High resolution dimmer circuit

A dimmer circuit includes a light emitting module, a first current source, a digital-to-analog converter, a switch, a second current source and a pulse width modulation generator. The light emitting module is for emitting light according to a driving current. The first current source includes a first terminal coupled to a second terminal of the light emitting module. The digital-to-analog converter is for generating a DC voltage according to a DC dimming code signal to control the first current source. The switch includes a first terminal coupled to a second terminal of the light emitting module. The second current source includes a first terminal coupled to a second terminal of the switch. The PWM generator is for generating a PWM voltage according to the PWM dimming code signal to control the second current source.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to a dimmer circuit, and more particularly to a high resolution dimmer circuit.

2. Description of the Prior Art

Various types of displays, such as liquid crystal display (LCD), organic light emitting diode (OLED) display, etc., can be implemented to electronic devices such as televisions, computers, and handheld devices. LCDs typically include a backlight to provide illumination to the liquid crystal layer, and circuitry to control the brightness and color of the pixels to render the desired image.

Light emitting diodes (LED) are widely used in displays due to their small size, low power consumption, high luminous efficiency, long lifespan and many other advantages. LED dimming technology mainly includes analog dimming and pulse-width modulation (PWM) dimming.

PWM dimming works by changing the duty cycle of the PWM current. For example, 50% brightness can be achieved by applying 100% amplitude of driving current at 50% duty cycle. Therefore, the display is not actually on at all time, but flickers at high frequency between on and off. Human eyes have persistent vision, so it would appear that the display is always on to human eyes. PWM dimming is to adjust the brightness of the display by controlling the frequency of on and off. If the display is turned on for a longer time and turned off for a shorter time in the same cycle, the overall screen would appear to be brighter. If the display is turned on for a shorter time and turned off for a longer time in the same cycle, the overall screen would appear to be darker. If the screen flickering frequency is lower than a certain frequency, it may discomfort the human eyes and cause other health issues. The analog dimming is to change the brightness of the screen by increasing or decreasing the power applied to the LED driving circuit. The brightness of the display can be adjusted only by adjusting the voltage or current. For example, applying 50% amplitude of the driving current can achieve 50% brightness. The advantage of analog dimming is that it is less burdensome for the human eyes. The disadvantage is that the brightness uniformity of the display is not as good as that of PWM dimming.

SUMMARY OF THE INVENTION

The embodiment provides a dimmer circuit for dimming according to a dimming code. The dimmer circuit includes a light emitting module for emitting light according to a driving current, a first current source, a digital-to-analog converter (DAC), a switch, a second current source and a pulse width modulation (PWM) generator. The light emitting module includes a first terminal for receiving a supply voltage, and a second terminal. The first current source includes a first terminal coupled to the second terminal of the light emitting module, a second terminal coupled to a ground terminal, and a control terminal. The digital-to-analog converter is coupled to the control terminal of the first current source, for generating a direct current (DC) voltage according to a DC dimming code signal to control the first current source. The switch includes a first terminal coupled to the second terminal of the light emitting module, a second terminal, and a control terminal. The second current source includes a first terminal coupled to the second terminal of the switch and a second terminal coupled to the ground terminal. The pulse width modulation (PWM) generator is coupled to the control terminal of the switch, for generating a PWM voltage according to a PWM dimming code signal to control the second current source. The DC dimming code signal includes the most significant bit (MSB) of the dimming code, and the PWM dimming code signal includes the least significant bit (LSB) of the dimming code.

The embodiment provides another dimmer circuit for dimming according to a dimming code. The dimmer circuit includes a light emitting diode, a first current source, a digital-to-analog converter (DAC), a switch, a second current source and a pulse width modulation (PWM) generator. The light emitting diode includes a first terminal for receiving a supply voltage, and a second terminal. The first current source includes a first terminal coupled to the second terminal of the light emitting module, a second terminal coupled to a ground terminal, and a control terminal. The digital-to-analog converter is coupled to the control terminal of the first current source, for generating a direct current (DC) voltage according to a DC dimming code signal to control the first current source. The switch includes a first terminal coupled to the second terminal of the light emitting module, a second terminal, and a control terminal. The second current source includes a first terminal coupled to the second terminal of the switch and a second terminal coupled to the ground terminal. The pulse width modulation (PWM) generator is coupled to the control terminal of the switch, for generating a PWM voltage according to a PWM dimming code signal to control the second current source. The DC dimming code signal includes the most significant bit (MSB) of the dimming code, and the PWM dimming code signal includes the least significant bit (LSB) of the dimming code.

The embodiment provides another dimmer circuit for dimming according to a dimming code. The dimmer circuit includes a light emitting module for emitting light according to a driving current, a first current source, a digital-to-analog converter (DAC), a second current source and a controller. The light emitting module includes a first terminal for receiving a supply voltage, and a second terminal. The first current source includes a first terminal coupled to the second terminal of the light emitting module, a second terminal coupled to a ground terminal, and a control terminal. The digital-to-analog converter is coupled to the control terminal of the first current source, for generating a direct current (DC) voltage according to a DC dimming code signal to control the first current source. The second current source includes a first terminal coupled to the second terminal of the light emitting module, a second terminal coupled to the ground terminal, and a control terminal. The controller is coupled to the control terminal of the second current source, for generating a control voltage according to a PWM dimming code signal to control the second current source. The DC dimming code signal includes the most significant bit (MSB) of the dimming code, and the PWM dimming code signal includes the least significant bit (LSB) of the dimming code.

DETAILED DESCRIPTION

FIG.1is a diagram of a dimmer circuit100of an embodiment of the present invention. The dimmer circuit100is for dimming according to a dimming code. The dimmer circuit100includes a light emitting module110for emitting light according to a driving current ILED, a first current source CS1, a digital-to-analog converter (DAC)120, a switch T1, a second current source CS2and a pulse width modulation (PWM) generator130. The light emitting module110includes a first terminal for receiving a supply voltage Vs, and a second terminal. The first current source CS1includes a first terminal coupled to the second terminal of the light emitting module110, a second terminal coupled to a ground terminal GND, and a control terminal. The digital-to-analog converter120is coupled to the control terminal of the first current source CS1, for generating a direct current (DC) voltage VDCaccording to a DC dimming code signal DCcode to control the first current source CS1. The switch T1includes a first terminal coupled to the second terminal of the light emitting module110, a second terminal, and a control terminal. The second current source CS2includes a first terminal coupled to the second terminal of the switch T1and a second terminal coupled to the ground terminal GND. The PWM generator130is coupled to the control terminal of the switch T1, for generating a PWM voltage VPWMaccording to a PWM dimming code signal PWMcode to control the second current source CS2. The DC dimming code signal DCcode includes the higher bits of the dimming code, and the higher bits include the most significant bit (MSB) of the dimming code. The PWM dimming code signal PWMcode includes the lower bits of the dimming code, and the lower bits include the least significant bit (LSB) of the dimming code. In application, the light emitting module110may include a light emitting diode LED1. A first terminal of the light emitting diode LED1can receive the supply voltage Vs, and the second terminal can be coupled to the first terminal of the first current source CS1. The switch T1can be an N-type transistor or other equivalent components.

The driving current ILEDfor driving the light emitting diode LED1is controlled by the first current source CS1and the second current source CS2. The DC current IDCand the PWM current IPWMare added together to form the driving current ILED, and the brightness of the light emitting diode LED1is determined by the driving current ILED. The first current source CS1is directly controlled by the DC voltage VDCto provide the DC current IDC. When the DC voltage VDCincreases, the DC current IDCwould also increase. As the result, the brightness of the light emitting diode LED1would also increase. The PWM voltage VPWMcan control the switch T1, thereby controlling the second current source CS2and providing the PWM current IPWM. Specifically, when the PWM voltage VPWMis at the high level, the switch T1is turned on to generate the PWM current IPWM. When the PWM voltage VPWMis at the low level, the switch T1is turned off and the PWM current IPWMis turned off. In other words, the higher the duty ratio of the PWM voltage VPWMis, the longer time the PWM current IPWMcan be turned on, which results in higher brightness of the light emitting diode LED1. The control method for the dimmer circuit100to generate the driving current ILEDaccording to the dimming code is described in detail in the following paragraphs.

FIGS.2A-2Bare diagrams of the dimming codes and the corresponding driving current ILEDof the dimmer circuit100inFIG.1. The dimming code shown inFIG.2Ais a 16-bit code, including a 12-bit DC code and a 4-bit PWM code. The DC code includes the higher 12 bits of the dimming code, which includes the most significant bit of the dimming code. The PWM code includes the lower 4 bits of the dimming code, which includes the least significant bit of the dimming code. The vertical axis shown inFIG.2Bis the drive current ILED, and the horizontal axis is the period, which is divided into 16 time slots. When the dimming code is input to the dimmer circuit100, the DC code and the PWM code can be processed separately. For example, the dimming code32(binary: 0000000000100000) can be converted into a DC code equal to 2, and the dimmer circuit100can generate DC current IDCof 2 mA for 16 time slots. The PWM code is equal to 0, so PWM current IPWMis generated. The dimming code31(binary: 0000000000011111) can be converted into a DC code equal to 1 and a PWM code equal to 15. The dimmer circuit100can generate DC current IDCof 1 mA for 16 time slots, and PWM current IPWMof 1 mA for 15 time slots. The dimming code18(binary: 0000000000010010) can be converted into a DC code equal to 1 and a PWM code equal to 2. The dimmer circuit100can generate DC current IDCof 1 mA for 16 time slots, and PWM current IPWMof 1 mA for 2 time slots. Dimming code17(binary: 0000000000010001) can be converted into DC code equal to 1 and PWM code equal to 1. The dimmer circuit100can generate DC current IDCof 1 mA for 16 time slots, and PWM current IPWMof 1 mA for 1 time slot, and so on. The total current summed by the DC current IDCand the PWM current IPWMis the driving current ILED. By this way, the brightness of the light emitting diode LED1generated by the dimming code32is higher than that of the dimming code31, and the brightness of the light emitting diode LED1generated by the dimming code18is higher than that of the dimming code17.

The DC code can be converted into a DC dimming code signal DCcode and input to the digital-to-analog converter120, and the digital-to-analog converter120converts the DC dimming code signal DCcode into the DC voltage VDCwhich can control the first current source CS1to provide DC current IDC. The PWM code can be converted into a PWM dimming code signal PWMcode and input to the PWM generator130. Then, the PWM generator130converts the PWM dimming code signal PWMcode into the PWM voltage VPWM. The PWM voltage VPWMswitches the switch T1rapidly, so that the second current source CS2provides a PWM current IPWMwith PWM waveform.

FIG.3is a diagram of the waveforms of the DC current IDCand the PWM current IPWMof the dimmer circuit100ofFIG.1. The waveform formula of the DC current IDCis as follows:

ILEDMAXis the maximum driving current of the light emitting diode LED1. DC_code is the DC code value. DC_resolution is the number of bits of the DC code. In this embodiment, the DC code is 12 bits. DC Hightime is the duration of the DC current at high level. In this case, it equals to 100% of the period, and the period can be 16 time slots.

The waveform formula of the PWM current IPWMis as follows:

ILEDMAXis the maximum driving current of the light emitting diode LED1. PWM_code is the PWM code value. PWM_resolution is the number of bits of the PWM code. DC_resolution is the number of bits of the DC code. PWM Hightime is the duration of the PWM current at high level. In this embodiment, the DC code is 12 bits and PWM code is 4 bits. The period can be 16 time slots. The waveform shown inFIG.3can be obtained using the above formulas.

A 16-bit dimming code is considered to be high resolution (having more bits) in the field. If solely applying the analog dimming method to a 16-bit dimming code to adjust the brightness of the light emitting diode LED1, it can easily produce uneven color. Furthermore, the circuit structure of the 16-bit digital-to-analog converter (DAC) requires more transistors, and the circuit area of the DAC is too large to be effectively integrated into a small-size chip. On the other hand, if solely applying the PWM dimming method to a 16-bit dimming code to adjust the brightness of the light emitting diode LED1, it cannot effectively provide the human eyes with linear brightness perception. Although the duty cycle of the driving current can be adjusted linearly, the visual perception does not change linearly relative to brightness. The visual perception changes logarithmically. Therefore, by integrating analog dimming and PWM dimming in the embodiment, the two mechanisms can be complemented to achieve the best effect. The present invention includes, but is not limited to, 16 bits, and other numbers of bits shall fall within the scope of the present invention.

FIG.4is a diagram of a dimmer circuit200of an embodiment. The dimmer circuit200is for dimming according to a dimming code. The dimmer circuit200includes a light emitting module210for emitting light according to a driving current ILED, a first current source CS1, a digital-to-analog converter (DAC)220, a switch T1, a second current source CS2and a pulse width modulation (PWM) generator230. The light emitting module includes a first terminal for receiving a supply voltage Vs, and a second terminal. The first current source CS1includes a first terminal coupled to the second terminal of the light emitting module210, a second terminal coupled to a ground terminal GND, and a control terminal. The digital-to-analog converter220is coupled to the control terminal of the first current source CS1, for generating a direct current (DC) voltage VDCaccording to a DC dimming code signal DCcode to control the first current source CS1. The switch T1includes a first terminal coupled to the second terminal of the light emitting module210, a second terminal, and a control terminal. The second current source CS2includes a first terminal coupled to the second terminal of the switch T1and a second terminal coupled to the ground terminal GND. The PWM generator230is coupled to the control terminal of the switch T1, for generating a PWM voltage VPWMaccording to a PWM dimming code signal PWMcode to control the second current source CS2. The DC dimming code signal DCcode includes the higher bits of the dimming code, and the higher bits include the most significant bit (MSB) of the dimming code. The PWM dimming code signal PWMcode includes the lower bits of the dimming code, and the lower bits include the least significant bit (LSB) of the dimming code. The switch T1can be an N-type transistor or other equivalent components. The DC current IDCand the PWM current IPWMare summed up to form the driving current ILED, and the brightness of the light emitting diode LED1is determined by the driving current ILED.

The difference between the dimmer circuit200and the dimmer circuit100is that the dimmer circuit200includes headroom control mechanism. The light emitting module210may include a light emitting diode LED1and a headroom control transistor T2. The first terminal of the light emitting diode LED can receive the supply voltage Vs. The first terminal of the headroom control transistor T2is coupled to the second terminal of the light emitting diode LED. The second terminal is coupled to the first terminal of the first current source CS1, and the control terminal is for receiving a headroom control voltage Vhrc. The headroom control mechanism can control the voltage of the light emitting module210to a roughly fixed value or less than a threshold value to reduce power consumption. The headroom control voltage Vhrc needs to be dynamically adjusted according to the voltage at the second terminal of the light emitting module210to control forward voltage sliding of the light emitting diode LED1.

In other words, the headroom control voltage Vhrc is a feedback mechanism. When the PWM voltage VPWMvaries, the voltage jitter of the light emitting module210can be somewhat large. At this time, the dimmer circuit200can adjust the headroom control voltage Vhrc to cause the voltage of the light emitting module210maintained at a roughly fixed value. In other words, the headroom control transistor T2can be regarded as a variable resistor of a low dropout voltage regulator, and its resistance is adjustable by the headroom control voltage Vhrc to reduce the power consumption of the dimmer circuit200and prolong the lifespan of the light emitting diode LED1.

FIG.5is a diagram of a dimmer circuit400of an embodiment. The dimmer circuit400is for dimming according to a dimming code. The dimmer circuit400includes a light emitting module410for emitting light according to a driving current ILED, a first current source CS1, a digital-to-analog converter (DAC)420, a switch T1, a second current source CS2and a pulse width modulation (PWM) generator430. The light emitting module includes a first terminal for receiving a supply voltage Vs, and a second terminal. The first current source CS1includes a first terminal coupled to the second terminal of the light emitting module410, a second terminal coupled to a ground terminal GND, and a control terminal. The digital-to-analog converter420is coupled to the control terminal of the first current source CS1, for generating a direct current (DC) voltage VDCaccording to a DC dimming code signal DCcode to control the first current source CS1. The switch T1includes a first terminal coupled to the second terminal of the light emitting module410, a second terminal, and a control terminal. The second current source CS2includes a first terminal coupled to the second terminal of the switch T1and a second terminal coupled to the ground terminal GND. The PWM generator430is coupled to the control terminal of the switch T1, for generating a PWM voltage VPWMaccording to a PWM dimming code signal PWMcode to control the second current source CS2. The DC dimming code signal DCcode includes the higher bits of the dimming code, and the higher bits include the most significant bit (MSB) of the dimming code. The PWM dimming code signal PWMcode includes the lower bits of the dimming code, and the lower bits include the least significant bit (LSB) of the dimming code. In application, the light emitting module410may include a light emitting diode LED1. A first terminal of the light emitting diode LED1can receive the supply voltage Vs, and the second terminal can be coupled to the first terminal of the first current source CS1. The switch T1can be an N-type transistor or other equivalent components. The DC current IDCand the PWM current IPWMare summed up to form the driving current ILED, and the brightness of the light emitting diode LED1is determined by the driving current ILED.

The difference between the dimmer circuit400and the dimmer circuit100is that the PWM generator430may include a digital circuit440, such as a look-up table. The look-up table can be implemented as hardware, such as a read-only memory, application specific integrated circuit (ASIC) or other forms of digital circuits. The digital circuit440can be used to generate dithering. By adding dithering (i.e., spread spectrum clock generation) to the PWM voltage VPWM, the quantization error and the audible frequency interference can be reduced, thereby making the dimmer circuit400generating more accurate brightness. The digital circuit440can also apply the same technique (i.e., spread spectrum clock generation) to reduce the electromagnetic interference (EMI) generated by the PWM voltage VPWM.

The digital circuit440can also be used to generate phase-shift for the PWM voltage VPWM, so that the rising edges and falling edges of the PWM voltages VPWMof a plurality of dimmer circuits400can be staggered. For example, a 16-channel driving device has a plurality of dimmer circuits400integrated into an integrated circuit. If all PWM voltages VPWMof the plurality of dimmer circuits400rise or fall simultaneously, the circuit voltage would change rapidly, causing the circuit to exceed its maximum loading. Adding phase-shift to the PWM voltage VPWMcan avoid the above-mentioned situation, thereby making the dimmer circuit400to operate more robustly.

FIG.6is a diagram of the PWM voltage VPWMof the dimmer circuit400ofFIG.5. The upper portion ofFIG.6is the PWM voltage VPWMwithout dithering, and the lower portion is the PWM voltage VPWMwith dithering. For example, the digital circuit440can apply the spread spectrum clock generation (SSCG) technique to break up the frequency spectrum of the PWM voltage VPWM. As shown in the lower portion ofFIG.6, both period N and period N+1 have 3+2 waveforms. This type of waveform can reduce quantization errors and audible frequency interference. In addition, the digital circuit440can also produce other types of waveforms in the same principle to reduce the high-frequency electromagnetic interference (EMI) in the dimmer circuit400. Thus, applying the digital circuit440to add dithering can make the driving current ILEDmore robust, reducing the flickers of the light emitting diode LED1and producing more accurate brightness.

A supplemental description here is for explaining spread spectrum clock generation (SSCG). Spread spectrum clock generation technique is an application of frequency modulation. The basic principle of the spread spectrum clock generation is to slightly modulate the frequency of a signal, so that the energy of the signal is dispersed into a small controllable range. After the modulation, the peak energy of each harmonic in the spectrum would be attenuated. Therefore, the applying spread spectrum clock generation can effectively reduce the electromagnetic interference or audible frequency interference of the signal.

FIG.7is a diagram of a dimmer circuit500of an embodiment. The dimmer circuit500is for dimming according to a dimming code. The dimmer circuit500includes a light emitting module510for emitting light according to a driving current ILED, a first current source CS1, a digital-to-analog converter (DAC)520, a switch T1, a second current source CS2and a controller530. The light emitting module includes a first terminal for receiving a supply voltage Vs, and a second terminal. The first current source CS1includes a first terminal coupled to the second terminal of the light emitting module510, a second terminal coupled to a ground terminal GND, and a control terminal. The digital-to-analog converter520is coupled to the control terminal of the first current source CS1, for generating a direct current (DC) voltage VDCaccording to a DC dimming code signal DCcode to control the first current source CS1. The second current source CS2includes a first terminal coupled to the second terminal of the light emitting module510, a second terminal coupled to the ground terminal GND, and a control terminal. The controller530is coupled to the control terminal of the second current source CS2, for generating a control voltage Vc according to a PWM dimming code signal PWMcode to control the second current source. The DC dimming code signal DCcode includes the higher bits of the dimming code, and the higher bits include the most significant bit (MSB) of the dimming code. The PWM dimming code signal PWMcode includes the lower bits of the dimming code, and the lower bits include the least significant bit (LSB) of the dimming code. In application, the light emitting module510may include a light emitting diode LED1. A first terminal of the light emitting diode LED can receive the supply voltage Vs, and the second terminal can be coupled to the first terminal of the first current source CS1. The DC current IDCand the PWM current IPWMare summed up to form the driving current ILED, and the brightness of the light emitting diode LED1is determined by the driving current ILED.

The difference between the dimmer circuit500and the dimmer circuit100is that the second current source CS2of the dimmer circuit500is an adjustable current source, and the controller530controls the PWM current IPWMthrough the control voltage Vc. Therefore, the implementation of the dimmer circuit500can omit the switch T1. The controller530can also include a digital circuit, such as a look-up table or other equivalent circuit structure. The controller530can convert the PWM dimming code signal PWMcode into the control voltage Vc. Further, the controller530can also be used to generate phase-shift for the control voltage Vc, so that the rising edges and falling edges of control voltages Vc of a plurality of dimmer circuits500can be staggered. For example, a 16-channel driving device can have a plurality of dimmer circuits500integrated into an integrated circuit. If all PWM voltages VPWMof the plurality of dimmer circuits500rise or fall simultaneously, the circuit voltage would change rapidly, causing the circuit to exceed its maximum loading. Adding phase-shift to the control voltage Vc can avoid the above-mentioned situation, thereby making the dimmer circuit500to operate more robustly.

Furthermore, the controller530can generate the control voltage Vc with different amplitudes, and control the second current source CS2to output the PWM current IPWMwith different amplitudes and pulse widths, so as to generate the effect of dithering. By adding dithering to the control voltage Vc, the audible frequency interference can be reduced, thereby making the dimmer circuit500generating more accurate brightness. The controller530can also apply the same technique to reduce the electromagnetic interference (EMI) in the dimmer circuit500, so as to make the driving current ILEDmore stable, reducing flickering of the light emitting diode LED1.

In summary, the dimmer circuits of the above-mentioned various embodiments of the present invention can divide a high resolution dimming code into a DC code and a PWM code, which respectively include higher bits and lower bits of the dimming code. As described, this can simplify control variables, reduce voltage variation and reduce error rate. The layout size of the dimmer circuit can also be reduced, thus reducing the area occupied in an integrated circuit. The dimmer circuit of the embodiment can also implement a digital circuit to add dithering to suppress electromagnetic interference and other interferences, thereby improving the accuracy of brightness control.