 5, the output of the differential amplifier 460 increases and therefore a pulse width of the output signal of the pulse width modulation comparator 480 decreases. The output of the pulse width modulation comparator 480 is input to a gate of a transistor Q5. If a pulse width of a gate input signal decreases, an ON time of the transistor Q5 decreases and therefore an output voltage of the boost converter 490 increases. This output voltage increases an input voltage of the current source 450 and increases an operating current flowing to the blue LD. Thus, the light output of the blue LD increases. The detection current increases in proportion to the increased light output, such that the feedback operation is stopped at an operating point when the feedback voltage is equal to the reference voltage.

As described above, the present invention can output a voltage required by a light source by controlling an external power supply voltage in response to a feedback voltage of a light receiver. Thus, the present invention can control the light source by adjusting an operating current to be supplied to the light source according to a converted output voltage such that the light output is constantly maintained even in a variation in an ambient temperature or deterioration in the light source.

A power converter controls the power supply voltage using a pulse width modulation scheme. Power consumption is reduced inside an IC for driving the light source. When an operating voltage is less or more than the power supply voltage, improved power conversion efficiency can be obtained. According to simulation results of the green laser drive circuit in accordance with the exemplary embodiment of the present invention, 85% of power supplied from the power supply is applied to the light source, the improved power conversion efficiency is 35% more than the existing power conversion efficiency of about 50%.

The light output power of the light source can be adjusted by varying a resistance value of a resistor, such that a largest light output can be set. A variation in the largest light output can be adjusted using an input modulation signal. Various analog modulation operations as well as a pulse operation according to a waveform of an input modulation signal are possible.

An IC and method for driving a light source can be implemented in accordance with the exemplary embodiments of the present invention. Although the exemplary embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions, and substitutions are possible, without departing from the scope of the present invention. Therefore, the present invention is not limited to the above-described embodiments, but is defined by the following claims, along with their full scope of equivalents.

Claims

1. An integrated circuit for driving a light source in a light source display, comprising: a light source to output light;a light receiver to receive at least a part of the light output from the light source and to perform conversion thereof to a current signal;an automatic power controller to perform a control operation that constantly maintains an intensity of the light output by the light source in response to the current signal output from the light receiver; anda power converter to perform conversion to an output voltage required by the light source by control of an external power supply voltage in response to a signal output from the automatic power controller and to provide an operating voltage of the light source.

2. The integrated circuit of claim 1, wherein the power converter is provided with a buck converter when the operating voltage of the light source is less than the power supply voltage and is provided with a boost converter when the operating voltage of the light source is more than the power supply voltage.

3. The integrated circuit of claim 1, wherein the power converter comprises: a differential amplifier to compare an output voltage of the automatic power controller with a preset reference voltage and yield a comparison result and output an error signal based on the comparison result;a sawtooth wave signal generator that outputs a sawtooth wave signal to be compared with the error signal;a pulse width modulation comparator that compares the error signal with the sawtooth wave signal and outputs a pulse signal in proportion to a voltage of the error signal; anda buck converter that performs a conversion to an output voltage required by the light source by controlling a supply of the power supply voltage in response to the pulse signal output by the pulse width modulation comparator.

4. The integrated circuit of claim 3, wherein the differential amplifier comprises: a time constant control capacitor that adjusts a response characteristic in response to an output voltage signal of the automatic power controller.

5. The integrated circuit of claim 3, wherein the buck converter comprises: a switch that controls the supply of the external power supply voltage in response to a pulse width modulation signal;a diode that allows an electric current output from the switch to flow only in one direction;an inductor that generates magnetic induction flux in response to a variation in the electric current output from the switch;a capacitor that charges and discharges an electric charge according to a current flow passing through the inductor; anda feed-forward capacitor, connected between the capacitor and the differential amplifier, that prevents oscillation of a feedback circuit.

6. The integrated circuit of claim 1, wherein the automatic power controller comprises: a current mirror that outputs a modulation current in response to an external modulation signal; anda first resistor that drops a voltage according to an electric current acquired by adding the modulation current from the current mirror and an electric current output from the light receiver.

7. The integrated circuit of claim 6, wherein the first resistor sets a largest light output according to a resistance value, such that the light output is adjusted again to a largest light output setting value or less in response to an external modulation signal input.

8. The integrated circuit of claim 6, wherein the automatic power controller further comprises: a current source that adjusts an operating current supplied from the light source in response to an output voltage of the first resistor.

9. The integrated circuit of claim 8, wherein the current source comprises a metal oxide semiconductor field effect transistor (MOSFET).

10. The integrated circuit of claim 6, wherein the current mirror comprises: a modulation signal input unit that receives the external modulation signal; anda second resistor that drops a voltage in response to the modulation signal from the modulation signal input unit,wherein the power supply voltage is input to sources of P-channel metal-oxide semiconductor (PMOS) input and output transistors in which a common gate is connected to the second resistor and a drain.

11. The integrated circuit of claim 1, wherein the light source comprises red and blue laser diodes and a green laser.

12. The integrated circuit of claim 1, wherein the green laser serves as a semiconductor laser and generates a second harmonic wave by passing pumped solid-state laser light to a single crystal.

13. The integrated circuit of claim 1, wherein the light receiver comprises a monitor photodiode.