Patent Application
20120062132
This patent application is based on Taiwan, R.O.C. patent application No. 099130937 filed on Sep. 13, 2010.
The present invention relates to a regulated voltage protection circuit, and more particularly, to a regulated voltage protection circuit of a light-emitting diode (LED) driving module.
Since an LED has numerous advantages, e.g., small size, short reaction time, low power consumption, high reliability, and high feasibility of mass production, the LED is widely applied to electronic apparatuses as light sources. For example, an LED serves as a backlight source of a liquid crystal display (LCD) to replace a conventional fluorescent tube.
For the LED strings 10 shown in
However, a circuit manufactured via the high-voltage fabrication process has a larger area than that circuit manufactured via a low-voltage fabrication process, and requires higher cost during manufacture. In addition, since the high-voltage fabrication process is incompatible with the low-voltage fabrication process, the driving circuit 12 is uneasily integrated with other system circuits of an LCD. Therefore, when the driving chip manufactured via the low-voltage fabrication process is applied to an LED string of a high-voltage source, damage created in the driving circuit due to EOS needs to be prevented.
Therefore, a new regulated voltage protection mechanism is needed to protect the driving circuit 12 manufactured via the low-voltage fabrication process so as to prevent internal circuit components of the driving circuit 12 from being undesirably affected by EOS caused by a voltage from the LED strings.
In view of the foregoing issues, according to the present invention, a regulated voltage protection circuit, applied to a LED driving module, in a driving circuit or outside a circuit chip, controls a voltage of an input pad to avoid EOS. Accordingly, the driving circuit is manufactured using a low-voltage manufacture process to integrate with other system circuits, to reduce circuit area and cost of the overall system in addition to increasing efficiency.
According to an embodiment of the present invention, the regulated voltage protection circuit provides regulated voltage protection to a driving module, which is coupled to a plurality of LED strings, and comprises a bias-voltage generating circuit and a clamping circuit. The bias-voltage generating circuit provides a bias voltage. The clamping circuit is simultaneously coupled to the plurality of LED strings and the driving module. According to the bias voltage, the clamping circuit generates a plurality of clamping voltages that are respectively transmitted to a plurality of input pads of a driving circuit.
According to another embodiment of the present invention, a display controller comprises an LED driving module comprising a plurality of LED strings, a driving circuit, a bias-voltage generating circuit, and a clamping circuit. Each of the LED strings comprises a plurality of LEDs in serial, and has one end coupled to a voltage source. The driving voltage drives the plurality of LED strings. The bias-voltage generating circuit provides a bias voltage. The clamping circuit is coupled to the plurality of LED strings and the driving circuit, and the clamping circuit generates a plurality of clamping voltages to a plurality of input pads of the driving circuit according to the bias voltage.
According to yet another embodiment of the present invention, a method for driving an LED comprises generating a bias voltage; clamping via the bias voltage a plurality of voltages from a plurality of LED strings to generate a plurality of clamped voltages; and transmitting the clamped voltages to an LED driving circuit.
In this embodiment, the regulated voltage protection circuit mainly comprises a bias-voltage generating circuit 20 and a clamping circuit 22. In Step 31, the bias-voltage generating circuit 20 provides a bias voltage Vbias to the clamping circuit 22, which is coupled to a plurality of LED strings 10. Each LED string includes a plurality of LEDs 100 connected in series. The outermost LED 100 of each LED string 10 has an anode coupled to a high-voltage source VDC, and a cathode coupled to the clamping circuit 22. The clamping circuit 22 is further coupled to a plurality of input pads 26 of a driving circuit 24. In Step 32, the clamping circuit 22 clamps voltages at the cathodes of the outermost LEDs of the LED strings 100 to generate a plurality of clamping voltages. In Step 33, the clamping voltages are used as feedback to the input pads 26 of the LED driving circuit 24, so that a voltage at each input pad 26 is kept below a predetermined voltage or an arbitrary rated voltage, so that the driving circuit 24 is protected from being undesirably affected or damaged by electrical overstress (EOS). The predetermined (or rated) voltage is dependent on the manufacturing process of the driving circuit 24. For example, when a 5 Volt (V) manufacture process is implemented, the predetermined voltage is accordingly determined as 5V. In this embodiment, the driving circuit 24 comprises a plurality of current sources respectively controlling luminance of the LED strings 10. The driving circuit 24 is a semiconductor integrated circuit (IC) of a low-voltage manufacture process and is integrated with other system circuits of an LCD to form a system-on-chip (SOC), e.g., a display control chip.
In this embodiment, the bias-voltage generating circuit 20 mainly comprises an NMOS bias-voltage transistor Ma and a regulated voltage circuit 200. The bias-voltage transistor Ma has a gate G coupled to the gates G of the clamping transistors M0 to Mn of the clamping circuit 22, a drain D electrically coupled to a voltage source V, and a source S coupled to ground via voltage-divider resistors R2 and R3. It is to be noted that, in this embodiment, the bias-voltage transistor Ma and the clamping transistors M0 to Mn share the same manufacture process, which means the bias-voltage transistor Ma and the clamping transistors M0 to Mn have the same threshold voltage. Accordingly, source voltages of the clamping transistors M0 to Mn are regulated by controlling a gate voltage and a source voltage of the bias-voltage transistor Ma. The regulated voltage circuit 200 is a programmable shunt regulator, e.g., a programmable shunt regulator TL431, which has three terminals—an anode A, a cathode K, and a reference voltage terminal VREF. More specifically, the anode A is coupled to ground, the cathode K is coupled to both the gate G of the bias-voltage transistor Ma and the voltage source V via a current limiting resistor R1, and the reference voltage terminal VREF is coupled to a central node of the voltage dividing resistors R2 and R3. The voltage at the gate G of the bias-voltage transistor Ma is adjusted to a desired voltage by matching the voltage source with the resistors.
Through the foregoing circuit connection configurations, a regulated voltage is generated between the gate G and the source S of the bias-voltage transistor Ma. For example, according to the design of the regulated voltage circuit, in the event that the voltage at the source S of the bias-voltage transistor Ma is 5V, the voltage at the gate G is (5+Vth)V, wherein Vth is the threshold voltage of the bias-voltage transistor Ma. Generally, the regulated voltage circuit 200 adjusts the voltage at the source S of the bias-voltage transistor Ma to be identical to a predetermined voltage of the input pads 26, and adjusts the voltage at the gate G to be equal to sum of the predetermined voltage and the threshold voltage Vth.
As mentioned above, the bias-voltage transistor Ma and the clamping transistor M0 to Mn share the same manufacture process, so that each of the clamping transistors M0, M1, and Mn has a bias voltage identical to that of the bias-voltage transistor Ma. For example, when the voltage at the gate G of the clamping transistor M0/M1/Mn is (5+Vth)V (i.e., Vth is its threshold voltage), the voltage at the source S maintains as 5V. Therefore, the configuration of the embodiment of the present invention avoids electrical overstress (EOS) of the driving circuit 24 because each voltage of the input pads 26 does not exceed the predetermined voltage (e.g., the foregoing 5V). For example, when one or several LEDs 100 of the LED strings 10 short-circuit due to component failure, the drain-source D-S voltage of the corresponding clamping transistor M0/M1/Mn is increased; however, the voltage at the source S of the clamping transistor M0/M1/Mn maintains as the predetermined voltage, protecting the driving circuit from EOS.
While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not to be limited to the above embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.
| Number | Date | Country | Kind |
|---|---|---|---|
| 099130937 | Sep 2010 | TW | national |
