BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a source driver and drive circuit thereof, and more particularly, to a source driver and drive circuit thereof capable of reducing impact of voltage coupling on channel current when a driving circuit is being turned on or off, to reduce channel current changes and luminance change.
2. Description of the Prior Art
In the art of light emitting diode (LED) driving, a passive matrix driving mode connects anodes (i.e. P-electrode) of light emitting diode pixels in each column of a matrix to each channel of light emitting diode source driver, while connecting cathodes (i.e. N-electrode) light emitting diode pixels in each row of the matrix to each scan line to a ground via each scan switch. When a specific column and a specific row are turned on, a light emitting diode pixel at the intersection emits light.
However, when a channel of the light emitting diode source driver is turned on, there are two coupling paths that affect other channels and thus affect brightness of light emitting diode pixels. In view of this, it is necessary to improve the conventional technology.
SUMMARY OF THE INVENTION
It is therefore an objective of the present invention to provide a source driver and drive circuit thereof capable of reducing impact of voltage coupling on channel current when a driving circuit is being turned on or off, to reduce channel current changes and luminance change.
The present invention discloses a source driver for driving a light emitting diode panel. The source driver includes a buffer including an output terminal; and a plurality of driving circuits coupled to the buffer. Each of the plurality of driving circuits includes a constant current transistor including a gate controlled by a node voltage of the output terminal of the buffer; and a compensation unit for compensating the node voltage of the output terminal of the buffer.
The present invention further discloses a driving circuit for driving a source driver of a light emitting diode (LED) panel. The driving circuit includes a constant current transistor, including a gate controlled by a node voltage of an output terminal of a buffer; and a compensation unit, for compensating the node voltage of the output terminal of the buffer.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various FIGS. and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a light emitting diode panel with passive matrix driving.
FIG. 2 is a schematic diagram of the source driver shown in FIG. 1.
FIG. 3 is a schematic diagram of operation of the source driver shown in FIG. 2.
FIG. 4 is a schematic diagram of another operation of the source driver shown in FIG. 2.
FIG. 5 is a schematic diagram of a source driver according to an embodiment of the present invention.
FIG. 6 is a schematic diagram of operation of the source driver shown in FIG. 5 according to an embodiment of the present invention.
FIG. 7 is a schematic diagram of another operation of the source driver shown in FIG. 5 according to an embodiment of the present invention.
FIG. 8 is a schematic diagram of another source driver according to an embodiment of the present invention.
FIG. 9 is a schematic diagram of operation of another drive circuit according to an embodiment of the present invention.
DETAILED DESCRIPTION
The term “comprising” as used throughout the specification and subsequent claims is an open-ended fashion and should be interpreted as “including but not limited to”. The descriptions of “first” and “second” mentioned in the entire specification and subsequent claims are only used to distinguish different components and do not limit the order of generation.
Please refer to FIG. 1, which is a schematic diagram of a light emitting diode (LED) panel 10 with passive matrix driving. As shown in FIG. 1, the light emitting diode panel 10 includes a source driver 102, a scanning circuit 104, channels C[1]-C[m], scanning lines S[1]-S[n], and scanning capacitors CS1-CSn, light emitting diode capacitors CLED 11-CLEDmn and corresponding light emitting diodes. The source driver 102 drives the channels C[1]-C[m], and the scanning circuit 104 connects the scanning lines S[1]-S[n] to a ground via corresponding switches. The scanning capacitors CS1-CSn are utilized that no voltage difference is formed across the light emitting diodes when the scanning lines S[1]-S[n] are not grounded. When the scanning circuit 104 connects specific scanning lines of the scanning lines S[1]-S[n] to the ground and the source driver 102 drives a specific channel of the channels C[1]-C[m], a light emitting diode at the intersection can be turned on.
For example, when the scanning circuit 104 connects the scan line S[1] to the ground and the source driver 102 drives the channel C[1], a voltage difference is formed across the light emitting diode capacitor CLED11 to conduct a corresponding light emitting diode. However, when the source driver 102 drives the channel C[1], a voltage change is coupled to other non-driven floating channel outputs via capacitive coupling paths on a light emitting diode board (e.g., coupled to the channel C[2] via nodes (1)>(2)>(3)>(4)>(5)). Meanwhile, the scan line S[1] is grounded, so that voltages across the light emitting diode capacitors CLED21-CLEDm1 are affected and thus conduction currents of the light emitting diodes are also affected. As a result, if more channels are being turned on at the same time, capacitive coupling on the LED board is stronger, resulting in greater impact on cross voltages, greater current change and greater brightness change.
On the other hand, please refer to FIG. 2, which is a schematic diagram of the source driver 102 shown in FIG. 1. As shown in FIG. 2, a node voltage VB of an output terminal of a buffer 200 controls the driving circuits DC[1]-DC[m] to drive the channels C[1]-C[m]. For example, in the driving circuit DC[1], a pulse width modulation (PWM) circuit 202 controls a pulse width modulation transistor MPWM to be turned on or off according to a pulse width modulation signal, to turn on or turn off the channel C[1]. Luminous brightness is determined by a pulse width of the pulse width modulation signal and a stable channel current provided by a constant current transistor MPS (i.e. a constant current source). When the pulse width modulation transistor MPWM is turned off, a node voltage VN[1] is instantaneously raised to a system voltage VDD, or when the pulse width modulation transistor MPWM is turned on, an amplifier 204 performs negative feedback and thus the node voltage VN[1] is instantaneously pulled down to a reference voltage VREF. Thus, the node voltage VB for a gate of the constant current transistor MPS is affected by voltage change of the node voltage VN[1] via a parasitic capacitor CGD. In this case, since channel currents will be affected by operations of other channels, if more channels operate at the same time, voltage coupling is stronger, resulting in greater impact on the constant current source, greater current change and greater brightness change.
For example, please refer to FIG. 3, which is a schematic diagram of operation of the source driver 102 shown in FIG. 2. As shown in the upper half of FIG. 3, take the scan line S[1] conducted as an example, when only the channel C[1] is being turned on, the pulse width modulation transistor MPWM is being turned on and the node voltage VN[1] is pulled down to the reference voltage VREF, the node voltage VB is slightly coupled downward and an instantaneous current of a channel current IC[1] only slightly increases. On the other hand, as shown in the lower half of FIG. 3, when the channels C[1]-C[m] are all being turned on at the same time, the node voltage VB is significantly coupled downward and instantaneous currents of channel currents IC[1]-IC[m] are increased substantially. In addition to the capacitive coupling paths on the board as shown in FIG. 1, current will significantly increase, so that brightness also changes significantly.
On the other hand, please refer to FIG. 4, which is schematic diagram of another operation of the source driver 102 shown in FIG. 2. As shown in the upper half of FIG. 4, take the scan line S[1] conducted as an example, when the channels C[1]-C[m-1] remain turned on, the channel C[m] is being turned off, the corresponding pulse width modulation transistor MPWM is being turned off and the node voltage VN[m] is raised to the system voltage VDD, the node voltage VB is slightly coupled upward and instantaneous currents of the channel currents IC[1]-IC[m-1] are reduced slightly. On the other hand, as shown in the lower half of FIG. 4, when only the channel C[1] remains turned on and the channels C[2]-C[m] are being turned off, the node voltage VB is significantly coupled upward, instantaneous current of the channel current IC[1] is reduced greatly, so that brightness also changes significantly.
In contrast, please refer to FIG. 5, which is a schematic diagram of a source driver 502 according to an embodiment of the present invention. The source driver 502 may replace the source driver 102 shown in FIG. 2 to implement the source driver 102 shown in FIG. 1. The source driver 502 is similar to the source driver 102, so components with similar functions and structures are denoted by the same symbols. The main difference between the source driver 502 and the source driver 102 shown in FIG. 2 is that each of the driver circuits DC[1]-DC[m] in the source driver 502 further includes a compensation unit 504 for compensating the node voltage VB of the output terminal of the buffer 200 when at least one driving circuit is being turned on or off. Specifically, when at least one driving circuit is being turned on, the compensation unit 504 raises the node voltage VB of the output terminal of the buffer 200, and when at least one driving circuit is being turned off, the compensation unit 504 reduces the node voltage VB of the output terminal of the buffer 200. For example, the compensation unit 504 in the driving circuit DC[1] may include compensation circuits IP[1] and IN[1] for raising and reducing the node voltage VB, respectively, wherein the compensation circuits IP[1] and IN[1] are illustrated as current sources in FIG. 5. In addition, the compensation unit 504 may compensate the node voltage VB of the gate of a corresponding constant current transistor MPS when a corresponding drive circuit is being turned on or off, but the compensation unit 504 may compensate the node voltage VB of the gate of a corresponding constant current transistor MPS when other drive circuits are being turned on or off.
In detail, please refer to FIG. 6, which is a schematic diagram of operation of the source driver 502 shown in FIG. 5 according to an embodiment of the present invention. As shown in FIG. 6, when the channels C[1]-C[m] are all being turned on at the same time (that is, at least one drive circuit and at least one corresponding channel are being turned on), the compensation circuits IP[1]-IP[m] outputs currents in synchronization with outputs of at least one channels, wherein output time lengths of the compensation circuit IP[1]-IP[m] may be adjusted to raise the node voltage VB to compensate for the voltage drop due to coupling of the node voltages VN[1]-VN[m] (compared with the lower half of FIG. 3, the embodiment of the present invention may reduce variation of the node voltage VB from a dashed line to a solid line, so as to reduce variation of the channel currents IC [1]-IC[m] from a dashed line to a solid line). It should be noted that if more channels are being turned on at the same time, capacitive coupling on the LED board is stronger, but the compensation circuits IP[1]-IP[m] corresponding to more channels compensate for the node voltage VB more, so as to compensate cooperatively to reduce channel current changes and brightness changes.
On the other hand, please refer to FIG. 7, which is a schematic diagram of another operation of the source driver 502 shown in FIG. 5 according to an embodiment of the present invention. As shown in FIG. 7, when the channel C[1] remains turned on and the channels C[2]-C[m] are being turned off (that is, at least one drive circuit and at least one corresponding channel are being turned off),
the compensation circuits IN[2]-IN[m] output currents after the channels are being turned off, wherein output time lengths of
the compensation circuits IN[2]-IN[m] may be adjusted to reduce the node voltage VB to compensate for the voltage raise due to coupling of the node voltages VN[2]-VN[m] (compared with the lower half of FIG. 4, the embodiment of the present invention may reduce variation of the node voltage VB from a dashed line to a solid line, so as to reduce variation of the channel current IC[1] from a dashed line to a solid line). It should be noted that if more channels are being turned off at the same time, capacitive coupling on the LED board is stronger, but the compensation circuits IN[2]-IN[m] corresponding to more channels compensate for the node voltage VB more, so as to compensate cooperatively to reduce channel current changes and brightness changes.
It is worth noting that the embodiment of the present invention compensates the node voltage VB of the output terminal of the buffer 200 to reduce the influence of voltage coupling on the channel current when at least one driving circuit is being turned on or off, so as to reduce the channel current change and the brightness change. Those skilled in the art may make modifications or alterations accordingly, but not limited to this. For example, FIG. 5 illustrates the compensation circuits IP[1] and IN[1] as current sources, but the compensation circuits IP[1] and IN[1] may be realized by comprising one of a metal oxide semiconductor field effect transistor, a diode, a source follower, an operational amplifier or a current source.
On the other hand, the compensation unit may further include other components. For example, please refer to FIG. 8, which is a schematic diagram of operations of a source driver 802 according to an embodiment of the present invention. The source driver 802 may replace the source driver 102 shown in FIG. 2 to implement the source driver 102 shown in FIG. 1. The source driver 802 is similar to the source driver 502, so components with similar functions and structures are denoted by the same symbols. The main difference between the source driver 802 and the source driver 502 is that a compensation unit 804 included in each of the driving circuits DC[1]-DC[m] in the source driver 802 further includes a resistor RVB coupled between the output terminal of the buffer 200 and the gate of the constant current transistor MPS, to greatly reduce coupling from the node voltage VN to the node voltage VB (the resistor RVB may function as a resistance capacitor filter (RC filter) to isolate the coupling from the node voltage VN to the node voltage VB), thereby reducing the influence of voltage coupling on other channels when the channel is being turned on or off. In this case, compared with the lower half of FIG. 3, as shown in the upper right of FIG. 8, the embodiment of the present invention may reduce variation of the node voltage VB from a dashed line to a solid line (smaller variation than the solid line in FIG. 6), to reduce variation of the channel current IC[1]-IC[m] from a dashed line to a solid line (smaller variation than the solid line in FIG. 6), so as to reduce brightness change. In addition, compared with the lower half of FIG. 4, as shown in the lower right of FIG. 8, the embodiment of the present invention may reduce variation of the node voltage VB from a dashed line to a solid line (smaller variation than the solid line in FIG. 7), to reduce variation of the channel current IC[1] from a dashed line to a solid line (smaller variation than the solid line in FIG. 7), so as to reduce brightness change.
On the other hand, the pulse width modulation circuit 202 may be implemented in any form. For example, please refer to FIG. 9, which is a schematic diagram of operations of a driving circuit DC according to an embodiment of the present invention. The driving circuit DC may be any one of the driving circuits DC[1]-DC[m] shown in FIG. 8. As shown in FIG. 9, the pulse width modulation circuit 202 controls a pulse width modulation transistor MPWM to be turned on or off according to a pulse width modulation signal PWM. A pulse width modulation circuit 202 receives the pulse width modulation signal PWM and generates an inverse signal via an inverter. A switch is coupled between a voltage VDD and the gate of the pulse width modulation transistor MPWM, and is controlled by the inverted signal, to control the gate of the pulse width modulation transistor MPWM to be at a high level (e.g. the system voltage VDD) and turned off when the pulse width modulation signal PWM is at a low level. The pulse width modulation circuit 202 further includes another switch coupled between an output terminal of the amplifier 204 and the gate of the pulse width modulation transistor MPWM, and is controlled by the pulse width modulation signal PWM, to form a negative feedback loop to lock a source voltage (i.e. a node voltage VN) of the pulse width modulation transistor MPWM at a reference voltage VREF when the pulse width modulation signal PWM is at a high level. When the pulse width modulation signal PWM is switched from a low level to a high level, a control signal PR controls a compensation circuit IP (to provide current) to raise the node voltage VB for compensation. When the pulse width modulation signal PWM is switched from the high level to the low level, a control signal PF controls a compensation circuit IN (to drain current) to reduce the node voltage VB for compensation. Other operations may be derived by referring to the above description, which will not be narrated here for brevity.
In summary, the present invention compensates the node voltage of the output terminal of the buffer when at least one driving circuit is being turned on or off, so as to reduce the influence of voltage coupling on the channel current, thereby reducing the channel current change and brightness change.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.
Patent Grant
11217152
