Patent Publication Number: US-6211661-B1

Title: Tunable constant current source with temperature and power supply compensation

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates generally to integrated circuits and, more particularly, to a tunable constant current source with temperature and power supply compensation. 
     2. Discussion of the Related Art 
     The generation of a constant current or constant voltage is one of the most critical portions of any analog integrated circuit. Upon generating the constant current or constant voltage source, all other analog circuits are biased off this constant source within the integrated circuit itself. Historically, a constant current source has been supplied by use of a band-gap reference network that is formed by a diode and resistor to generate a constant current source. 
     While a band-gap reference network works well at conventional supply voltages, such as 5 volts (VCC), as power supply voltages decrease, the band-gap implementation becomes ineffective to inoperative. This is due to the diode drop which is between about 0.7 to 0.8 volts which does not scale and leaves less headroom for current source cascading. In most instances, a band-gap reference network will work well down to a supply voltage of about 2.5 volts and will operate with significant jitter down to about 1.8 volts. However, with new lower voltage power supplies operating in a range of about 1.5 volts, the band-gap reference network is not a viable option. Additionally, the band-gap reference network has undesirable power supply tracking characteristics. These characteristics cause excessive error with respect to power supply noise (delta in output due to delta in power supply). 
     Since constant current sources are generally used for phase lock loops (PLL) which are generally used to generate clocks for microprocessors, the above disadvantages have drastic effects on microprocessor operation. Moreover, since about 95% of all microprocessors use phase lock loops for generating clock pulses, this effects a broad range of microprocessors operating today. Thus, low power supply microprocessors may not use band-gap reference networks to provide constant current sources. In addition, because of the high sensitivity by band-gap reference networks to power supply changes, this creates jitter in the phase lock loop which adds directly to the cycle time of the processor, thereby reducing the operating speed and efficiency of the microprocessor. 
     What is needed then is a tunable constant current source with temperature and power supply compensation that does not suffer from the above-mentioned disadvantages. This will, in turn, provide a current source that has a very low sensitivity to power supply changes, provide a low clock skew and jitter in a phase lock loop, thereby reducing cycle time so that the microprocessor may run faster, provide operating supply voltage ranges between about 0.5 volts to about 1.8 volts, provide a constant current within an integrated circuit that can be used for any analog application, such as a phase lock loop which is used for processor clock generation, and provide a constant current source that compensates for temperature variations efficiently. It is, therefore, an object of the present invention to provide such a tunable constant current source with temperature and power supply compensation in an integrated circuit. 
     SUMMARY OF THE INVENTION 
     This invention is directed to a constant current source with temperature and power supply compensation which is formed within an integrated circuit. The tunable constant current source basically includes a voltage regulator, a differential amplifier, a current source and a compensating load which all operate to provide a tunable constant current source at low power supply voltage levels. 
     In one preferred embodiment, a tunable constant current source having temperature and power supply compensation includes a voltage regulator, a differential amplifier, a current source and a compensating load. The voltage regulator provides a substantially constant bias voltage V B . The different amplifier receives the bias voltage V B  and maintains a load voltage V L  substantially equal to the bias voltage V B  by way of a negative feedback. The current source generates a substantially constant current IREF from the differential amplifier. The compensating load varies with temperature changes to maintain the current IREF substantially constant, whereby the tunable constant current source may operate in a supply voltage range between about 0.5 volts to about 1.8 volts. 
     In another preferred embodiment, a voltage regulator for use in a constant current source includes a negative feedback circuit, a gross temperature compensation circuit, a bias circuit and a temperature compensation circuit. The negative feedback circuit generates a substantially constant bias voltage V B . The gross temperature compensation circuit forms a portion of the negative feedback circuit and has a plurality of decodable branches with one of the plurality of branches selected to operate with the negative feedback circuit based upon an operating temperature of a circuit that employs the voltage regulator. The bias circuit operates to bias the negative feedback circuit with a reference voltage REFP. The temperature compensation circuit varies the reference voltage REFP during slight variations in the operating temperature of the circuit. 
     Use of the present invention provides a tunable constant current source with temperature and power supply compensation. As a result, the aforementioned disadvantages associated with the current band-gap reference network for use in providing a constant current source have been substantially reduced or eliminated. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Still other advantages of the present invention will become apparent to those skilled in the art after reading the following specification and by reference to the drawings in which: 
     FIG. 1 illustrates a block diagram of a tunable constant current source according to the teachings of the preferred embodiment of the present invention; 
     FIG. 2 illustrates a schematic diagram of a voltage regulator of FIG. 1 according to the teachings of the preferred embodiment of the present invention; 
     FIG. 3 illustrates a schematic diagram of a compensating load of FIG. 1 according to the teachings of the preferred embodiment of the present invention; 
     FIG. 4 illustrates a schematic diagram of a differential amplifier of FIG. 1 according to the teachings of the preferred embodiment of the present invention; and 
     FIG. 5 is a graph illustrating the constant current response versus power supply voltage of the tunable constant current source according to the teachings of the preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description of the preferred embodiment concerning a tunable constant current source with temperature and power supply compensation is merely exemplary in nature and is not intended to limit the invention or its application or uses. Moreover, while the present invention is described in detail below with reference to use with particular circuit configurations, such as a phase lock loop (PLL), it will be appreciated by those skilled in the art that the present invention is clearly not limited to use of a tunable constant current source in only a phase lock loop which is merely for exemplary purposes. 
     Referring to FIG. 1, a tunable constant current source  10  having temperature and power supply compensation according to the teachings of the preferred embodiment of the present invention is shown. The tunable constant current source  10  includes a voltage regulator  12 , a differential amplifier  14 , a current source  16  and a compensating load  18 . The voltage regulator  12  generates a constant bias voltage V B  which is supplied to an input of the differential amplifier  14 . The differential amplifier  14  utilizes a negative feedback  20  to hold the load voltage V L  substantially equal to the bias voltage V B  by generating an output voltage supplied to the current source  16 . In other words, the differential amplifier  14  biases the current source  16  to generate the current necessary to generate (I REF )(Z L )=V L . Therefore, by holding the load voltage V L  substantially equal to the bias voltage V B (V L =V B ), this holds the load voltage V L  constant so that the compensating load  18  is able to sink a constant current I REF . 
     This constant current I REF  is used as the output of the tunable constant current source  10  with mirrored versions of this current utilized to feed various analog circuitry in a conventional phase lock loop (PLL) or in other circuits as well. This enables the phase lock loop to act as a supply clock to a microprocessor which operates at a low voltage power supply, such as 1.5 volts. Thus, in order to fix the load voltage V L , the current source  16  must deliver a constant current (I REF ), such that (I REF )(Z L )=(V L )=(V B ). The output of the differential amplifier  14  is then fed to as many current mirrors as needed where the mirrored currents will be constant with value (M) (I REF ), where M is the relative size of the mirrored current source devices with respect to the current source  16  in the negative feedback loop  20  of the differential amplifier  14 . 
     Turning to FIG. 2, a schematic diagram of the voltage regulator  12  is shown in further detail. The voltage regulator  12  consists of a negative feedback network  22 , a gross temperature compensating circuit  24 , a bias/gain circuit  26  and a temperature compensation circuit  28 . In general, the voltage regulator  12  uses the negative feedback network  22  to hold the bias node (V B )  30  constant over changes in VDDP. The scaleable design parameters for the negative feedback network  22  is the ratio of the width of PO divided by the length of RP (WPO/LRP) and the width of NO divided by the length of RN (WNO/LRN), as well as the length of RB relative to the width of P 1 , P 2  and N 4  where the longer the length of a resistor, the larger the resistance and the larger the width of a MOSFET, the larger the current transmitted by the MOSFET. The pFETS P 1  and P 2 , the nFET N 4  and the resistor RB in the gain/bias circuit  26  are also scalable to provide the correct gain for dREFP/dVDDP. The pFET P 0  and the resistor RP are also scalable to give the right amount of VDDP compensation to node GATE  32 . Similarly, the sizes of the resistor RN and the nFET N 0  are also scalable to provide for the right amount of compensation to node gate  32 . 
     The additional nFETS N 1 , N 2  and N 3  in the gross temperature compensation circuit  24  are used to account for a gross shift in process or large changes in operating temperature. Four different nFET sizes are used and decoded orthogonally, such that one nFET is selected for low temperature, another selected for high temperature and another for nominal temperature. The diode DI 0 , resistor RT, pFET PT and nFET NT in the temperature compensation circuit  28  provide for compensation for small changes in operating temperature, such as ±10° C. In this regard, as temperature increases, the diode voltage across DI 0  decreases, thereby decreasing the drain current of nFET NT and the voltage across resistor RT and pFET PT, which reduces the current added to the bias/gain circuit  26 . 
     The voltage regulator  12  is essentially a biased network, such that the bias node (V B )  30  will stay constant as a function of VDDP, so as VDDP ramps up or down in a DC manner, the bias voltage V B  at bias node  30  will remain unchanged. The negative feedback circuit  22  consists of resistor RN, pFET P 0 , resistor RP and one of the branches of the gross temperature compensating circuit  24  which is either nFET N 0 , N 1 , N 2  or N 3 . The gate  32  of pFET P 0  is biased by the feedback network  22  such that a change in VDDP changes the voltage across resistor RP to bias the current through the drain of pFET P 0  such that pFET P 0  maintains the current feedback characterization through the feedback network  22 . In other words, as VDDP is ramped up or down, the bias node (V B )  30  will stay substantially constant or have an umbrella shape (see FIG. 5) with a low sensitivity to supply voltage changes. This negative feedback at the gate node  32  biases the nFET N 0 , N 1 , N 2  or N 3  whichever is selected via the decoder  34 . In this way, as current changes in P 0 , this changes current through RP thereby changing the gate voltage at gate node  32  which feeds back to the gate of N 0 , assuming this branch is currently operating, to change the current across resistor RN to control the bias voltage V B  at bias node  30 . 
     For example, assuming that the nFET N 0  branch of the gross temperature compensating circuit  24  is employed, maintenance of the constant bias voltage V B  is provided by way of two inverters. These inverters consist of nFET N 0  and resistor R 1  and pFET P 0  with resistor RP. As the power supply VDDP changes, there will be a change in the voltage REFP that has a relationship of 8 to 1 so that you do not have a 1 to 1 change in REFP to VDDP. This ratio is provided by the bias/gain circuit  26 . The voltage at REFP will thus change based upon changes in VDDP, such that the voltage across resistor RP and accordingly the voltage at gate node  32  will change as VDDP changes. This change in gate voltage at gate node  32  will change the gate voltage in nFET N 0  which will change the current through resistor RN to control the bias voltage V B  at bias node  30 . Thus, the negative feedback or circuit  22  will generate a change in current across resistor RN such that as the gate voltage at gate node  32  goes up, voltage at the gate of nFET N 0  goes up to create more current through resistor RN, thereby pulling the bias voltage V B  at bias node  30  down while also pulling the voltage at the gate node  32  down, via this negative feedback. In other words, in the negative feedback system  22 , as the gate voltage at gate node  32  changes, this changes the voltage at the gate of nFET N 0 , thus increasing or decreasing its current to change the voltage across resistor RN which is providing a negative feedback return to gate node  32  to hold the bias voltage V B  constant. It should further be noted that this negative feedback network or circuit  22  is very scalable or tunable by simply changing the nFET N 0  and pFET P 0  combination to get a new bias voltage V B  at a new desired value. 
     Referring to the gross temperature compensation circuit  24 , this circuit consists of four individual branches with each branch including a pair of nFET transistors in series with one nFET transistor in each branch in communication with the decoder  34 . The decoder  34  enables which branch in the temperature compensation circuit  24  that will be used in combination with the negative feedback network  22 . In this regard, the nFET N 0  branch is for operation at nominal temperatures of about 0° C. The nFET N 1  branch is directed to high operating temperatures of about 80° C. The nFET N 2  branch is directed to low operating temperatures of about −50° C. The nFET N 3  branch is directed to high operating temperatures utilizing worst case hardware. Accordingly, based upon the operating condition of the particular circuit or the hardware utilized, the decoder  34  will merely be configured to select the appropriate branch for operation in combination with the negative feedback network circuit  22 . 
     The bias/gain circuit  26  utilizes resistor R B , nFET N 4 , and pFETS P 1  and P 2  to set up the proper voltage divider to provide a correct bias at REFP for the pFET P 0 . The temperature compensating circuit  28  is used for maintaining a constant bias voltage V B  through small temperature changes of about ±10° C. The temperature compensation circuit  28  utilizes the diode DI 0  and the nFET NT to control temperature compensation for the bias voltage at REFP to provide a broader operating window over slight temperature ranges. 
     Turning to FIG. 3, the compensating load  18  is shown in further detail. The compensating load  18  consists of a very large nFET transistor NT and a resistor R in series. Since the resistor value for resistor R will vary slightly with variations in temperature change, this will create a change in I REF . By adding the nFET NT in series, the nFET NT threshold voltage will move in the opposite direction of the resistor current. Therefore, as temperature goes up, the current through the resistor R will go down, but the threshold voltage of nFET NT will also go down, and as a result, there will be a large voltage across resistor R to compensate for the change in the temperature. This reduces the current through the resistor R, but increases the voltage across the resistor R, thus compensating for temperature changes to provide for a stabile I REF . The compensating load  18  is preferably configured to operate between about −50° C. to about +80° C. 
     Turning now to FIG. 4, a schematic diagram of the differential amplifier  14  is shown in further detail. The differential amplifier  14  is a standard and conventional differential amplifier. In this regard, nFET N 2  acts as a current source, while nFET N 0  and nFET N 1  steer the current between pFET P 4  and pFET P 5 . Thus, if V bias (V B ) is greater than V load (V L ), current is steered through nFET N 0  and pFET P 4  so that there is a voltage drop at output  36 . If V load (V L ) is greater than V bias (V B ), current is steered through nFET N 1  and pFET P 5 , such that voltage goes up at output  36 . nFET N 2  is also biased by both pFET P 4  and pFET P 5  such that if V bias (V B ) is greater than V load (V L ), this turns nFET N 2  off slightly to further increase or decrease the output swing, thereby increasing the gain of the overall differential amplifier  14 . 
     The current source  16  is a standard cascaded current source which can consist of either two p-channel MOSFETS in series or one p-channel MOSFET. The MOSFET is then biased with the output voltage of the differential amplifier  14  at its gate with the source being VDDP and the drain being V L . 
     Referring now to FIG. 5, the responsiveness of the constant current source  10  is shown. In this regard, the X axis illustrates the change in power supply while the Y axis illustrates the change in current (mA). Referring to response  38 , this illustrates a response of the constant current source  10  when the nFET N 0  is used in the voltage regulator  12 . In this regard, the response  38  is centered substantially about the 1.5 volt area so that the response or curve  38  is substantially flat in this region. Thus, any slight fluctuations in the power supply, from the supply voltage of 1.5 volt, will provide substantially the same current output. 
     Assuming the constant current source  10  is operating with the voltage regulator utilizing the nFET N 0  at 80° C., the response  40  will now be observed. This shows that the response shifts off-center such that with any slight change in power supply voltage from 1.5 volt, there is also a change in the current which is an undesirable effect. Therefore, by providing the tuning bits or various branches in the gross temperature compensating circuit  24 , upon adjusting for the high temperature branch or nFET N 1 , the response now shifts to a lower current, but with the response  42  being centered substantially along the 1.5 volt power supply. In this way, any change again in power supply voltage will essentially provide a constant current. It should further be noted that a reduction in current supply between response  40  and  42  does not pose a problem for a phase lock loop or other analog circuitry since the main criteria is that the source provide a constant current with variations in power supply voltage. 
     While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.