Patent Publication Number: US-6667604-B2

Title: Power supply circuit with continued power generation after switch turn-off

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-227680 filed on Jul. 27, 2001. 
     FIELD OF THE INVENTION 
     The present invention relates to a power supply circuit utilized for a vehicular control device and continues power generation after switch turn-off. 
     BACKGROUND OF THE INVENTION 
     In a power supply circuit used for a vehicular control device, large capacitance is required for an output capacitor in a constant voltage circuit. Moreover, connectors in the power supply circuit may cause problems due to inadequate connection. 
     A power supply circuit that counters the above problem is proposed in U.S. Pat. No. 6,084,384 (JP-A-11-266547). As shown in FIG. 4, this power supply circuit includes a battery  61 , the first power supply line BATT, the second power supply line VB and a switching device  62 , which includes an ignition switch. The first power supply line BATT is continuously supplied with power from the battery  61 . The second power supply line VB is supplied with power from the battery  61  only when the switching device  62  is closed. A primary constant voltage circuit  63  is connected to the second power supply line VB and an auxiliary constant voltage circuit  64  is connected to the first power supply line BATT. The constant voltage circuits  63  and  64  supply power to the third power supply line DL. 
     The primary circuit  63  includes a transistor  63   a  and a constant voltage control IC  63   b . The auxiliary circuit  64  includes a transistor  64   a  and a constant voltage control IC  64   b . A halt control circuit  65  is provided to continue operations of the auxiliary circuit  64  for a predetermined period after power supply to the second power supply line VB is cut off. 
     In the power supply circuit, power is normally supplied from the primary circuit  63  to the third power supply line DL. After the switching device  62  is opened, the power is supplied from the auxiliary circuit  64  to the third power supply line DL. If the second power supply line VB is momentarily shut down, a constant voltage is supplied by the auxiliary circuit  64 . The output voltage of the auxiliary circuit  64  is adjusted lower than that of the primary circuit  63 . 
     However, when the power supply circuit is implemented on an IC chip, a voltage drop in output voltage may occur. When the switching device  62  is opened and constant voltage generation by the primary circuit  63  is halted, the auxiliary circuit  64  starts a constant voltage generation. During the period between the time that the switching device  62  is opened and the time that the auxiliary circuit  64  starts providing a sufficient voltage, a voltage drop may occur. The dropped voltage may trigger a low voltage reset. 
     As shown in FIG. 5, when the switching device  62  is switched from ON (closed) to OFF (opened), the primary circuit  63  enters the non-operating state and the auxiliary circuit  64  enters the operating state. It takes for a while until the output voltage of the auxiliary circuit  64  rises to a sufficient level. As a result, the output voltage of the power supply circuit drops during that period. 
     In some vehicular control devices, a plurality of constant voltage circuits are provided in a power supply circuit to generate constant voltages in each section. In recent years, the constant voltage is lowered to cut back power consumption of an onboard battery. For instance, a power supply circuit that produces different constant voltages for sensors and a CPU has been introduced. In such a power supply circuit, constant voltage generation for each constant voltage circuit is controlled by opening and closing a switching device such as an ignition switch. Since requirements for reduction in power consumption and for constant voltage variation will increase, it is preferable that the circuit configuration is more simplified. 
     SUMMARY OF THE INVENTION 
     The present invention therefore has an objective to provide a power supply circuit that implements desired constant voltage generation with a simple configuration. 
     The present invention has another objective to simplify the configuration of the power supply circuit that outputs a variety of constant voltages. 
     The power supply circuit of the present invention includes a constant voltage circuit provided in a power supply line that is continuously supplied with power from a power source. When a power supply switching device is closed, the constant power supply circuit performs constant voltage generation. The power supply circuit also includes a timer circuit. The timer circuit controls the constant voltage circuit to continue the constant voltage generation for a predetermined period (allowable period) after the switching device is opened. 
     If an instantaneous power interruption occurs and the interrupted period is shorter than the allowable period, the constant voltage generation continues. Therefore, the constant voltage is outputted without interruption. When the interrupted period reaches the allowable period, the constant voltage generation is halted. 
     The power supply circuit of the present invention does not require a large-capacitance output capacitor to handle the instantaneous power interruption. Since the power supply circuit of the present invention does not require the switching, the voltage drop is prevented. Therefore, a desired constant voltage generation by the simple power supply circuit is possible. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objectives, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
     FIG. 1 is a circuit diagram showing a power supply circuit according to the first embodiment of the present invention; 
     FIG. 2 is a timing chart showing operations of the power supply circuit according to the first embodiment of the present invention; 
     FIG. 3 is a circuit diagram showing a power supply circuit according to the second embodiment of the present invention; 
     FIG. 4 is a circuit diagram showing a power supply circuit according to the related art; and 
     FIG. 5 is a timing chart showing a voltage drop occurring in the power supply circuit of the related art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiments of the present invention will be explained with reference to the accompanying drawings. 
     [First Embodiment] 
     Referring to FIG. 1, a power supply circuit  10  is directly connected to a battery  1  to be continuously supplied with power from the battery  1  via the first power supply line BATT. It is also connected to the battery  1  through an ignition (IG) switch  2  to be supplied with power from the battery  1  via the second power supply line VB only when the IG switch  2  is closed. The IG switch  2  is a power supply switching device and can be other types of switching device such as a relay. 
     The power supply circuit  10  includes a constant voltage circuit  20  and a timer circuit  30 . A power voltage is inputted to the constant voltage circuit  20  via a diode  11  in the first line BATT or a diode  12  in the second line VB. Then, the constant voltage circuit  20  generates a predetermined constant voltage VOM (e.g., 5 V). An output capacitor  13  is provided for the constant voltage circuit  20 . 
     The constant voltage circuit  20  includes a pnp transistor  21  and a npn transistor  22 . The pnp transistor  21  is connected in the first line BATT. The npn transistor  22  is connected to the base of the pnp transistor  21 . The constant voltage VOM is generated by regulating a base current of the transistor  21  via the transistor  22 . An output terminal of an operational amplifier  24  is connected to the base of the transistor  22  via a switch  23 . Non-inverting and inverting terminals of the amplifier  24  are connected to a reference voltage supply  25  and a middle point of a voltage divider comprising resistors  26  and  27 , respectively. The resistors  26  and  27  are utilized to feed back a fraction of the constant voltage VOM to the amplifier  24 . 
     The timer circuit  30  includes a voltage divider constructed of resistors  31  and  32 . The voltage divider is connected to the second line VB. Non-inverting and inverting terminals of a comparator  33  are connected to a middle point of the voltage divider and a reference voltage supply  34 , respectively. An output of the comparator  33  is divided into two; one is directly inputted to an OR circuit  36  and the other is inputted to the OR circuit  36  via a delay circuit  35 . The switch  23  is opened or closed based on an output of the OR circuit  36 . In other words, constant voltage generation is controlled by the output of the OR circuit  36 . 
     The delay circuit  35  delays inputting an output signal of the comparator  33  to the OR circuit  36  for a certain period of time. A conventional circuit such as a flip-flop can be used for the delay circuit  35 . A delay time THD of the delay circuit  35  is set based on an instantaneous interruption tolerance dose or a capacity of the output capacitor  13 . The comparator  33  corresponds to a determination circuit for determining a condition of the switch  23  (open or closed). The OR circuit corresponds to a logic circuit that outputs a signal to halt constant voltage generation. 
     Referring to FIG. 2, the IG switch  2  is assumed to switch from OFF to ON and the voltage VB starts to increase from 0 V to 14 V at time t 1 . At time t 2 , the voltage VB reaches a threshold voltage Vth. As a result, a voltage measured at the non-inverting terminal of the comparator  33  exceeds the reference voltage and the output of the comparator  33  becomes high (H). At time t 2 , the output of the timer circuit  30  (OR circuit  36 ) becomes high (H). Because of this high signal, the switch  23  is closed and the constant voltage VOM starts rising to a higher level. The timing at which the output of the delay circuit  35  rises is later by the delay time THD (e.g., 20 msec) than the timing at which the output of the comparator  33  rises. 
     It is assumed that at time t 3 , the voltage VB is instantaneously interrupted due to the IG switch or power supply terminal connector having a connection failure. During the period of TA, the output of the comparator  33  becomes low. If the instantaneous power interruption is temporary and the period TA is shorter than the delay time THD, the output of the timer circuit  30  remains high and the constant voltage VOM generation continues. 
     When the IG switch switches to OFF at time t 4 , the voltage VB decreases. When the voltage VB becomes lower than the threshold voltage Vth at the time t 5 , the output of the comparator  33  becomes low. At the time t 6 , which is later by the delay time THD than the time t 5 , the outputs of the delay circuit  35  and timer circuit  30  become low. Because of this low signal, the switch  23  switches to OFF and the constant voltage generation is halted. Therefore, the constant voltage VOM falls to a low level. 
     In FIG. 2, the same threshold voltage Vth is used in both cases that the voltage VB is rising and falling for expediency. In real application, the threshold voltage Vth preferably has hysteresis. It is preferable to switch the threshold voltage Vth to a lower level for the rising voltage and to a lower lever for the falling voltage. 
     The power supply circuit  10  does not require a large-capacitance output capacitor to counter to the instantaneous power interruption. 
     The two power supply lines BATT and VB are provided to supply the battery voltage to the constant voltage circuit  20  via the diodes  11  and  12 . This makes the power supply circuit  10  highly reliable. 
     [Second Embodiment] 
     Referring to FIG. 3, a power supply circuit  40  has the same configuration as the power supply circuit  10  of the first embodiment except for a constant voltage circuit  50 . The first constant voltage circuit  20  generates the first constant voltage VOM (5 V) and the second constant voltage circuit  50  generates the second constant voltage VOS (3.3 V). The output capacitors for the constant voltage circuits  20  and  50  are not shown in FIG. 3 for convenience. 
     A power supply voltage necessary for constant voltage generation is inputted to the first and second constant voltage circuits  20  and  50 . The voltage is inputted to the circuit  20  and  50  via a power supply line BATT that is continuously supplied with power from the battery  1 . 
     The constant voltage circuit  50  has a pnp transistor  51  and a npn transistor  52 . The transistor  51  is connected in the first power supply line BATT and the transistor  52  is connected to the base of the transistor  51 . A constant voltage VOS (3.3 V) is generated by regulating a base current of the transistor  51  via the transistor  52 . An output terminal of an operational amplifier  54  is connected to the base of the transistor  52  via a switch  53 . A constant voltage supply is connected to the non-inverting terminal of the amplifier  54 . A middle point of the voltage divider constructed of resistors  56  and  57  is connected to the inverting terminal of the amplifier  54 . 
     The switch  53  is opened or closed based on the constant voltage VOM outputted from the constant voltage circuit  20 . If VOM=5 V, the switch  53  is closed and the constant voltage VOS is generated. If VOM=0 V, the switch  53  is opened and the generation of constant voltage VOS is halted. 
     In this embodiment, start and halt of the constant voltage generation by the constant voltage circuit  50  are appropriately controlled by the constant voltage VOM. Complicated configuration is not required to determine whether the constant voltage generation needs to be started or halted. Therefore, the power supply circuit  40  has simple configuration even though it outputs multiple constant voltages. 
     Since the power supply circuit  40  has the timer circuit  30 , the constant voltage generation continues in the case of instantaneous power interruption. The constant voltage circuit  50  can also have the same function. The power supply circuit does not require a complicated configuration to provide desired constant voltage generation. 
     The present invention should not be limited to the embodiment previously discussed and shown in the figures, but may be implemented in various ways without departing from the spirit of the invention. For instance, the constant voltages (first and second constant voltages) VOM and VOS can be the same voltages or different voltages. The constant voltage VOS can be more than one. In this case, a plurality of the constant voltage circuits having the same configuration as the constant voltage circuit  50  is provided as required. Start and halt of constant voltage generation by the constant voltage circuits are controlled based on the constant voltage VOM. 
     In FIGS. 1 and 3, the power supply lines BATT and VB are connected to the diodes  11  and  12 . The configuration can be modified as long as power required for constant voltage generation is supplied to the constant voltage circuit  20  via the first line BATT.