This Application claims priority of China Patent Application No. 201110297992.5, filed on Sep. 27, 2011, the entirety of which is incorporated by reference herein.
1. Field of the Invention
The disclosure relates to a low dropout (LDO) regulator, and more particularly to a LDO regulator with high stability.
2. Description of the Related Art
Voltage regulators are commonly used in the power management systems of computers, mobile phones, automobiles and many other electronic products. Generally, voltage regulators are configured to convert unstable power supply voltage into stable power supply voltage. A low dropout (LDO) regulator has a low input-to-output voltage difference between an input terminal where an unstable power supply voltage is inputted and an output terminal where a stable power supply voltage is outputted. “Dropout voltage” refers to the input-to-output voltage difference, whereby the regulator ceases to regulate against further reductions in the input voltage. Ideally, the dropout voltage should be as low as possible, to reduce the power consumption while still maintaining regulation performance.
In the conventional LDO regulator design, a low frequency pole, at about 200 KHz to 500 KHz, is usually generated at the feedback terminal Because the low frequency pole falls within the operation frequency band of the LDO regulator, the stability of the LDO regulator is seriously downgraded. However, stability is an important factor of the LDO regulator.
Therefore, a novel LDO regulator, which can push the pole to a high frequency band while still maintaining high stability, is highly required.
Voltage regulators are provided. An embodiment of a voltage regulator comprises a pass transistor, an operational amplifier, and a voltage divider circuit. The pass transistor receives a supply voltage to generate a regulated output voltage according to a control signal. The operational amplifier generates the control signal according to a feedback voltage. The voltage divider circuit generates the feedback voltage at a feedback node according to the regulated output voltage. The voltage divider circuit comprises a string of resistors and a stabilization element. The string of resistors is coupled to the pass transistor and comprises a plurality of resistors. The stabilization element is coupled to the string of resistors and receives the regulated output voltage.
Another embodiment of a voltage regulator comprises a first transistor, an operational amplifier, and a voltage divider circuit. The first transistor receives a supply voltage to generate a regulated output voltage at an output node according to a control signal. The operational amplifier generates the control signal according to a difference between a reference voltage and a feedback voltage. The voltage divider circuit generates the feedback voltage at a feedback node according to the regulated output voltage. The voltage divider circuit comprises a string of resistors and a stabilization element. The string of resistors is coupled to the first transistor and comprises a plurality of resistors. The second transistor is coupled to the resistors and comprises a gate coupled to the output node.
Another embodiment of a voltage regulator comprises a pass transistor and a voltage divider circuit. The pass transistor receives a supply voltage to generate a regulated output voltage according to a control signal. The control signal is generated according to a feedback voltage. The voltage divider circuit generates the feedback voltage at a feedback node according to the regulated output voltage. The voltage divider circuit comprises a string of resistors and a stabilization element. The string of resistors is coupled to the pass transistor and comprising a plurality of resistors. The resistors and a plurality of parasitic capacitance generate a pole in a low frequency region at the feedback node. The stabilization element is coupled to the resistors and pushes the pole to a high frequency region.
A detailed description is given in the following embodiments with reference to the accompanying drawings.
The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.
According to one embodiment of the invention, the voltage divider circuit 103 comprises a string of resistors 131 and a stabilization element 132. The string of resistors 131 comprises a plurality of resistors (not labeled). The stabilization element 132 is coupled to the resistors and comprises a control node (not shown) receiving the regulated output voltage VOUT for stabilizing operations of the voltage regulator 100. To be more specific, the stabilization element 132 generates a high frequency pole, with a frequency much higher than the operation frequency band of the voltage regulator, at the feedback node FB. The high frequency pole is generated by the stabilization element 132 by pushing a pole, which would cause the system to operate unstably in a conventional voltage regulator, to a high frequency region, so as to maintain the stability of the voltage regulator 100.
The operational amplifier 202 comprises two input nodes 202a and 202b for respectively receiving the reference voltage VREF and the feedback voltage VFB, and generates the control signal Ctrl according to a difference between the reference voltage VREF and the feedback voltage VFB. The voltage divider circuit 203 comprises a string of resistors 203a and a stabilization element 203b. The string of resistors 230a at least comprises resistors R1 and R2. The resistor R1 is coupled between the pass transistor 201 and the stabilization element 203b, the stabilization element 203b is coupled between the resistor R1 and the feedback node FB, and the resistor R2 is coupled between the feedback node FB and a ground node 204.
According to an embodiment of the invention, the stabilization element 203b may include, e.g. the transistor 232 shown in
where the R1 represents the resistance of the resistor R1, the R2 represents the resistance of the resistor R2, the Cgs represents the capacitance of the capacitor Cgs, and the Cgd represents the capacitance of the capacitor Cgd.
Suppose that R1=R2=1MΩ and Cgs=Cgd=500 fF, the frequency of the pole as derived from Eq. 1 would be 300 KHz. Because an operation frequency band of a voltage regulator is generally distributed from 200 KHz to 500 KHz, the low frequency pole would seriously affect the stability of the voltage regulator 200 if there is no stabilization element 203b.
Therefore, in one embodiment of the invention, the transistor 232 is coupled at the feedback node FB so as to stabilize the operations of the voltage regulator 200. As previously described, because the transistor 232 operates in the linear region , the turn-on resistance rON of the transistor 232 is very small. Therefore, the transistor 232 may be regarded as a small resistor for direct current (DC) and barely affect the DC component in the regulated output voltage VOUT. In another perspective, regarding the alternative current (AC) component, the transistor 232 may further reduce the resistance at the feedback node FB when looking upward from the feedback node FB, thereby pushing the pole (that is, the above-mentioned low frequency pole), created by the parasitic capacitance Cgs and Cgd and at the feedback node FB, from the low frequency region to the high frequency region.
Based on the values derived above, the drain-source current of the transistor 232 may be:
where Vth is the threshold voltage of the transistor 232, μ is the charge carrier effective mobility, Cox is the unit capacitance of the gate oxide, W is the gate width of the transistor 232 and L is the gate length of the transistor 232.
The input impedance of the AC voltage Vi may further be derived from Eq. 2 as:
where rin is the input impedance of the transistor 232 when looking upward from the feedback node FB. Because the gain A and the transconductance gm are generally very large, the input impedance rin is very small as shown in Eq. 3, when the transistor 232 is coupled to the feedback node FB, the frequency of the low frequency pole created at the feedback node FB becomes:
As shown in Eq. 4, because the input impedance rin is very small, the low frequency pole created at the feedback node FB will be pushed to a high frequency region and becomes a high frequency pole. Since the frequency of the high frequency pole is much higher than the operation frequency band (as described above, usually in several KHz) of the voltage regulator 200, the high frequency pole will not affect the stability of the voltage regulator 200. In addition, because the circuit area required for a transistor is small, the increased circuit area due to the addition of the transistors, as the stabilization element 203b to the voltage regulator 200, is small. Note that in another embodiment of the invention, the stabilization element 203b may also comprise more than one transistor. By taking the programmable advantages of the transistors, the stability and the ability to resist process variation may further be improved. In addition, the gate of the transistor 232 may not have to be directly connected to the output node VOUT as shown in
While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. Those who are skilled in this technology can still make various alterations and modifications without departing from the scope and spirit of this invention. Therefore, the scope of the present invention shall be defined and protected by the following claims and their equivalents.
Number | Date | Country | Kind |
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201110297992.5 | Sep 2011 | CN | national |