Abstract:
A substrate bias generator in which a junction point of the capacitance and the diode of a charge pump is connected to the ground point of the circuit (and of the further circuit on the substrate for which the bias is generated) via two or more series-connected transistors. During the charging period of the capacitance the transistors are (fully) conductive, hence the capacitance is optimally charged as the conductive transistors cause no (or hardly any) voltage drop. During the pumping cycle all transistors are diode-connected, to bring about a negative voltage with respect to the ground point at the junction point. This negative voltage is limited to the sum of the threshold voltages of the diode-connected transistors.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a circuit for generating a bias voltage for another circuit which is integrated on a semiconductor substrate. The first-mentioned circuit comprises an oscillator for generating control pulses and at least one charge pump to which electrical pulses derived from the control pulses are applied. The charge pump comprises a series arrangement of a capacitance and a diode. The electrical pulses are applied to a first electrode of the capacitance, whose second electrode is connected to the diode associated with the capacitance. An output of the charge pump is connected to the substrate and the junction point of the capacitance and the diode of the charge pump is connected to the ground point of the integrated circuit via a channel of an insulated-gate switching transistor whose gate is connected to a control circuit which receives the control pulses. 
     Such a circuit is known from U.S. Pat. No 4,438,346. In the prior art circuit, the control electrode of the transistor, which connects the junction point of the capacitance and the diode of the charge pump to the ground point, is connected to a junction point of two series-arranged, diode-connected transistors which interconnect the ground point and a junction point carrying the negative substrate voltage. Hence, the control electrode is at a negative potential when there are no control pulses, thus causing the transistor to remain in the cut-off state if the voltage at the junction point in the charge pump decreases to a value which lies more than one threshold voltage of said transistor below ground potential. Thus, during a pumping cycle efficient use is made of the charge stored in the capacitance. However, in order to charge the capacitance, the negatively-biassed transistor must be rendered conductive. In said circuit this is achieved by means of control pulses which are applied to the control electrode of the transistor via a capacitor and which exceed the supply voltage. 
     For generating such control pulses, a relatively complex control circuit is needed in which the required voltage levels of the control pulses can be generated by means of bootstrap techniques. 
     However, the said U.S. Patent also describes steps, whereby the control pulses, generated by the relatively complex control circuit, are no longer needed. The control electrode of the switching transistor is connected to the ground point via the junction point of the capacitance and the diode of the charge pump. However, this circuit, which is known per se, has the disadvantage that the capacitance is charged to a maximum of V DD  -2 V TH  (V DD  is the supply voltage and V TH  is the threshold voltage of the field-effect transistors. The capacitance is usually formed by interconnecting the main electrodes of a field-effect transistor). However, at this low supply voltage the charge pump cannot pump much charge (or no charge at all if V DD  &lt;2 V TH ). 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a circuit for generating a substrate bias which does not require a complicated control circuit for generating control pulses of relatively high amplitude (for example, higher than the supply voltage) and which comprises a charge pump which operates efficiently, even at a relatively low supply voltage (for example, fractionally higher than 2 V TH ). 
     For that purpose, the invention is characterized in that the switching transistor is connected in series with at least another (i.e. a second) switching transistor whose insulated-gate electrode receives the electrical pulses for the charge pump, the control pulses being applied to the gate electrode of the first-mentioned switching transistor after having been inverted by the control circuit. The control circuit connects the gate electrode of the first-mentioned switching transistor to its main electrode (source) when a control pulse is applied to the control circuit. In the circuit in accordance with the invention, the capacitance of the charge pump is charged to V DD  -V TH , which is advantageous, especially, at a relatively low supply voltage (for example, 2 or 3 V TH ). During the pumping cycle of the charge pump a voltage to -2 V TH  can be generated because two transistors, which are diode-connected during the pumping cycle, are arranged in series. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will now be described, by way of example, with reference to the accompanying drawing, in which: 
     FIG. 1 is an embodiment of the invention, and 
     FIG. 2 is a further embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A circuit for generating a substrate bias, as shown in the relevant Figure, comprises an oscillator 10 for the generation of control pulses, a first and a second charge pump 1 and 2, respectively, and a control circuit 3. Oscillator 10 is a ring oscillator and it comprises seven, known inverting amplifier stages 10a, b, c, d, e, f and g, which each comprise two complementary field-effect transistors. The output of amplifier stage a is connected to a first electrode of a capacitance C1 of the first charge pump 1, which further comprises a diode-connected field-effect transistor N1 whose control electrode (gate) is connected to a main electrode (drain) and to an output A. Output A of the circuit is connected to the substrate (not shown) on which a further integrated circuit has been provided, for which further circuit the negative substrate bias V BB  appearing at output A is generated. Junction point B of capacitance C1 and transistor N1 is connected to the ourput of charge pump 2 which comprises a capacitance C2 and a transistor N2. Transistor N2 is diode-connected in known manner and capacitance C2 receives electrical pulses which appear at the output of the amplifier stage 10b. Hence, capacitances C1 and C2 receive (control) pulses which are substantially in phase opposition. 
     Junction point C of capacitance C2 and transistor N2 is connected to ground point M via two series-connected transistors N3 and N4. A source electrode of transistor N4 is connected to ground point M and the gate electrode is connected to the output of the amplifier stage 10b. A main electrode (drain) of transistor N3 is connected to junction point C, the other main electrode (source) of transistor N3 and the main electrode (drain) of transistor N4 are connected to a junction point D. The control electrode (gate) of transistor N3 is connected to the output of control circuit 3 which comprises an inverting amplifier with two complementary transistors P1 and N5, The input of this inverting amplifier is connected to the output of the amplifier stage 10a. The source electrode of transistor P1 is connected to the supply voltage V DD  and the source electrode of transistor N5 is connected to junction point D. 
     The circuit shown operates as follows. If the output of the amplifier stage 10a is at a low level (low potential), the output of control circuit 3 and the output of amplifier stage 10b will be at a high potential (just below V DD ). Due to the high potential at its control electrode, transistor N3 will be conductive as will the transistor N4 which receives the high output potential of amplifier stage 10b at its control electrode. Since transistors N3 and N4 are conductive, capacitance C2 will be charged. Capacitance C2 (and capacitance C1) is formed in known manner by a field-effect transistor whose main electrodes are interconnected. During the charging of capacitance C2, a charge Q is stored in the said capacitance, Q=C 2 . (V DD  -V TH ), where C2 is the value of capacitance C2, V DD  is the supply voltage, and V TH  is the threshold voltage of the transistor arranged to constitute the capacitance C2. As illustrated, the control electrodes of the transistors which are used as capacitances C1 and C2 are preferably connected to the relevant diode N2 or N1. Preferably, the capacitance C2 (and C1) is constituted by a P-channel transistor, the (inevitable) stray capacitances being connected to the output of amplifier stage 10b (and 10a, respectively) as shown in the drawing, and not to junction point C (and B). Consequently, they do not load charge pump 2 (and 1), which would be very disadvantageous. 
     The charging period of capacitance C2 ends as soon as the output level of amplifier stage 10a increases from a low potential to a high potential. Transistors P1 and N5 of control circuit 3 will be turned off and turned on, respectively, causing the control electrode and the source electrode of transistor N3 to be interconnected after the control electrode has been disconnected from the power supply V DD . The ratio of transistors P1 and N5 is chosen (for example, 2.5/10 and 2/2, respectively) so that the control electrode of transistor N3 is connected to the source electrode thereof prior to the pumping cycle of charge pump 2. The output level of amplifier stage 10b will decrease from a high potential to a low potential and, hence, connect, in effect, the control electrode of transistor N4 to ground point M. Junction point C of charge pump 2 is now connected to ground point M via two transistors N3 and N4 which are arranged as diodes. During the pumping cycle, which is effected when the potential at the output of amplifier stage 10b goes from a high to a low level, the potential at junction point C will decrease to a level below the ground potential (of ground point M) until the two series-arranged diodes N3 and N4 become conductive. Thus, the negative potential at junction point C is limited to -2 V THN , V THN  being the threshold voltage of the N-channel transistors N3 and N4. Further, charge pumps 1 and 2 cooperate in known manner so that they can generate a substrate bias of -2 V at a supply voltage V DD  of 2 V. 
     FIG. 2 shows a further embodiment of the invention which, apart from an additional part 3&#39;, is identical to the circuit shown in FIG. 1. For that reason, all corresponding components of FIGS. 1 and 2 bear the same reference numerals. In FIG. 2, an additional switching transistor N3&#39; has been provided between the switching transistors N3 and N4, and it is controlled in the same way as transistor N3. During the charging period of capacitance C2, the switching transistors N3&#39;, N3 and N4 are turned on. The output of amplifier stage 10a is at a low potential, hence the control electrodes of switching transistors N3 and N3&#39; are connected to the power supply V DD  via the P-channel transistors P1 and P1&#39;, respectively. If the output of amplifier stage 10a goes from a low to a high level, the transistors P1 and P1&#39; will be turned off and the transistors N5 and N5&#39; will be turned on. This will result in the control electrodes of switching transistors N3 and N3&#39; being connected to the respective source electrodes thereof so that junction point C is connected to ground point M via three diode-connected transistors N3, N3&#39; and N4. The additional part 3&#39; enables the potential at junction point C to decrease to -3 V TH  below ground point potential (M) during the pumping cycle. The use of such an additional part (or two, three etc.) is effective only when the supply voltage V DD  is such that |V DD  |≧|3 V TH  | (4 V TH  or 5 V TH  etc.), where V DD  is the supply voltage and 3 V TH  (4 V TH , 5 V TH ) is the (maximum) negative voltage of point C at which the three (four, five, etc.) series-arranged, diode-connected transistors (N3, N4, N3&#34;&#39;, (N3&#34;, N3&#34;) will become conductive during the pumping cycle. 
     A circuit for generating a substrate bias in accordance with the invention is used, preferably, in a circuit which is integrated in a semiconductor substrate, which circuit has been fabricated, at least in part, in an N-well on a P-type semiconductor substrate, and which must also remain operative at a low supply voltage of, for example, 2 V. Especially in the case of integrated static-memory circuits, comprising memory cells having high-value resistors and N-channel transistors, the use of the circuit in accordance with the invention is advantageous, as, because of this, the information content of the relevant memory cells is not disturbed by input signals which exhibit undersirable negative voltage peaks (for example, values to -1 or -1.5 V) as occur in TTL-circuits, which voltage peaks bring about a charge injection in the N-well.