Voltage regulator provided with a current limiter

[0002] The invention relates to a voltage regulator for converting an input voltage, which may be affected by a ripple, into an output voltage which is substantially not affected by a ripple, comprising an input terminal for receiving the input voltage, an output terminal for supplying the output voltage in response to the input voltage, and current limiting means for limiting the maximum absolute value of an output current supplied from the output terminal.

[0003] Such a voltage regulator is known from Japanese patent abstract JP 2-136029 A. The known voltage regulator comprises a current mirror with an input and an output and, a bipolar transistor whose base is connected to the current mirror and whose emitter forms the output terminal of the voltage regulator. The known voltage regulator further comprises a voltage divider which consists of two resistors connected in series. The voltage divider is connected between the emitter of the bipolar transistor and a supply voltage terminal. The known voltage regulator further comprises a comparator, a first current source which supplies a comparatively small current, and a second current source which supplies a comparatively large current. A switch is connected in series with the second current source. The comparator is connected by a first input to the common junction point of the two resistors connected in series, and is connected by a second input to a reference voltage source, and is connected by an output to a control electrode of the switch. In a normal operational state of the voltage regulator, the switch is in the conducting state. The current supplied to the input of the current mirror in that case is determined by the sum of the currents supplied by the first and the second current source. This current is delivered from the output of the current mirror to the base of the bipolar transistor. The bipolar transistor amplifies this current and delivers the amplified current to the voltage divider. As the current through the voltage divider rises, the voltage at the first input of the comparator will become greater than the voltage at the second input of the comparator at a given moment. As a result of this, the voltage at the output of the comparator changes, such that the switch switches from the conducting state to a non-conducting state. In this state, the current supplied to the input of the current mirror is dependent on the first current source only. As a result, the current supplied from the output of the current mirror is reduced, so that the current supplied from the emitter of the bipolar transistor to the voltage divider is limited.

[0004] A disadvantage of the known voltage regulator is that the current limitation is achieved in a comparatively complicated manner.

[0005] It is an object of the invention to provide a voltage regulator which reduces the above mentioned disadvantage.

[0006] According to the invention, the voltage regulator mentioned in the opening paragraph is for this purpose characterized in that the current limiting means comprise a current limiting transistor with a main current path, and in that the current limiting means are designed such that, if the voltage across the main current path is higher than a given threshold voltage of the current limiting transistor, at which the current limiting transistor acts as a current source, the maximum absolute value of the output current is limited.

[0007] The invention is based on the recognition that the transistor is in its linear operational range as long as a voltage across the main current path of a transistor lies below a certain limit, so that the transistor behaves as a resistor, and on the recognition that, as the voltage across the main current path rises, there comes a moment when the voltage across the main current path exceeds said limit, so that the transistor starts behaving as a current source. The transistor thus acts as a current limiting transistor. The current limiting transistor may be constructed, for example, with a field effect transistor. When the drain-source voltage of the field effect transistor is smaller than the difference between the gate-source voltage and the so-called threshold voltage V_t, the field effect transistor is in its linear operational range. When the drain-source voltage of the field effect transistor is higher than the difference between the gate-source voltage and the so-called threshold voltage Vt, the field effect transistor is in its saturation range, wherein the field effect transistor acts as a constant-current source. The current limiting transistor may alternatively be constructed with a bipolar transistor. When the collector-emitter voltage of the bipolar transistor is below the so-called saturation voltage, the transistor is in saturation and behaves more or less as a resistor. When the collector-emitter voltage of the bipolar transistor is greater than the so-called saturation voltage, the bipolar transistor is not in the saturated state. The bipolar transistor then acts as a constant-current source.

[0008] Further advantageous embodiments of the invention are defined in claims 2 and 3.

[0009] The invention will be explained in more detail with reference to the attached drawing, in which:

[0010] FIG. 1 is a circuit diagram of a first embodiment of a voltage regulator according to the invention; and

[0011] FIG. 2 is a circuit diagram of a second embodiment of a voltage regulator according to the invention.

[0012] Corresponding components or elements have been given the same reference symbols in these Figures.

[0013] FIG. 1 shows a circuit diagram of a first embodiment of a voltage regulator according to the invention. The voltage regulator is supplied from a supply voltage source SV which is connected between a supply voltage terminal V_SSand a further supply voltage terminal V_DD. The voltage regulator has an input terminal 1 for receiving an input voltage V_iand an output terminal 2 for supplying an output voltage V_oin response to the input voltage V_i. A load Z_Lis connected between the output terminal 2 and the supply voltage terminal V_SS. An output current I_osupplied from the output terminal 2 flows through the load Z_L. The voltage regulator further comprises a first current mirror CM₁with field effect transistors T₁₁and T₁₂, a second current mirror CM₂with field effect transistors T₁₃and T₁₄, field effect transistors T₁to T₈, current limiting field effect transistor T_CL, tail resistor R_TL, and a voltage divider which is implemented with a series arrangement of a resistor R₁and a resistor R₂, which series arrangement is connected between the output terminal 2 and the supply voltage terminal V_SS. The gates of transistors T₁₁and T₁₂and the drain of transistor T₁₁are interconnected and form the input of the first current mirror CM₁. The drain of transistor T₁₂forms the output of the first current mirror CM₁and is connected to the output terminal 2. The sources of transistors T₁₁and T₁₂are interconnected and form a reference connection point of the first current mirror CM₁and are connected to the input terminal 1. The gates of transistors T₁₃and T₁₄and the drain of transistor T₁₃are interconnected and form the input of the second current mirror CM₂. The drain of transistor T₁₄forms the output of the second current mirror CM₂and is connected to the input of the first current mirror CM₁. The sources of transistors T₁₃and T₁₄are interconnected and form a reference connection point of the second current mirror CM₂. The sources of transistors T₆, T₇, T₈and of current limiting transistor T_CLare connected to the supply voltage terminal V_SS. The drain of transistor T₆and the gates of transistors T₆, T₇, T₈and of the current limiting transistor T_CLare connected to a current reference terminal T₁₃. The drain of current limiting transistor T_CLis connected to the reference connection point of the second current mirror CM₂. The sources of transistors T₃, T₄, and T₅are connected to the further supply voltage terminal V_DD. The drain of transistor T₃and the gates of transistors T₃and T₄are connected to the drain of transistor T₁. The gate of transistor T₁is connected to a voltage reference terminal V_RF. The source of transistor T₁is connected to the drain of transistor T₇. The drain of transistor T₄and the gate of transistor T₅are connected to the drain of transistor T₂. The source of transistor T₂is connected to the drain of transistor T₈. The tail resistor R_TLis connected between the source of transistor T₁and the source of transistor T₂. The gate of transistor T₂is connected to the common junction point of the resistors R₁and R₂.

[0014] The circuit operates as follows. The voltage across the resistor R₂is controlled so as to be equal to the reference voltage which is offered between the voltage reference terminal V_RFand the supply voltage terminal V_SS. As a result of this, the output voltage V_obetween the output terminal 2 and the supply voltage terminal V_SSis equal to said reference voltage multiplied by the sum of the values of the resistors R₁and R₂and divided by the value of resistor R₂. Since the reference voltage is free from ripple, the output voltage V_ois also free from ripple. The ripple which may be present on the input voltage Vi accordingly does not extend itself to the output voltage V_o. For an optimum operation, however, the input voltage V_ishould always be greater than the output voltage V_o. As long as the voltage regulator is in a normal operating condition, i.e. no current limitation takes place, the output current I_owill rise as the impedance of the load Z_Ldecreases. In this normal operating condition, the current limiting transistor T_CLis in its linear operating range. The current limiting transistor T_CLthus acts as a resistor. As the output current I_orises, there will come a moment when the voltage U between the drain and the source of the current limiting transistor T_CLbecomes so great that the current limiting transistor T_CLchanges from its linear operating range to its so-called saturation region. The current limiting transistor T_CLacts as a constant current source as a result of this. The current which is supplied by transistor T₅cannot be controlled upwards any further because in that case the potential at the drain of transistor T₅will rise quickly, which will render the source-drain voltage of transistor T₅so low that the transistor T₅changes from the saturation region to the linear operating region. Since the current to the input of the second current mirror CM₂is limited thereby, the output current I_ois also limited via the second current mirror CM₂and via the first current mirror CM₁. The tail resistor R_TLserves to improve the stability of the voltage regulator, so that there is no risk of undesirable oscillations occurring.

[0015] FIG. 2 shows a circuit diagram of a second embodiment of a voltage regulator according to the invention. An advantage of this second embodiment over the first embodiment of FIG. 1 is that the reference voltage between the voltage reference terminal V_RFand the supply voltage terminal V_SSmay be chosen to be lower. For this purpose, all transistors having a p-conductivity type are replaced by transistors having an n-conductivity type, except for transistors T₁₁and T₁₂, and all transistors having an n-conductivity type are replaced by transistors having a p-conductivity type. A third current mirror CM₃is added, composed with field effect transistors T₁₅and T₁₆. The drain of transistor T₁₅and the gates of transistors T₁₅and T₁₆are interconnected and form the input of the third current mirror CM₃, which is connected to the output of the second current mirror CM₂. The drain of transistor T₁₆forms the output of the third current mirror CM₃and is connected to the input of the first current mirror CM₁. The sources of transistors T₁₅and T₁₆are interconnected and form a reference connection terminal of the third current mirror CM₃, which is connected to the supply voltage terminal V_SSof the voltage regulator.

[0016] The operation of the circuit of FIG. 2 is equivalent to the operation of the circuit of FIG. 1.

[0017] A further advantage of a voltage regulator according to the invention is that the output voltage V_ocan be substantially equal to the input voltage V_i.

[0018] The differential pair T₁, T₂may be replaced by some other type of differential stage, for example a cascoded differential stage. The voltage regulator may either be constructed from discrete components or be implemented in an integrated circuit. The voltage regulator may be constructed with field effect transistors as well as with bipolar transistors. A combination of field effect transistors and bipolar transistors may also be used. It is also possible to replace all p-type transistors with n-type transistors, provided all n-type transistors are replaced with p-type transistors at the same time.