Abstract:
A digital direct access arrangement (DAA) circuitry may be used to terminate the telephone connections at the user&#39;s end to provide a communication path for signals to and from the phone lines. Briefly described, a means for providing a proper hookswitch transition for a variety of international phone standards is provided. The invention may also be utilized with means for transmitting and receiving a signal across a capacitive isolation barrier. More particularly, a DAA circuitry may be utilized which satisfies many or all hookswitch transition standards without the use of additional discrete devices. The hookswitch transition standards may be satisfied by ramping down the current flowing through the hookswitch prior to transitioning the hookswitch state. In this manner the hookswitch current change as a function of time (di/dt) may be decreased. Thus, the current through the hookswitch may be actively controlled prior to switching the hookswitch from an off-hook condition to an on-hook condition. By controlling the current drawn from the phone lines through the hookswitch, the maximum voltage seen at the phone company exchange may be decreased.

Description:
[0001]    This application is a continuation-in-part application of U.S. Ser. Nos. 08/841,409, 08/837,702 and 08/837,714 all filed on Apr. 22, 1997; and a continuation-in-part application of U.S. Ser. Nos. 09/034,455, 09/035,779, 09/034,620, and 09/035,175 all filed on Mar. 4, 1998; and a continuation-in-part application of U.S. Ser. No. 09/098,489 filed on Jun. 16, 1998, all of which are expressly incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD OF THE INVENTION  
         [0002]    This invention relates to the field of digital access arrangement circuitry. More particularly, this invention relates to digital access arrangement circuitry for connecting to a variety of phone line standards. The digital access arrangement circuitry may further include isolation barrier utilizing a capacitor coupled isolation barrier.  
         BACKGROUND  
         [0003]    Direct Access Arrangement (DAA) circuitry may be used to terminate the telephone connections at a phone line user&#39;s end to provide a communication path for signals to and from the phone lines. DAA circuitry includes the necessary circuitry to terminate the telephone connections at the user&#39;s end and may include, for example, an isolation barrier, DC termination circuitry, AC termination circuitry, ring detection circuitry, and processing circuitry that provides a communication path for signals to and from the phone lines.  
           [0004]    Generally, governmental regulations specify the telephone interface requirements and specifications for a variety of parameters including pulse dialing transitions, spark quenching, AC termination, DC termination, ringer impedance, ringer threshold, etc. For example, Federal Communications Commission (FCC) Part 68 governs the interface requirements for telephones in the United States. However, the interface requirements world wide are not standardized, and thus, in countries other than the United States the applicable standards may include the CTR21, TBR21, NET4, JATE, and various country specific PTT specifications. Because the interface requirements are not standardized from country to country, often different DAA circuitry is required for use in each country in order to comply with the appropriate standard. The requirement for different DAA circuitry, however, limits the use of one phone line interface in a variety of countries. Thus, for example, a modem in a laptop computer configured for interfacing with a phone line in one country may not necessarily operate properly in another country. Further, the requirement for different DAA circuitry in various countries hinders the design of a single integrated cost effective DAA solution for use world wide.  
           [0005]    As mentioned above, the telephone interface requirements generally include specifications for the pulse dialing (also called decadic dialing) transitions and spark quenching presented to the telephone line. In general, pulse dialing comprises a repetitive series of on-hook and off-hook transitions. FIG. 1 shows the standard two-wire public network lines, the TIP line  8  and the RING line  8 . The TIP line and the RING line may be conventionally connected to a diode bridge  11 . The diode bridge presents the proper polarity line signal to the hookswitch circuit  12  independent of the TIP and RING polarity. The hookswitch circuit  12  operates as a switch to “seize” or “collapse” the TIP and RING phone lines to allow the maximum loop current (I loop ) that is available from the phone line to flow. In an on-hook condition (i.e. the user is not transmitting data to or from the phone line), the hookswitch circuit  12  may be switched open. In an off-hook condition, the hookswitch circuit  12  may be switched closed to allow a loop current flow I LOOP . The remaining DAA circuitry is shown as block  10 . The phone company exchange is connected to the other side of the TIP and RING lines and may be characterized as a voltage source  16 , an inductor  14  have an inductance L and a resistor  13 . As the hookswitch opens and closes, the loop current flow I LOOP  will change and the voltage across the inductor  14  will change.  
           [0006]    [0006]FIG. 1A illustrates the voltage across the inductor as the state of the hookswitch changes. FIG. 1A shows an exemplary series of hookswitch transitions. Such a series of transitions may be seen at a period of 100 msec, for example, during pulse dialing. It will be noted that the voltage across the inductor spikes as shown by dashed lines  22  during transitions from an off-hook condition to an on-hook condition (i.e. the loop current I LOOP  transitions from a steady state off-hook value to zero). The voltage spike  22  results from the inductor V-I relationship V=L(di/dt) that results since the phone company exchange is characterized as an inductive source. If the loop current suddenly drops, large voltage spike will occur across the effective inductance of the phone company exchange, and thus, across the TIP and RING lines. The maximum inductor voltage is specified in various countries and is sometimes referred to as the “spark quenching” specification. For example, in Australia (one of the more demanding specifications), the line inductance is specified as  4 H and voltage across such an inductance may not exceed 230V. These voltage spikes may also result in undesirable voltage sparks across the hookswitch.  
           [0007]    In addition to the spark quenching specifications, for pulse dialing some countries have specifications which require the transition from off-hook to on-hook to occur slowly. Thus, another country dependent specification exists which may require the hookswitch transition to be controlled.  
           [0008]    One prior art approach to limit the instantaneous current change which occurs when the hookswitch is changed from off-hook to on-hook is shown in FIG. 1B. As shown in FIG. 1B, a resistor  32  and capacitor  30  are provided around the hookswitch circuit  12 . The purpose of the resistor  32  and capacitor  30  is to provide a current path around the hookswitch when the hookswitch is opened. The RC effect of the resistor  32  and capacitor  30  is to slow the current change from the steady state off-hook value to the on-hook zero value. Thus, the di/dt term will be decreased and the maximum voltage seen across the inductor will drop since the spike  22  will decrease. Other prior art approaches may include the use of a voltage clamp (such as an MOV device) placed across the TIP and RING lines. The use of these additional discrete external devices add to the DAA system costs and complexity.  
           [0009]    It is desirable, therefore, to provide a DAA circuitry that may be suitable for use in many or all countries without the need for use of additional external discrete devices to satisfy off-hook and on-hook transition standards.  
           [0010]    Further, it is also desirable that the DAA circuitry act as an isolation barrier since an electrical isolation barrier must exist in communication circuitry which connects directly to the standard two-wire public switched telephone network and that is powered through a standard residential wall outlet. For example, in order to achieve regulatory compliance in the United States with Federal Communications Commission Part 68, which governs electrical connections to the telephone network in order to prevent network harm, an isolation barrier capable of withstanding 1000 volts rms at 60 Hz with no more than 10 milliamps current flow, must exist between circuitry directly connected to the two wire telephone network and circuitry directly connected to the residential wall outlet.  
           [0011]    Thus, there exists a need for reliable, accurate and inexpensive DAA circuitry for satisfying the hookswitch transition standards for multiple country phone line standards and a DAA circuitry which also provides the necessary electrical isolation barrier.  
         SUMMARY OF THE INVENTION  
         [0012]    The above-referenced problems are addressed by the present invention, which provides a reliable, inexpensive, DAA circuit that may be utilized with multiple telephone interface standards and which also provides an isolation system that is substantially immune to noise that affects the timing and/or amplitude of the signal that is transmitted across the isolating element, thus permitting an input signal to be accurately reproduced at the output of the isolation system.  
           [0013]    The present invention provides digital direct access arrangement (DAA) circuitry that may be used to terminate the telephone connections at the user&#39;s end to provide a communication path for signals to and from the phone lines. Briefly described, the invention provides a means for providing a proper hookswitch transition for a variety of international phone standards. The invention may also be utilized with means for transmitting and receiving a signal across a capacitive isolation barrier. More particularly, a DAA circuitry may be utilized which satisfies many or all hookswitch transition standards without the use of additional discrete devices. The hookswitch transition standards may be satisfied by ramping down the current flowing through the hookswitch prior to transitioning the hookswitch state. In this manner the hookswitch current change as a function of time (di/dt) may be decreased. Thus, the current through the hookswitch may be actively controlled prior to switching the hookswitch from an off-hook condition to an on-hook condition. By controlling the current drawn from the phone lines through the hookswitch, the maximum voltage seen at the phone company exchange may be decreased.  
           [0014]    In one embodiment, a communication system is provided. The communication system may comprise phone line side circuitry that may be coupled to phone lines and powered side circuitry that may be coupled to the phone line side circuitry through an isolation barrier. The communication system may further include a hookswitch transition signal, and current ramping circuitry coupled to the hookswitch transition signal within the phone line side circuitry, the current ramping circuitry ramping downward the current drawn from the phone line in response to the hookswitch transition signal prior to the hookswitch completely changing states.  
           [0015]    In another embodiment, a method of operating a communication system that may be coupled to a phone line is provided. The method may include coupling an isolation barrier between powered circuitry and phone line side circuitry, drawing current at a first current level from the phone line through the hookswitch circuitry, providing hookswitch circuitry within the phone line side circuitry, and decreasing the current drawn through the hookswitch to a second level prior to changing the hookswitch from an off-hook state to an on-hook state, the second current level being less than the first current level.  
           [0016]    In yet another emodiment, a hookswitch transition circuit within a communication system that may be connected to phone lines is provided. The hookswitch transition circuit may comprise a hookswitch control signal, and at least one variable current circuit coupled to the hookswitch control signal, the at least one variable current circuit responsive to the hookswitch control signal to decrease a current drawn from the phone lines prior to changing the state of a hookswitch.  
           [0017]    In another emodiment, a method of controlling the current change in phone line side circuitry is provided. The method includes providing a signal indicative of a desire to change a hookswitch from an off-hook state to an on-hook state, and adjusting downward the current drawn from a phone line in response to the signal prior to changing the hookswitch from the off-hook state to the on-hook state.  
           [0018]    A method of controlling current in a phone line is also provided in another embodiment. The method may include actively controlling at least one current circuit of a DAA integrated circuit in response to a hookswitch transition signal, and substantially decreasing the current in the phone line as a result of the active control prior to achieving an on-hook state.  
           [0019]    An integrated circuit compatible with a plurality of phone line standards having hookswitch transition requirements is also provided. The integrated circuit may include a hookswitch signal, and at least one current control circuit coupled to the hookswitch signal, the current control circuit coupled to at least one output of the integrated circuit, the current control circuit operating prior to the completion of a hookswitch transition to enable a decrease in a current level on the phone. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0020]    So that the manner in which the herein described advantages and features of the present invention, as well as others which will become apparent, are attained and can be understood in detail, a more particular description of the invention summarized above may be had by reference to the embodiments thereof which are illustrated in the appended drawings, which drawings form a part of this specification.  
         [0021]    It is noted, however, that the appended drawings illustrate only exemplary embodiments of the invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0022]    [0022]FIG. 1 is a general schematic illustrating a hookswitch coupled to a phone company exchange and additional DAA circuitry.  
         [0023]    [0023]FIG. 1A illustrates the voltage seen during hookswitch transitions.  
         [0024]    [0024]FIG. 1B illustrates one prior art approach to limit voltage spikes or sparks.  
         [0025]    [0025]FIG. 2A is a block diagram of a telephone set illustrating a typical application of the present invention.  
         [0026]    [0026]FIG. 2 is a general block diagram of digital DAA circuitry including phone line side circuitry, an isolation barrier, and powered side circuitry according to the present invention.  
         [0027]    [0027]FIG. 3 is a general block diagram of transmit and receive signal paths within digital DAA circuitry according to the present invention.  
         [0028]    [0028]FIG. 4 is a general circuit diagram of digital DAA circuitry implemented with two integrated circuits (ICs), a capacitive isolation barrier, and external circuitry according to the present invention.  
         [0029]    [0029]FIG. 5 is a conceptually diagram of a circuit according to the present invention.  
         [0030]    [0030]FIG. 6 illustrates a hookswitch circuit and DAA which may be controlled according to the present invention.  
         [0031]    [0031]FIG. 7 illustrates a circuit for ramping a component of the loop current prior to transitioning the hookswitch.  
         [0032]    [0032]FIG. 8 illustrates a circuit for ramping a component of the loop current prior to transitioning the hookswitch.  
         [0033]    [0033]FIG. 9 illustrates a circuit for ramping a component of the loop current prior to transitioning the hookswitch.  
         [0034]    [0034]FIGS. 10 and 10A are timing diagrams of the circuits of FIGS. 7, 8 and  9 . 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0035]    In order to provide a context for understanding this description, FIG. 2A illustrates a typical application for the present invention: a telephone that includes circuitry powered by a source external to the phone system. A basic telephone circuit  118  is powered by the “battery” voltage that is provided by the public telephone system and does not have a separate power connection. Many modern phones  110 , however, include radio (cordless), speakerphone, or answering machine features that require an external source of power  112 , typically obtained by plugging the phone (or a power supply transformer/rectifier) into a typical 110-volt residential wall outlet. In order to protect public phone system  114  (and to comply with governmental regulations), it is necessary to isolate “powered circuitry”  116  that is externally powered from “isolated circuitry”  118  (isolated circuitry may also be called phone line side circuitry) that is connected to the phone lines, to prevent dangerous or destructive voltage or current levels from entering the phone system. (Similar considerations exist in many other applications as well, including communication, medical and instrumentation applications in which this invention may be beneficially applied.) The required isolation is provided by isolation barrier  120 . The signal that passes through the isolation barrier  120  is an analog voice signal in a typical telephone application, but it may also be a digital signal or a multiplexed signal with both analog and digital components in various applications. In some applications, communication across isolation barrier  120  may be unidirectional (in either direction), but in many applications, including telephony, bidirectional communication is required. Bidirectional communication may be provided using a pair of unidirectional isolator channels, or by forming a single isolation channel and multiplexing bidirectional signals through the channel. The primary requirements placed on isolation barrier  120  are that it effectively prevents harmful levels of electrical power from passing across it, while accurately passing the desired signal from the powered side  122  to the isolated side  124 , or in the reverse direction if desired.  
         [0036]    [0036]FIG. 2 is a general block diagram of digital DAA circuitry  110  including phone line side circuitry  118 , an isolation barrier  120 , and powered side circuitry  116  according to the present invention. The isolation barrier  120  may include one or more capacitors and allow for the transmission of digital information between the isolation interface  1614  in the phone line side circuitry and the isolation interface  1610  in the powered side circuitry. The phone line side circuitry  118  may be connected to phone lines of a telephone network system, and the powered side circuitry  116  may be connected to external controllers, such as digital signal processors (DSP), that may be part of a communication device, such as a phone or modem.  
         [0037]    The powered side circuitry  116 , which may be implemented as an integrated circuit (IC), may communicate with the external controller through a digital interface  1606  and a control interface  1608 . For example, the digital interface  1606  may have a number of external pins providing a serial port interface to the external controller, such as a master clock input pin (MCLK), a serial port bit clock output (SCLK), a serial port data IN pin (SDI), a serial port data OUT pin (SDO), a frame sync output pin (FSYNC_bar) (it is noted that the suffix “_bar” is used to denote a signal that is typically asserted when at a low logic level), and a secondary transfer request input pin (FC). Similarly, the control interface  1608  may have a number of external pins providing control and status information to and from the external controller, such as a ring detect status pin (RGDT_bar), an off-hook status pin (OFHK_bar), a reset pin (RESET_bar), and multiple mode select pins (MODE). In addition, the digital interface  1606  and the control interface  1608  are connected to the isolation interface  1610  so that control, status, signal and other desired information may be transmitted to and received from the phone line side circuitry  118  across the isolation barrier  120 .  
         [0038]    The phone line side circuitry  118 , which may be implemented as an integrated circuit (IC), may communicate with the phone lines through hybrid and DC termination circuitry  1617  (the DC termination circuitry provides an internal power supply voltage), and determine ring-detect and off-hook status information through off-hook/ring-detect block  1620 . In addition, the hybrid and DC termination circuitry  1617  and the off-hook/ring-detect block  1620  are connected to the isolation interface  1614  so that control, status, signal and other desired information may be transmitted to and received from the powered side circuitry  116  across the isolation barrier  120 .  
         [0039]    In the embodiment depicted, the hybrid portion of the hybrid and DC termination circuitry  1617  has an output pin QE 2  and an input pin (RX) that may connect to external telephone interface circuitry such as hook-switch circuitry and a diode bridge. The hybrid circuitry may function to split the differential signal existing on the phone, which typically includes both transmit and receive analog information, into an internal transmit signal (TX INT ) and receive signal (RX INT ). It is noted that the QE 2  output pin is used to transmit analog information to the phone lines, and that the RX pin is labeled to indicate that it is used to receive analog information from the phone lines. These external pin signals are different than the internal analog transmit signal (TX INT ) and analog receive signal (RX INT ).  
         [0040]    The hybrid and DC termination circuitry  1617  may have a number of external pins that also connect to external telephone interface circuitry such as hook-switch circuitry and a diode bridge as shown in FIGS. 2 and 4. For example, the hybrid and DC termination circuitry  1617  may have a DC termination pin (DCT), a voltage regulator pin (VREG), two external resistor pins (REXT and REXT 2 ), two filter pins (FILT and FILT 2 ) and an isolated ground pin (IGND). The DC termination circuitry terminates the DC voltage on the phone line and provides an internal power supply for the phone line side circuitry  118 . The DC termination pin (DCT) receives a portion of the phone line DC current with the remainder flowing through pins QE 2  and QB 2 , depending upon the termination mode and DC current level. The voltage regulator pin (VREG) allows external regulator circuitry, such as a capacitor, to be connected to the DC termination circuitry  1617 . External resistors and a capacitor may be connected to the two external resistor pins (REXT and REXT 2 ) to set the real and complex AC termination impedance respectively. The filter pin FILT (along with the capacitor C 5 ) sets the time constant for the DC termination circuit. The filter pin FILT 2  sets the off hook/on hook transient responses for pulse dialing. The isolated ground pin (IGND) may be connected to the system ground for the powered side circuitry  116  through a capacitor within the isolation barrier  120  and may also be connected to the phone line through a ground connection within external diode bridge circuitry.  
         [0041]    The off-hook/ring-detect block  1620  may have external input pins allowing status information to be provided concerning phone line status information (RNG 1 , RNG 2 ), such as ring and caller identification signals. For example, the first ring detect pin (RNG 1 ) may connect to the tip (T) lead of the phone line through a capacitor and resistor, and the second ring detect pin (RNG 2 ) may connect to the ring (R) lead of the phone line through a capacitor and resistor. In addition, off-hook/ring-detect block  1620  may have external output pins (QB, QE) that control external off-hook circuitry to enter, for example, an off-hook state or a limited power mode to get caller identification information. More particularly, the output pins (QB, QE) may be connected to the base and emitter, respectively, of a bipolar transistor within external hook-switch circuitry.  
         [0042]    [0042]FIG. 3 is a general block diagram of internal transmit (TX) and receive (RX) signal paths within digital DAA circuitry  110  according to the present invention. In the embodiment depicted, information may be communicated in either direction across the isolation barrier  120 . It is noted that FIG. 3 does not depict all of the functional blocks within powered side circuitry  116  and phone line side circuitry  118 . It is also noted that the blocks depicted may be implemented as numerous additional blocks carrying out similar functions.  
         [0043]    In the embodiment of FIG. 3, communications from the phone line side circuitry  118  to the powered circuitry  116  are considered receive signals. Within phone line side circuitry  118 , a delta-sigma analog-to-digital converter (ADC)  1710  receives an internal analog receive signal (RX INT ), which may be provided for example by hybrid circuitry  1617 . The output of delta-sigma ADC  1710  is oversampled digital data stream in a pulse density modulation format. The decoder/encoder circuitry  1708  processes and formats this digital information as desired before sending it across the isolation barrier  120  as encoded digital information. For example, decoder/encoder  1708  may multiplex control data with the digital stream before it is sent across the isolation barrier  120 . This control data may be any desired information, such as ring detect signals, off-hook detect signals, other phone line status information or data indicative of the country in which the DAA will be utilized (so that the appropriate phone line interface standards will be satisfied). Within powered side circuitry  116 , the decoder/encoder  1706  decodes this encoded digital information received across the isolation barrier  120 . The digital filter  1702  processes this decoded digital stream and converts it into internal digital receive data (RX D ) that may be provided through the digital interface  1606  to an external controller.  
         [0044]    Communications from the powered side circuitry  116  to the phone line side circuitry  118  are considered transmit signals. Within powered side circuitry  116 , a delta-sigma modulator  1704  receives an internal digital transmit signal (TX D ), which may be provided for example from an external controller through digital interface  1606 . The output of delta-sigma modulator  1704  is an oversampled digital data stream in a pulse density modulation format. The decoder/encoder circuitry  1706  processes and formats this digital information as desired before sending it across the isolation barrier  120  as encoded digital information. For example, decoder/encoder  1706  may multiplex control data with the digital stream. This control data may be any desired information, such as ring detect signals, off-hook detect signals, or other phone line status information. In addition, decoder/encoder  1706  may add framing information for synchronization purposes to the digital stream before it is sent across the isolation barrier  120 . Still further, decoder/encoder  1706  may format the digital data stream so that a clock signal may be recovered within the phone line side circuitry  118 , for example. Within phone line side circuitry  118 , the decoder/encoder  1708  may recover a clock signal and may decode the encoded digital information received across the isolation barrier  120  to obtain framing, control or status information. The digital-to-analog converter (DAC)  1712  converts the decoded digital stream and converts it into internal analog transmit data (TX INT ) that may be provided as an analog signal through the hybrid circuitry  1617  and ultimately to the phone lines.  
         [0045]    [0045]FIG. 4 is a general circuit diagram of digital DAA circuitry  110  implemented with two integrated circuits (ICs) and a capacitive isolation barrier  120  according to the present invention. The DAA circuitry  110  may be coupled to phone line TIP and RING lines as shown. In particular, powered side circuitry  116  may include a powered side integrated circuit (IC)  1802 A, and phone line side circuitry  118  may include a phone line side IC  1802 B. External discrete devices may be coupled to the TIP line, RING line, phone line side IC  1802 B and powered side IC  1802 A. The external circuitry may include circuitry, such as hookswitch circuitry  1804 , diode bridge circuitry  1806 , and impedance circuitry  1820 . During an on-hook condition, typical prior art hookswitches may typically be turned off thus not allowing loop current to be drawn from the phone line. Prior art hookswitches may include bipolar and/or relay switches. During an off-hook condition, the switches may be placed in saturation and act as a switch that “seizes” or “collapses” the phone line, i.e. draws all the available phone line current. The communication system disclosed herein allows for the hookswitch devices to draw loop current from the phone line in both on-hook and off-hook conditions. Thus, even though an on-hook condition occurs, current may be obtained through the hookswitch devices. This feature allows circuitry which operates during on-hook conditions to still receive power from the phone line. Moreover because the hookswitch devices are utilized for drawing power in both on-hook and off-hook conditions, the use of additional switches dedicated to drawing the power during on-hook conditions is not required.  
         [0046]    In the embodiment depicted in FIG. 4, external pins  1809  of the powered side IC  1802 A are connected to an external digital signal processor (DSP) and to a external application specific IC (ASIC) or controller. The isolation barrier  120  may include a first capacitor (C 1 ) connecting an external signal (C 1 A) pin on the powered side IC  1802 A to an external signal (C 1 B) pin on the phone line side IC  1802 B. In addition, the isolation barrier  120  may have a second capacitor (C 2 ) connecting the isolated ground (IGND) pin on the phone line side IC  1802 B to the system ground (GND) pin on the powered side IC  1802 A. In addition, the isolated ground (IGND) pin may be connected to node  1812  within diode circuitry  1806  (and thereby be connected to the phone line) and the remaining ground connections of the external circuitry of the phone line side circuitry  118 . Typical component values for the various external capacitors, resistors, transistors, and diodes for the circuit of FIG. 4 are shown in Table 1 and Table 2. As used in the Tables, when a device is listed as “Not Installed” the device may be considered to be an open circuit. Table 1 illustrates external components that may be used for a global DAA (i.e. for use in multiple countries, including the U.S.) while Table 2 illustrates simplified circuitry for meeting U.S. FCC and CTR21 requirements only. As discussed below in more detail, the components C 15 , R 14 , Z 2 , and Z 3  of Table 1 may be not installed for all countries except the Czech Republic.  
                             TABLE 1                           Global External Component Values                Symbol   Value                       C1, C2   150 pF, 2 kV, , ±20%           C3, C6, C10, C16   0.1 μF, 16 V, ±20%           C4, C11, C23, C28, C29   NOT INSTALLED           C5   0.1 μF, 50 V, ±20%           C7, C8   1800 pF, 300 V, ±5%           C9   22 nF, 300 V, ±20%           C12   0.22 μF, 16 V, ±20%           C13   0.47 μF, 16 V, ±10%           C14   0.68 uF, 16 V ±10%           C15   1.0 μF, 250 V, ±20%           C18, C19   12 nF, 16 V, ±10%           C20   0.01 uF, 16 V, ±10%           C21   NOT INSTALLED           C22   1800 pF, 50 V, ±10%           C24, C25   1000 pF, 2000 V, ±10%           R1, R4, R21, R22, R23   NOT INSTALLED           R2   402 Ω, {fraction (1/16)} W ±1%           R3   NOT INSTALLED           R5   36 kΩ, {fraction (1/16)} W, ±5%           R6   120 kΩ, {fraction (1/16)} W ±5%           R7, R8, R15, R16, R17, R19   4.87 KΩ, ¼ W ±1%           R9, R10   15 kΩ, {fraction (1/10)} W ±5%           R11   10 kΩ, {fraction (1/16)} W ±1%           R12   78.7 Ω, {fraction (1/16)} W ±1%           R13   215 Ω, {fraction (1/16)} W ±1%           R14   7.5 kΩ, ¼ W ±5%           R18   2.2 kΩ, {fraction (1/10)} W ±5%           R24   150 Ω, {fraction (1/16)} W ±5%           Q1, Q3   A42, NPN 300 V           Q2   A92, PNP 300 V           Q4   2N2222 NPN 40 V ½ W           FB1, FB2   Ferrite Bead           RV1   Sidactor 275 V, 100A           D1-D4   1N4004           Z1   Zener Diode 43 V           Z2, Z3   Zener Diode 5.6 V                      
 
         [0047]    [0047]                         TABLE 2                           FCC/CTR21 Only External Component Values            Symbol   Value               C1, C2   150 pF, 2 kV, ±20%       C3, C6, C10, C16   0.1 uF, 16 V, ±10%       C5   0.10 μF, 50 V ±20%       C7, C8   1800 pF, 250 V, ±10%       C9   22 nF, 250 V, ±20%       C12   0.22 uF, 16 V, Tant, ±10%       C13   0.47 uF, 16 V, ±10%       C18, C19   12 nF, 16 V, ±10%       C20   0.01 uF, 16 V, ±10%       C22   1800 pF, 50 V ±10%       C24, C25   1000 pF, 2 kV ±10%       C4, C11, C14, C15, C17, C21, C23,   NOT INSTALLED       C28, C29       R2   402 Ω, {fraction (1/16)} W ±1%       R5   36 kΩ, {fraction (1/16)} W ±5%       R6   120 kΩ, {fraction (1/16)} W ±5%       R1, R3, R4, R14, R12, R13, R21, R22, R23   NOT INSTALLED       R9, R10   15 kΩ, {fraction (1/10)} W ±5%       R7, R8, R15, R16, R17, R19   4.87 kΩ, ¼ W ±1%       R11   10 kΩ, {fraction (1/16)} W ±1%       R18   2.2 kΩ, {fraction (1/10)} W ±5%       R24   150 Ω, {fraction (1/16)} W ±5%       Q1, Q3   A42, NPN 300 V       Q2   A92, PNP 300 V       Q4   2N2222, NPN,           40 V ½ W       FB1, FB2   Ferrite Bead       RV1   Sidactor 275 V, 100A       D1-D4   1N4004       Z1   Zener Diode 43 V       Z2, Z3   NOT INSTALLED                    
         [0048]    An exemplary embodiment of the present invention will be discussed below with reference to a configuration according to FIG. 4 as configured as shown in Table 1. It will be recognized, however, that the concepts of the present invention may be implemented in other configurations. According to the present invention, DAA circuitry may be utilized which satisfies many or all hookswitch transition standards. The hookswitch transition standards may be satisfied by ramping down the current flowing through the hookswitch prior to transitioning the hookswitch state. In this manner the hookswitch current change as a function of time (di/dt) may be decreased. Thus, the current through the hookswitch may be actively controlled prior to switching the hookswitch from an off-hook condition to an on-hook condition. By controlling the current drawn from the phone lines through the hookswitch, the maximum voltage seen at the TIP and RING lines may be decreased and pulse dialing specifications which require the transition to an on-hook condition to occur slowly may be satisfied.  
         [0049]    The technique for transitioning the state of a hookswitch as disclosed herein may be conceptually seen with respect to FIG. 5. As shown in FIG. 5, the TIP and RING lines are coupled to a hookswitch  500  through a diode bridge  11 . The hookswitch may then be coupled to a phone line side DAA integrated circuit  1802 B which may include a variable current source  502 . The variable current source  502  may be affected by an on-hook control signal  504 . The on-hook control signal  504  may also be provided to a delay element  506  to provide a delayed on-hook control signal  508 . In operation, the circuit may initially be in an off-hook condition (hookswitch  500  is closed) and the current drawn through the hookswitch may be at a steady state (i.e. the current through the current source  502  is at a relatively high off-hook steady state). Then an on-hook state is desired, the on-hook control signal  504  will change to an on-hook state. Thus, the control signal  504  operates as a signal indicative of a hookswitch transition. In response to an on-hook state at the on-hook control  504 , the current through current source  502  is ramped down. However, the delay element  506  causes results in the delayed on-hook control signal  508  to indicate an on-hook state to the hookswitch  500  at some time after the current ramp down has begun. In this manner, the ramp down of the current may occur in a more controlled slow manner and commenced prior to the hookswitch  500  opening.  
         [0050]    The current through current source  502  does not have to be completely ramped down prior to the opening of the hookswitch  500 . Rather, the current need only be dropped to a level sufficiently low so that the current change (di/dt) when the hookswitch  500  opens does not exceed a level that results the failure to meet pulse dialing and spark quenching specifications.  
         [0051]    Moreover, though described conceptually in FIG. 5 with respect to a hookswitch which opens instantaneously in response to a control signal, the hookswitch may be constructed in a manner such that the switch transitions from a fully closed state to a fully opened state over some time period. Thus, the on-hook control signal  504  may be applied to both the hookswitch  500  and the current source  502  at the same time. In this example, the current ramp down by current source  502  may commence simultaneously with the beginning of the change of state of the hookswitch  500 . The benefits of the techniques disclosed herein will still be achieved since the current level is still ramped down to some extent prior to the hookswitch reaching it open circuit condition.  
         [0052]    Example circuitry for achieving a current ramp when transition from off-hook to on-hook conditions in shown in FIG. 6. FIG. 6 illustrates the phone line side DAA integrated circuit  1802 B and the surrounding external hookswitch circuitry using the same nomenclature and circuit connections as shown in FIG. 4. As seen in FIG. 8, the TIP and RING lines are provided to the diode bridge  1820 . The diode bridge is coupled to the phone line side DAA integrated circuit  1802 B through the hookswitch circuitry which includes transistors Q 1 , Q 2 , Q 3  and Q 4  and associated resistors. The hookswitch circuitry shown herein is merely exemplary, and many other hookswitch circuits may utilize the techniques of the present invention. The phone line side DAA integrated circuit  1802 B is indicated by the dashed line and includes input/output pins QE, QB, QE 2 , IGND, FILT, FILT 2  and REF. The DAA integrated circuit  1802 B includes an I HOOK  current source  600 , an I DCT ′ current source  604 , an I CHIP  current source  606  and an I QB  current source  604 . The current I HOOK  operates to control the activation of transistor Q 2 . When the current I HOOK  is zero the hookswitch is in an on-hook state and transistor Q 2  is off. When the current I HOOK  is on, transistor Q 2  is activated and current flows through Q 2 . When the current I HOOK  is large enough (for example approximately 4 mA), transistor Q 2  is in saturation and the hookswitch is in the off-hook mode. During off-hook conditions, the loop current is the sum of the currents I HOOK , I DCT , I DCT ′, I QB , and I CHIP . As will be described below, the current I DCT ′ is created by current mirroring (32×) the current I DCT . In off-hook conditions, I QB  is similar in magnitude to the current I DCT . The current I CHIP  represents all other currents drawn on chip. The I HOOK  current is related to the currents I DCT , I DCT ′, and I QB  as described below in more detail.  
         [0053]    The current change over time change (di/dt) which occurs when the hookswitch transitions from an off-hook state to an on-hook state may be minimized by ramping down the currents I DCT ′, I HOOK , and I CHIP  prior to the hookswitch going completely on-hook (i.e. the hookswitch is opened and Q 2  is off). Ramping down the currents lowers the loop current flowing through the hookswitch prior to opening the hookswitch. Thus, by the time I HOOK  reaches a level sufficiently low to turn off Q 2 , and thus open the hookswitch, the total loop current will have already significantly dropped in a relatively slow and controlled manner. For example, a typical loop current may be approximately 100 mA in the off-hook mode and after approximately 1.5 to 2 msec may have dropped to 2 mA prior to the hookswitch completely opening. Thus, the loop current may decrease by 50% or more prior to the hookswitch opening, and more preferably by more than 75%.  
         [0054]    Circuitry for ramping down the currents I DCT ′, I CHIP , and I HOOK  may seen with respect to FIGS. 7, 8, and  9  respectively. As shown in FIG. 7, the current I DCT ′ may be generated by use of current mirror transistors  706 ,  705 , and  708  which are sized to provide a current I DCT ′ that is 32 times the current I DCT . During off-hook operation the switch  704  is closed and the switch  702  is opened. Connected to switch  704  is a large resistance resistor  712  (2 MΩ) and connected to switch  702  is a smaller resistance resistor  714  (400 KΩ). Switch  704  is connected to the FILT pin of the phone line side DAA integrated circuit  1802 B and switch  702  is coupled between the FILT pin and the QE 2  pin as shown. A diode connected transistor  720  may be connected to the resistor  714  as shown. Coupled between the FILT pin and the QE 2  pin is an external capacitor C 12 . As shown in Table 1, C 12  may have a capacitance of 0.22 uF. As mentioned above, in the steady-state off-hook operation switch  702  is open and  704  is closed. This provides a path to the gate of transistor  708  to generate the 32× mirror current through transistor  708 . When a transition to an on-hook state is signaled to switches  704  and  702  (such as for example by an on-hook control signal  504  as shown in FIG. 5), switch  704  is opened and switch  702  is closed. This will result in a change in the gate voltage of transistor  708  (and thus correspondingly the current I DCT ′) that is dependent upon the time constant of the internal resistor  714  and the external capacitor C 12 . The di/dt of the current I DCT ′ is therefore affected by the values chosen for the resistor  714 , transistor  720  and the capacitor C 12 .  
         [0055]    Similarly, the I CHIP  may be ramped down as shown in FIG. 8. As shown in FIG. 8, a Vc supply voltage level is provided to the phone line side DAA integrated circuit  1802 B at the QE 2  pin. Coupled between the FILT 2  pin and the QE 2  pin is an external capacitor C 12  (for example 0.47 uf as shown in Table 1). Coupled to Vc is a plurality of p-channel chip bias transistors  802  which provide bias currents to the various circuits of the phone line side DAA integrated circuit  1802 B. These bias currents together result in the current I CHIP . During off-hook operation, the switch  806  is closed and the switch  804  is opened. Coupled to switch  806  is an internal resistor  808  (for example 500 KΩ) and coupled to switch  804  is an internal resistor  810  (for example 400 KΩ). A diode connected transistor is connected to resistor  810  as shown. The voltage applied through switch  806  to the gates of transistors  802  when the circuitry is in an off-hook mode is generated with a differential amplifier  814  having a bandgap voltage of 1.25 V and the REF pin voltage as its two inputs as shown. In off-hook operation, the switch  806  is closed and the switch  804  is opened. When a transition to on-hook operation is desired (for example as signaled by the on-hook control signal  504 ), the switch  806  opens and the switch  804  closes. This will result in transistors  802  to begin to turn off and the current I CHIP  will begin to ramp down. The speed at which the transistors will turn off and the current ramps down will be dependent upon the time constant of external capacitor C 13 , transistor  820 , and the internal resistor  810 . The di/dt of the current I CHIP  is therefore affected by the values chosen for the resistor  714  and the capacitor C 12 .  
         [0056]    A circuit for controlling the current I HOOK  is shown in FIG. 9. As with the I DCT ′ and the I CHIP  currents, the current I HOOK  may be ramped down in response to the on-hook control signal  504 . As shown in FIG. 8, a Vc supply voltage level is provided to the phone line side DAA integrated circuit  1802 B at the QE 2  pin. The op amp  920  and associated circuitry coupled to the QE 2  and QB pins (internally and externally) generate a feedback loop which maintains QE 2  at approximately 2.6 V during off-hook conditions. Also shown in FIG. 9 are current sources  908 ,  910 ,  912  and  914 . Current sources  908  and  910  are proportional to the currents I DCT  and I DCT ′ respectively as shown. Thus as current source I DCT ′ is ramped downward as described above, current source  910  will be ramped downward. The current source  914  is a bias current I BIAS  that is generated from a bias device that is controlled such as transistor  802  of FIG. 8 is controlled, and therefore bias current I BIAS  will ramp downward similar to the manner that current source I CHIP  ramps downward as described with reference to FIG. 8. Switches  902 ,  904 , and  906  are responsive to a hookswitch transition signal such as the on-hook control signal  504 . More particularly, during on-hook states switches  902  and  906  are opened and switch  904  is closed. During off-hook states, switches  902  and  906  are closed and switch  904  is opened.  
         [0057]    In operation, the circuit of FIG. 9 operates such that in response to on-hook control signal  504  the switches  902 ,  904 , and  906  will switch states. Further, the current sources  910  and  914  will begin to ramp downward. This circuit changes will cause I HOOK  to begin to drop. For example with an off-hook loop current of 100 mA, I HOOK  may be 4 mA and Q 2  (see FIG. 6) may be saturated. As I HOOK  drops toward zero, Q 2  will first enter an active region and then reach an off state. At this point the hookswitch will be open.  
         [0058]    The operation of the switches  702 ,  704 ,  804 ,  806 ,  902 ,  904 , and  906  in relation to the off-hook or on-hook state may be seen with respect to FIG. 10. In FIG. 10, the hookswitch is idealized to being either on or off. FIG. 10A illustrates the embodiment discussed herein in which the hookswitch may include a transistor Q 2  which transitions from a saturation region (off-hook), to an active region to an off state (on-hook). As shown in FIG. 10, a on-hook control signal  504  changes from a state indicating on-hook conditions are desired to a state indicating off-hook conditions are desired at time t 1 . At that time switches  702 ,  804 , and  904  change from closed to opened states and switches  704 ,  806 ,  902  and  906  change from open to closed states. The hookswitch also changes from being opened to being closed and the loop current I LOOP  rises as shown in FIG. 10. At time t 2  the on-hook control signal  504  changes to the on-hook control state. At that time switches  702 ,  804 , and  904  change from opened to closed states and switches  704 ,  806 ,  902  and  906  change from closed to open states. At time t 2  the loop current I LOOP  begins to ramp down over the period t RAMP  as shown in FIG. 10. At time t 3  the hookswitch is completely opened and the loop current I LOOP  drops to zero.  
         [0059]    As mentioned above, FIG. 10 conceptualizing the hookswitch as being instantaneously on or off. As shown in FIG. 10A, a timing diagram is shown in which a hookswitch includes a transistor such as transistor Q 2  which transitions from saturation to active to off states. FIG. 10A illustrates the on-hook control signal  504  switching to the off-hook state at time t 1  and then switching to the on-hook state at time t 2 . The transistor Q 2  is in saturation between times t 1  and t 3 . The transistor Q 2  enters an active state between times t 3  and t 4  and finally enters an off state at time t 4 . An exemplary time ranges for the time t 3 -t 2  may be 0-100 usec. An exemplary time range for the time t 4 -t 3  (i.e. the time in the active state) may be 1-5 msec. With a 100 mA off-hook loop current, exemplary values for I HOOK  may be 4 mA at point  988 , 8 uA at point  990  and 7 uA at point  992 . At point  992  the value of I HOOK  will have dropped sufficiently low that the supply voltage will collapse and the current through the hookswitch will then drop to close to zero. Exemplary values for I CHIP  may be 5 mA at point  980  and dropping close to zero at point  982  according to a time constant affected by the value of capacitor C 13 . Exemplary values for I DCT ′ may be 85 mA at point  984  and dropping close to zero at point  986  according to a time constant affected by the value of capacitor C 12 . Thus, it may be seen that the substantial components of the loop current may be substantially decreased (individually and collectively) between the time t 2  at which the on-hook control signal  504  changes and time t 4  when the hookswitch is completely open.  
         [0060]    Further modifications and alternative embodiments of this invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the manner of carrying out the invention. It is to be understood that the forms of the invention herein shown and described are to be taken as the presently preferred embodiments. Various changes may be made in the shape, size and arrangement of parts. For example, equivalent elements may be substituted for those illustrated and described herein, and certain features of the invention may be utilized independently of the use of other features, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Moreover, the various aspects of the inventions disclosed herein may be used in combination or separately as will also be apparent to those skilled in the art.