Device for voltage-noise rejection and fast start-up

A device for voltage-noise rejection and fast start-up is provided. It comprises a low-pass filter connected to a voltage source, a voltage-controlled switch connected in parallel with the low-pass filter, and an auxiliary start-up element connected to a DC-only voltage output. By using a transistor operating in the triode region and a capacitor with suitable capacitance, it is suitable for integration to form a low-frequency low-pass pole to suppress the noise in the reference current. The auxiliary start-up element overcomes the large turn on time caused by the low frequency low-pass pole. As there is no static current during normal operation, the power consumption for the device is low.

FIELD OF THE INVENTION

The present invention generally relates to a device for noise rejection in a reference voltage source, and more specifically to a device with a start-up mechanism to compensate the delay in start-up caused by a noise rejection filter.

BACKGROUND OF THE INVENTION

As the deep sub-micron technology improves, the device geometry has been greatly reduced, which leads to the reduction of the operating voltage, as well as the signal amplitude of the circuit. Under such circumstances, noise is becoming more prominent than before. To maintain the signal-to-noise ratio (SNR) under an acceptable level to keep the circuit functions, various solutions have been proposed to reduce the noise appearing in the circuit. This is especially true for the noise appearing in the reference voltage sources.

The conventional techniques to suppress the reference noise are to use a large capacitor and a large resistor on the signal path to form a low-pass pole for filtering the noise. However, in the IC design, a large capacitor and a large resister will occupy a large chip area, which is not economically viable. Therefore, these approaches are rarely used in practice, except in a limited application where integration and monolithic design takes less priority than the needs of the reference noise suppression. Even in those applications, there exists additional problem, such as the large circuit switching on/off delay.

Hakkinen, Rahkonene, and Kostamovaara proposed in the article “An integrated programmable Low-Noise Charge Pump” (Proceedings of ICECS, 1999) a design of an integrated programmable charge pump based on an op-amp current mirror, as shown inFIG. 1. The design is applicable to the charge pump of the lock-phase loop, which has strict requirements in terms of noise level. The prior art uses an op-amp101to construct a feedback circuit, and, based on the negative feedback mechanism, the noise of the current source IDis suppressed by the loop gain provided by the op-amp101. However, the prior art does not suppress the noise in the reference source Vref. The reference noise is converted by a resistor103and injected into the circuit.

U.S. Patent Publication 2003/0169872 disclosed a voltage reference filter for subscriber line interface circuit, as shown inFIG. 2. As shown therein, the direct current (DC) component of the reference signal is first filtered by a high-pass RC filter (capacitor201and resistor202). The alternating current (AC) component passes the amplifiers203,204, and then a subtracting circuit205is used to subtract the AC component from the reference signal to obtain the DC component of the reference signal. However, for the high-pass filter to be effective, the RC value must be sufficiently large, which may even require the use of external elements. In addition, this prior art employs multiple amplifiers and hence consumes a considerable amount of static current. Therefore, although this prior art is applicable to the subscriber line, it is not generally applicable to other systems that demand low manufacturing cost, small device geometry and high integration.

FIG. 3shows a conventional current mirror. A current mirror is a circuit to copy a current flowing through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of the loading. The current being copied can be a varying signal current. Current mirrors also allow current signals to have a fanout greater than one and each output can be scaled using appropriate W/L ratios. Another important function of the current mirror is to reverse the current direction. As shown inFIG. 3, a reference current IREFis mirrored to transistor M2through transistor M1, then mirrored to transistor M4through transistor M3, and finally flows to the circuits requiring the current.

Reference current IREFis a fixed current generated by another circuit block, which, in general, is a bandgap reference voltage generator. As shown inFIG. 3, the noise in reference current IREFwill flow along with the DC component to be mirrored from M1to M4, and enters the operating circuit through M4. Along the current flow, more noise current from M1, M2, and M3will be added. All the noise currents, if not suppressed, will greatly affect the noise characteristic of the operating circuit.

To suppress these noises, a conventional method is to place a large capacitor at the gate of M4, as the capacitor C1shown inFIG. 3. Capacitor C1and transistor M3form a low-pass pole to perform a first-order filtering for the noise. The location of the pole is gm3/C1, where gm3is the conductance of transistor M3. For example, if gm3is 0.9 mA/V, a capacitor C1of 10 pF integrated into an IC can create a 14 MHz pole. However, to filter noise of even lower frequency, the required capacitance will be even larger, which makes the integration even harder.

Another technique is to place a resistor with large resistance between M3and C1. Similarly, the resistor and C1form a low-pass pole to perform a first-order filtering for the noise. Although the integrated resistor occupies a smaller area, it still requires a rather large resistance and large capacitance to generate a pole of sufficiently low frequency. Therefore, the overall integrated area is still considerably large.

FIG. 4shows a simplified block diagram of the operation of a filter used in conjunction with a voltage source. Vinis a source consisting of a DC component and high-frequency noise component. Assume that a very low frequency pole is introduced such that the low-pass filter filters out the high-frequency noise component while allowing the DC component to pass; hence the Voutconsists of only the DC component. However, the changes in Vout, for example, on and off, cannot react as immediately as the changes in Vinbecause of the large time constant introduced by the filter.

SUMMARY OF THE INVENTION

The present invention has been made to overcome the above-mentioned drawback of conventional techniques to reduce the reference noise. The primary object of the present invention is to provide a device for noise rejection in the reference current to reduce the impact of the noise to the SNR.

Another object of the present invention is to provide a reference noise rejection device that uses a fast start-up mechanism to overcome the delay problem caused by the low frequency low-pass pole.

Yet another object of the present invention is to provide a reference noise rejection device that is small in chip area and without static current consumption.

To achieve the above objects, the present invention provides a device for voltage-noise rejection and fast start-up, which comprises a low-pass filter connected to a voltage source, a voltage-controlled switch connected in parallel with the low-pass filter, and an auxiliary start-up element connected to a DC-only voltage output. By using a transistor operating in the triode region with large resistance and a capacitor with mediate capacitance. It is possible to integrate these devices and to form a low-frequency low-pass pole for suppressing the noise in the reference current. The auxiliary start-up element overcomes the switch delay caused by the low frequency low-pass pole.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 5shows a schematic view of a block diagram of the present invention. As shown inFIG. 5, a device for voltage-noise rejection and fast start-up of the present invention includes a low-pass filter510, a voltage-controlled switch520, and an auxiliary start-up element530. The low-pass filter510is connected to a voltage source Vinfor filtering out the high-frequency noise to produce a DC-only Vout, so that the voltage at node B has the same DC voltage at node A but without the noise. The voltage-controlled switch520is connected in parallel with low-pass filter510at A and B for bypassing low-pass filter510when the circuit starts up. Once the circuit is fully turned on, voltage-controlled switch520will be open to allow low-pass filter510to operate. The auxiliary star-up element530is connected to B and outputs a control voltage to voltage-controlled switch520.

FIG.6shows a schematic view of a block diagram of the auxiliary start-up element ofFIG. 5. As shown inFIG. 6, the auxiliary start-up element530further includes a voltage-controlled current source531, a load532, and a threshold voltage detector533. The voltage-controlled current source531is connected to B to convert the voltage to a current. The load532is connected to the output of voltage-controlled current source531and a ground end666to convert the current from voltage-controlled current source531to a voltage to provide to the threshold voltage detector533. The threshold voltage detector533detects the voltage of the load532and sends out a voltage to open the voltage-controlled switch520when the voltage on load532exceeds the threshold.

The auxiliary start-up element530can further include an auxiliary voltage-controlled switch534, as shown inFIG. 7, which shows another example of a block diagram of the auxiliary start-up element ofFIG. 5. The purpose of the auxiliary voltage-controlled switch534is to switch open the load532when the circuit is fully turned on so that the voltage-controlled current source531will be turned off to save the current consumption.

The operation of the auxiliary start-up element530is as follows. When the voltage at B reaches the steady state, the current output from voltage-controlled current source531is I. With load532having the resistance R, the voltage on load532will be I*R when the voltage at B reaches the steady state. Therefore, the threshold voltage detector533must be designed to send out a control voltage to switch open the voltage-controlled switch520when the voltage from load532is equal to I*R.

FIG. 8shows an equivalent circuit of an embodiment of the present invention. As shown inFIG. 8, a transistor M5and a capacitor C1form low-pass filter510ofFIG. 5. Similarly, transistor M6forms the voltage-controlled switch520ofFIG. 5, and the circuit of M7-M10forms the auxiliary start-up element530ofFIG. 5.

In comparison with the circuit shown inFIG. 3, in the circuit shown inFIG. 8, a transistor M5operating in the triode region is used to act as a large resistor. Transistor M5can be designed to have a small aspect ratio, W/L, such that it has a large turn-on resistance. For example, in the 0.18 um process, a transistor with W/L=0.25 um/10 um can have a resistance of 1.1 MΩ. In conjunction with a 10 pF capacitor C1, a low-pass pole of 16 kHz can be obtained. Furthermore, a plurality of transistors can be used to increase the resistance if a single transistor is insufficient.

The operation of transistors M6, M7-M10is described as follows. When the circuit is turned off, S1is opened, and S2B-S4Bare closed; therefore, M6becomes conductive with a small resistance and M7is non-conductive. When the circuit receives the start-up signal, S1is closed, and S2B-S4Bare opened. Because M6has a small resistance, the low-pass pole that C1forms is moved to a higher frequency, and the gate voltages on M4and M7will change rapidly as the gate voltage changes on M3. Hence, the current of M4and M7will be turned on immediately. When the current of M7increases, the drain voltage of M7also increases. When M7is close to fully turned on, the drain voltage reaches the threshold to drive INV1and INV2. The gate voltage of M6changes from low to high, and M6becomes open-circuit. The low-pass pole returns from high frequency to low frequency for noise rejection. At this point, the circuit returns to normal operation.

It is worth noticing that the gate of M6and the gate M8are connected. That is when M6is open-circuited, so is M8. In this case, there will be no current flowing through M7. Therefore, when the circuit is fully turned on, there will be no static current flowing through M7. Therefore, transistor M7only consumes current during the start up process.

In addition, M6is open-circuited only when there is sufficient current on M7. A certain delay can be added to the feedback control loop of M6to guarantee that M6will not be turned off before the start-up is over. Therefore, a small capacitor C2is added to the drain of M7to increase the charge time of M7drain in order to prolong the conductive time of M6.

With the present invention, the noise in the reference current can be rejected and the delay in start-up can be avoided. In addition, the present invention is mostly implemented with transistors, which is suitable for integration. As there is no static current during normal operation, the power consumption is low.