Abstract:
An indicator drive circuit includes a current mirror circuit including a first transistor located at a control side and a second transistor located at a controlled side, a current through the current mirror circuit being switched by an external control signal, a current source adapted to feed a control current to the first transistor of the current mirror circuit, and a switching element adapted to be driven by a current through the second transistor of the current mirror circuit and to switch on or off an indicator light source. The current source includes two transistors cascode-connected.

Description:
[0001]    The present application is based on Japanese patent application No. 2011-003829 filed on Jan. 12, 2011, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to an indicator drive circuit for turning on or off an indicator with a light source such as an LED. 
         [0004]    2. Description of the Related Art 
         [0005]    For example, in order to switching an indicator driven by a battery power supply of 12V by using a control signal of 5V system, a vehicle needs an indicator drive circuit for shifting the level of the control signal so as to control a switching element. A conventional indicator drive circuit uses, e.g., a MOS transistor with a high output as the switching element for turning on or off electrical current from the battery power supply, and a control part of 5V system controls the electrical current to be fed into a resistance between a gate and a source of the MOS transistor, so an LED connected to the MOS transistor is turned on/off (for example, see JP-A-06-326579). 
       SUMMARY OF THE INVENTION 
       [0006]    However, in the conventional indicator drive circuit, when the drive current of the switching element such as a power MOS transistor being operated by the battery power supply is fed through a current mirror circuit of the control part being operated by the 5V system, electrical current flowing through the current mirror circuit may vary in conjunction with the variation of the power supply voltage of the control part. Thus, the LED as a light source of the indicator may cause a fluctuation or flicker in brightness. 
         [0007]    Therefore, it is an object of the invention to provide an indicator drive circuit that prevents a light source of an indicator from flickering by variation of power supply voltage. 
         [0000]    (1) According to one embodiment of the invention, an indicator drive circuit comprises: 
         [0008]    a current mirror circuit comprising a first transistor located at a control side and a second transistor located at a controlled side, a current through the current mirror circuit being switched by an external control signal; 
         [0009]    a current source adapted to feed a control current to the first transistor of the current mirror circuit; and 
         [0010]    a switching element adapted to be driven by a current through the second transistor of the current mirror circuit and to switch on or off an indicator light source, 
         [0011]    wherein the current source comprises two transistors cascode-connected. 
         [0012]    In the above embodiment ( 1 ) of the invention, the following modifications and changes can be made. 
         [0013]    (i) One of the two cascode-connected transistors of the current source is turned off when the indicator light source is turned off. 
         [0014]    (ii) The two cascode-connected transistors include a unipolar transistor. 
         [0015]    (iii) The unipolar transistor includes a PMOS transistor. 
         [0016]    (iv) The two cascode-connected transistors include a bipolar transistor. 
         [0017]    (v) The indicator light source comprises a light-emitting diode (LED). 
       Points of the Invention 
       [0018]    According to one embodiment of the invention, an indicator drive circuit is constructed such that a current source to feed a control current into a current mirror circuit is composed of two PMOS transistors that are cascode-connected. Thereby impedance can be increased compared to a conventional circuit that the current source is composed of a single PMOS transistor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The preferred embodiments according to the invention will be explained below referring to the drawings, wherein: 
           [0020]      FIG. 1  is a circuit diagram showing an indicator drive circuit according to an embodiment of the invention; 
           [0021]      FIG. 2  is a circuit diagram showing a current source shown in  FIG. 1  in detail, in which two PMOS transistors are cascode-connected; and 
           [0022]      FIG. 3  is a graph showing DC characteristics when MOS transistors are cascode-connected. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    In the embodiment, with regard to constructional elements having the same function in each of the drawings, the same references will be used, and detail explanation will be omitted. 
       Configuration of Indicator Drive Circuit 
       [0024]      FIG. 1  is a circuit diagram showing an indicator drive circuit according to an embodiment of the invention. The indicator drive circuit  1  includes a control part  10  and an output part  20 . 
         [0025]    The control part  10  includes an inverter  11  connected to a control input terminal  13 , that operates with, for example, a power supply VCC of 5V, an NMOS transistor Q 1  that is driven by the inverter  11 , an npn transistor Q 2  that is one bipolar transistor constituting a current mirror circuit  200 , and PMOS transistors Q 3 , Q 4  that supplies a control current to the npn transistor Q 2  as a current source  100 . 
         [0026]    The inverter  11  to which the power supply VCC of 5V is connected via a wiring (not shown) inverts a control signal (L/H) input via the control input terminal  13  so as to output to the NMOS transistor Q 1 . 
         [0027]    To a gate of the NMOS transistor Q 1  that is source-grounded, an output of the inverter  11  is connected. A drain of the NMOS transistor Q 1  is connected to a collector of the npn transistor Q 2  that is emitter-grounded. Namely, the control current that the current source  100  allows to flow into the current mirror circuit  200  is switched via the NMOS transistor Q 1  that is switched on or off by the control signal. 
         [0028]    The current source  100  that allows the control current to flow into the current mirror circuit  200  has a configuration that two PMOS transistors Q 3 , Q 4  are cascode-connected. 
         [0029]    Namely, the current source  100  including two PMOS transistors Q 3 , Q 4  has a configuration that a drain of the PMOS transistor Q 3  and a source of the PMOS transistor Q 4  are connected so that the PMOS transistors Q 3 , Q 4  are arranged in series, and the power supply VCC is connected to a source of the PMOS transistor Q 3  via a power supply terminal  12 . In addition, a drain of the PMOS transistor Q 4  is connected to a drain of the NMOS transistor Q 1  and a collector of the npn transistor Q 2 . 
         [0030]    A predetermined reference voltage REF is applied to a gate of the PMOS transistor Q 3 . The reference voltage REF is configured to have such electrical potential that the PMOS transistor Q 3  is switched always on, and is generated, for example, based on a zener voltage of a constant voltage circuit including a resistor  14  connected between the power supply VCC and a GND in series, and a zener diode  15 . 
         [0031]    An output of the inverter  11  is divergingly connected to a gate of the PMOS transistor Q 4  that is another PMOS transistor of the current source  100 . 
         [0032]    Here,  FIG. 2  is a circuit diagram showing the current source  100  shown in  FIG. 1  in detail, in which two PMOS transistors Q 3 , Q 4  are cascode-connected. As shown in  FIG. 2 , the power supply VCC is connected to back gates B of both PMOS transistors Q 3 , Q 4 . Due to this, the electrical potential just below a channel of the PMOS is fixed to a high potential of 5V. 
         [0033]    Further, the current source  100  can have a configuration that, for example, two pnp bipolar transistors are cascode-connected. 
         [0034]    Short circuit is caused between a base and a collector of the npn transistor Q 2 , and the base is connected to a base of an npn transistor Q 6  of an output part  20  explained next. Both of the two npn transistors Q 2 , Q 6  are emitter-grounded, thereby the current mirror circuit  200  is configured, in which electrical currents flowing along the transistors Q 2 , Q 6  coordinate with each other. Further, if current amplification factors of the npn transistors Q 2 , Q 6  are equal to each other, it results in that the same amount of electrical current as that supplied from the two PMOS transistors Q 3 , Q 4  flows into the npn transistor Q 6 . 
         [0035]    Further, here, of the two npn transistors Q 2 , Q 6  constituting the current mirror circuit  200 , the npn transistor Q 2  located at a side of the control part  10  is referred to as a transistor at a control side, and the npn transistor Q 6  located at a side of the output part  20  is referred to as a transistor at a controlled side. 
         [0036]    The output part  20  has a configuration that a battery power supply VBB of vehicles (for example, 12V) is connected thereto via a power supply terminal  22 , and an LED  30  that is a light source of the indicator and a resistor  31  that limits electrical current flowing along the LED  30  are connected to an output terminal  23 . In addition, the output part  20  includes a PMOS transistor Q 5  that is a switching element allowing the LED  30  to be switched on or off and the above-mentioned npn transistor Q 6  at the controlled side constituting the current mirror circuit  200 . 
         [0037]    The battery power supply VBB is connected to a source of the PMOS transistor Q 5  that is a switching element so that a drive current from the battery of vehicles is supplied thereto. In addition, a resistor  21  between a gate and a source of the PMOS transistor Q 5  is pulled-up in the power supply line of the battery power supply VBB. A gate of the PMOS transistor Q 5  is connected to a collector of the npn transistor Q 6  at a controlled side of the current mirror circuit  200 . 
       Operation of the Indicator Drive Circuit 
       [0038]    When the control signal applied to the control input terminal  13  is a low level L (turn-off control), a signal of a high level H inverted by the inverter  11  is applied to a gate of the PMOS transistor Q 4  constituting the current source  100 , the PMOS transistor Q 4  is switched off. Due to this, the control current that the PMOS transistor Q 3  allows to flow into the npn transistor Q 2  at a control side of the current mirror circuit  200  is blocked. Consequently, the current does not flow into the npn transistor Q 6  at a controlled side of the current mirror circuit  200 . 
         [0039]    In addition, likewise, when the control signal is the low level L, the NMOS transistor Q 1  is switched on by a gate voltage of the high level H inverted by the inverter  11 . At this time, base currents of the two npn transistors Q 2 , Q 6  constituting the current mirror circuit  200  flow into the GND via the NMOS transistor Q 1 , thus both of the npn transistors Q 2 , Q 6  are switched off. 
         [0040]    As mentioned above, by the control signal of the low level L, the npn transistor Q 2  at a control side of the current mirror circuit  200  is switched off, and at the same time the control current from the current source  100  is also blocked. Consequently, an electrical current does not flow into the npn transistor Q 6  at a controlled side of the current mirror circuit  200  and the resistor  21 , thereby the PMOS transistor Q 5  is switched off so as to switch off the LED  30 . 
         [0041]    Further, in the embodiment, the PMOS transistor Q 4  that is one of PMOS transistors cascode-connected is switched off by the signal of the high level H obtained by inverting the control signal of the low level L at the time of the light-out control, thus even if a noise current is induced in the control part  10  by, for example, an influence of Electromagnetic Interface (EMI) or the like, the current mirror circuit  200  is not affected. Consequently, it can be prevented that the LED  30  erroneously lights up at the time of the light-out control by the influence of EMI or the like. 
         [0042]    On the other hand, when the control signal applied to the control input terminal  13  is a high level H (turn-on control), a signal of a low level L inverted by the inverter  11  is applied to the gate of the PMOS transistor Q 4 , and the PMOS transistor Q 4  is switched on. At the same time, the NMOS transistor Q 1  is switched off by a gate voltage of a low level L that the inverter  11  outputs. 
         [0043]    At this time, a predetermined control current is supplied to the npn transistor Q 2  at a control side of the current mirror circuit  200  via the two PMOS transistors Q 3 , Q 4  that are cascode-connected. Consequently, the same amount of electrical current flows also into the npn transistor Q 6  at a controlled side, and an on-voltage is generated in the resistance  21  between the gate and source of the PMOS transistor Q 5 . The PMOS transistor Q 5  that is a switching element is switched on, thereby a current-carrying circuit is formed between the battery power supply VBB and the LED  30  so as to switch off the LED  30 . 
       Effects of the Embodiment 
       [0044]    According to the above-mentioned indicator drive circuit  1 , the current source  100  that allows the control current to flow into the current mirror circuit  200  is configured by the two PMOS transistors Q 3 , Q 4  that are cascode-connected, thereby impedance can be increased in comparison with a conventional circuit in which, for example, the current source is configured by a single PMOS transistor Q 3 . 
         [0045]    Here,  FIG. 3  is a graph showing DC characteristics (relationship between a drain voltage Vd and a drain current Id) in the case that the two MOS transistors are cascode-connected. Further, in  FIG. 3 , for comparison, the DC characteristics in case of a single MOS transistor are shown by a dashed line. As shown in  FIG. 3 , it is understood that the MOS transistors cascode-connected has larger impedance (resistance component ΔVd/ΔId in  FIG. 3 ) in a saturated region than the single MOS transistor. 
         [0046]    Thus, even if a voltage of the power supply VCC shifts in the form of DC, by the large resistance component of the two PMOS transistors Q 3 , Q 4  cascode-connected, an amount of variation of the control current supplied to the npn transistor Q 2  at a control side can be reduced in comparison with the conventional case. In addition, even if an AC noise is generated in the power supply VCC, by the large inductance component of the two PMOS transistors Q 3 , Q 4  cascode-connected, variation of the current supplied to the npn transistor Q 2  at a control side can be reduced in comparison with the conventional case. 
         [0047]    Consequently, an influence of voltage variation of the power supply VCC on the current that flows along the current mirror circuit  200  can be reduced, and it can be prevented that the LED  30  that is a light source of an indicator fluctuates or flickers in brightness by variation of the power supply VCC. 
         [0048]    Although the invention has been described with respect to the specific embodiments for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.