Abstract:
A power supply circuit for a vehicle audio system supplies a regulated voltage to a control unit through a series diode and full battery voltage to a power amplifier through a normally-off series power-FET. When the audio system is activated, the control unit turns on the series power-FET to permit operation of the power amplifier. If the control unit detects a sudden voltage drop or an engine crank signal, it prevents audible loudspeaker sounds by immediately turning off the series power-FET and issuing a shut-down command to the power amplifier. If the battery voltage is reversed, the series diode isolates the control unit, and the series power-FET remains off.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a vehicle audio system, and more particularly to a power supply circuit therefor. 
       BACKGROUND OF THE INVENTION 
       [0002]    The audio system in a motor vehicle is subject to various power fluctuations and irregularities that have the potential to damage the system circuitry and/or cause undesired system operation. For example, the system must be protected from reverse voltages that occur when the battery cables are incorrectly installed. And power fluctuations that produce audible loudspeaker sounds can occur with little or no warning. Accordingly, what is needed is a power supply arrangement that protects the audio system and eliminates undesired system operation. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention is directed to an improved power supply circuit for an audio system including a power amplifier that operates at full battery voltage and a control unit that operates at a lower regulated voltage. Battery voltage is supplied to the control unit voltage regulator through a series diode and to the power amplifier through a normally-off series power-FET. When the audio system is activated, the control unit turns on the series power-FET to permit operation of the power amplifier. If the control unit detects a sudden voltage drop or an engine crank signal, it prevents audible loudspeaker sounds by immediately turning off the series power-FET and issuing a shut-down command to the power amplifier. If the battery voltage is reversed, the series diode isolates the control unit, and the series power-FET remains off. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a diagram of a vehicle electrical system including an audio system having a microprocessor-based control unit and a power supply circuit according to this invention; 
           [0005]      FIG. 2  is a flow diagram representing an operation of the control unit of  FIG. 1  according to this invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0006]    Referring to the drawings, and particularly to  FIG. 1 , the reference numeral  10  generally designates a portion of a vehicle electrical system, including a 12-volt storage battery  12 , an engine starter motor  14 , a start switch  16 , and an audio system  18 . The starter motor  14  and start switch  16  are depicted because closure of start switch  16  for engine cranking is typically the most severe load borne by battery  12 , and produces a sudden and substantial drop in the voltage supplied to audio system  18  by battery  12 . 
         [0007]    The audio system  18  includes a power supply circuit  20 , a power amplifier  22 , a voltage regulator  24 , a vacuum fluorescent (VF) display  26 , a microprocessor-based control unit  28 , a tuner  30 , and one or more loudspeakers  32 . The power supply circuit  20  supplies filtered battery voltage (i.e., 12-volts) to power amplifier  22 , voltage regulator  24 , and display  26 ; and voltage regulator  24  supplies a lower regulated voltage (5-volts, for example) to control unit  28  and tuner  30 . Battery voltage filtering is achieved by the combination of series inductor  34  and the filter capacitors  36 ,  38  and  40 . A power field-effect-transistor (FET)  42  couples the inductor  34  to filter capacitor  36  to provide a filtered 12-volt supply voltage for power amplifier  22  at terminal  44  when power FET  42  is conductive (on). A first power diode  46  couples inductor  34  to filter capacitor  38  to provide a filtered 12-volt supply voltage for voltage regulator  24  at terminal  48 ; and a second power diode  50  couples inductor  34  to filter capacitor  40  to provide a filtered 12-volt supply voltage for display  26  at terminal  52 . 
         [0008]    The power FET  42  is a normally-off device, and control unit  28  selectively biases FET  42  on through driver circuit  54  to provide a low-resistance circuit path between inductor  34  and terminal  44 . In this way, power amplifier  22  can be electrically isolated from storage battery  12  when the audio system  18  is not activated and when the battery voltage suddenly drops, due to activation of starter motor  14  for example. In the case of a sudden drop in battery voltage, isolating terminal  44  from storage battery  12  prevents filter capacitor  36  from discharging through the starter motor  14 , and provides sufficient reserve energy to enable power amplifier  22  to carry out its power-down function. The first power diode  46  and filter capacitor  38  perform a similar function for voltage regulator  24 , as do the second power diode  50  and filter capacitor  40  for display  26 . Theoretically, of course, a third power diode could be used in place of the FET  42 , but the voltage drop across FET  42  is much less than the voltage drop across a forward-biased diode, so that substantially full battery voltage is available to power amplifier  22  at terminal  44  during operation of the audio system  18 . Additionally, the maximum current requirement of power amplifier  22  would produce significantly higher heat dissipation in a diode than in FET  42  due to its low on-resistance. On the other hand, the input voltage and current requirements of voltage regulator  24  and display  26  are substantially less than that of power amplifier  22 , so that the forward voltage drops and heat dissipation of diodes  46  and  48  do not pose significant design considerations. Additionally, FETs could theoretically be used in place of the second and third power diodes  46  and  50 , but the power diodes  46  and  50  function adequately at a substantially lower cost. 
         [0009]    In some cases, the power diode  50  may be omitted to reduce the power supply cost, particularly in applications where it is not important for the VF display  26  to be operative during engine cranking. Also, if a liquid-crystal-diode (LCD) display is used in place of the VF display  26 , it can be powered by the voltage regulator  24 ; in this case, the second power diode  50  and capacitor  40  can be omitted. 
         [0010]    The audio system  18  operates in one of two modes depending on the state of the vehicle ignition switch (not shown). When the ignition switch is off, voltage regulator  24  supplies control unit  28  a low current stand-by operating voltage (SB) on line  56 , sufficient to supply time-of-day data to display  26  via line  58 . In the stand-by mode, the other functions of audio system  18  are inactive, and the power FET  42  remains in its normal off state. Reverse battery voltage occurring during the stand-by mode is isolated from power amplifier  22  by FET  42 , and from voltage regulator  24  and display  26  by the first and second power diodes  46  and  50 , respectively. When the ignition switch is on (run or accessory), voltage regulator  24  supplies operating voltage to tuner  30  via line  60 , and main power (MP) to control unit  28  via line  62 , to transition from the stand-by mode to the on mode. If the audio system  18  is activated, control unit  28  turns on FET  42  via driver  54 , sends a frequency signal to tuner  30  via line  64  if appropriate, and suitably activates display  26  via line  58 . The tuner output is supplied to power amplifier  22  via line  66 , and power amplifier  22  drives the loudspeakers  32  accordingly. When a shut-down (SD) function is desired, control unit  28  issues a “Shut-Down” command to power amplifier  22  via line  68  to activate muting and power-down functions. 
         [0011]    The flow diagram of  FIG. 2  represents a software routine periodically executed by control unit  28  during the on mode of audio system  18 . The battery terminal voltage VBAT (or a signal ratiometrically related to VBAT) is supplied to an A/D input port of control unit  28 , and block  70  of the flow diagram depicted in  FIG. 2  monitors the input signal to detect a sudden negative-going voltage transition such as can occur when the starter motor  14  is activated. If the voltage drop condition is detected, block  70  is answered in the affirmative and block  72  is executed to turn off FET  42 , and to signal power amplifier  22  to activate the Shut-Down function. As mentioned above the filter capacitor  36  provides sufficient reserve energy to enable power amplifier  22  to respond by muting the audio output to loudspeakers  32  and performing a controlled power down. When the voltage drop condition is no longer True, block  70  is answered in the negative and block  74  is executed to turn on FET  42  to resume normal operation of audio system  18 . Alternatively or additionally, a crank switch input (CR) may be supplied to control unit  28  as indicated in phantom in  FIG. 1 ; when a state change of the CR input indicates closure of crank switch  16 , block  72  is executed as described above to turn off FET  42  and command the shut-down of power amplifier  22 . 
         [0012]    In summary, the power supply circuit  20  provides a simple and cost-effective arrangement for protecting audio system  18  and eliminating undesired system operation due to supply voltage anomalies. While the invention has been described with respect to the illustrated embodiment, it is recognized that numerous modifications and variations in addition to those mentioned herein will occur to those skilled in the art. Accordingly, it is intended that the invention not be limited to the disclosed embodiment, but that it have the full scope permitted by the language of the following claims.