DRAM boosted voltage supply

Abstract

A circuit for providing an output voltage for a DRAM word line which can be used to drive memory word lines which can be as high as 2Vdd. Transistors in a boosting circuit are fully switched, eliminating reduction of the boosting voltage by Vtn through the transistors. The boosting capacitors are charge by Vdd. A regulator detects conduction current of a replica of a memory cell access transistor, shutting off the boosting circuit clock oscillator when the correct voltage to operate the access transistor has been reached.

Description

FIELD OF THE INVENTION

This invention relates to dynamic random access memories (DRAMs) and in particular to a boosted word line power supply charge pump and regulator for establishing word line voltage.

BACKGROUND TO THE INVENTION

High density commercial DRAMS typically use capacitive pump voltage boosting circuits for providing sufficiently high voltage to drive DRAM word lines. Regulation of the voltage has been poor, and danger exists of generating voltages above the limits imposed by reliability requirements of the device technology. Such circuits, where a supply voltage of V_dd is present, generate a maximum achievable voltage of 2V_dd=31 V_tn where V_tn is the threshold voltage of an N-channel field effect transistor (FET).

DESCRIPTION OF THE PRIOR ART

FIG. 1 illustrates a voltage boosting circuit according to the prior art and FIG. 2 illustrates clock signal waveforms used to drive the circuit.

A pair of 25-channel transistors 1 and 2 are cross-coupled to form a bistable flip-flop, the sources of the transistors being connected to voltage rail V_dd. The drain of each transistor, is connected to the gate of the respective other transistor, and form nodes 3 and 4 which are connected through corresponding N-channel transistors 5 and 6 configured as diodes, to one terminal of a capacitor 7. The other terminal of capacitor 7 is connected to ground.

A clock source is connected through an inverter 8 and via capacitor 9 to node 4, and another clock source is connected through an inverter 10 through capacitor 11 to node 3.

The clock source voltage at the output of inverter 8 is shown as waveform φ₂, varying between voltages V_dd and V_ss, and the clock source output at the output of inverter 10 is shown as waveform φ₁, varying between the voltages V_dd and V_ss.

The output terminal of the circuit supplies the voltage V_pp at the junction of the capacitor 7 and transistors 5 and 6.

Operation of the above-described circuit is well known. As the levels of φ₁ and φ₂ vary as shown in FIG. 2, capacitors 9 and 11 alternately charge between V_ss and V_dd and discharge to capacitor 7. The maximum achievable voltage at the output terminal is 2V_dd=31 V_tn, where V_tn is the threshold of operation of either of transistors 5 or 6.

It should be noted that the external supply voltage V_dd can vary between limits defined in the device specification, and also as a result of loading, both static and dynamic of other circuits using the same supply. The threshold voltage V_tn is sensitive to variations in semiconductor processing, temperature and chip supply voltage, and this contributes to significant variation in the boosted supply. Finally the boosted V_pp supply itself varies as a function of load current drawn from capacitor 7. Therefore the voltage at the output terminal, which is supposed to provide a stable word line voltage can vary substantially from the ideal. For example, it V_dd is excessively high, this can cause the output voltage to soar to a level which could be damaging to word line access transistor gate insulation, damaging the memory. If V_dd is low, it is possible that insufficient output voltage could be generated to drive the memory cell access transistors, making memory operation unreliable.

SUMMARY OF THE PRESENT INVENTION

The present invention is a circuit for providing an output voltage which can be used to drive memory word lines which can be as high as 2V_dd; it does not suffer the reduction of V_tn of the prior art circuit. Thus even if V_dd is low, the word line driving voltage even in the worst case would be higher than that of the prior art, increasing the reliability of operation of the memory.

The above is achieved by fully switching the transistors in a boosting circuit, rather than employing N-channel source followers as “diodes”. This eliminates reduction of the boosting voltage by V_tn.

Another embodiment of the invention is a circuit for detecting the required word line driving voltage and for regulating the voltage boosting pump by enabling the pump to operate if the boosted voltage is low, causing the word line driving voltages to increase, and inhibiting the plump if the voltage reaches the correct word line voltage. This is achieved by utilizing a sample transistor which matches the memory cell access transistor which is to be enabled from the word line. The word line driving voltage is applied to the sample transistor, and when it begins to conduct current indicating that its threshold of operation has been reached, a current mirror provides an output voltage which is used in a feedback loop to inhibit operation of the voltage pump. Since the sample transistor is identical to the memory access transistor, the exactly correct word line driving voltage is maintained.

Thus accurate regulation of the boosted word line voltage is produced, without the danger of damaging voltages. Because once the correct word line driving voltage is reached, the voltage pump is inhibited, there is no additional power required to charge voltage boosting capacitors higher than this point, saving power. Since the voltage that is exactly that required is generated, improved reliability is achieved because double boot-strap voltages on the chip are eliminated. The circuit is thus of high efficiency.

The first and second embodiments are preferred to be used together, achieving the advantages of both.

The same basic design could also be employed as a negative substrate back-bias voltage (V_bb) generator.

An embodiment of the invention is a boosted voltage supply comprising a D.C. voltage supply terminal, first and second capacitors, the first capacitor having one terminal connected to ground and its other terminal to an output terminal switching apparatus for connecting one terminal of the second capacitor alternately between the voltage supply terminal and ground and connecting the other terminal of the second capacitor alternately between the voltage supply terminal and the output terminal whereby a boosted voltage regulated to the D.C. voltage supply is provided at the output terminal.

Another embodiment of the invention is a dynamic random access (DRAM) word line supply comprising an increasing voltage supply for the word line for connection to the word line from time to time, a memory cell access transistor for connecting a memory cell capacitor to a bit line having a gate connected to the word line, a sample transistor similar to the memory cell access transistor, apparatus for applying the voltage supply to the sample transistor for turning on the sample transistor at a supply voltage related to the characteristics of the sample transistor, and apparatus for inhibiting increase of the voltage supply upon turn-on of the sample transistor, whereby a voltage supply having a voltage level sufficient to turn-on the memory cell access transistor is provided for connection to the word line.

BRIEF INTRODUCTION TO THE DRAWINGS

A better understanding of the invention will be obtained by reference to the detailed description below, in conjunction with the following drawings, in which:

FIG. 1 is schematic diagram of a prior art voltage boosting circuit,

FIG. 2 illustrates clock waveforms used to drive the circuit of FIG. 1,

FIG. 3 is a schematic diagram of an embodiment of the present invention,

FIG. 4 illustrates clock signal waveforms used to operate the circuit of FIG. 3,

FIG. 5 is a schematic diagram of a boosted clock generator, and

FIG. 6 is a partly schematic and partly block diagram illustration of another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 3, a capacitor 15 is connected in a series circuit between ground and through an N-channel field effect transistor FET 16, configured as a diode, with gate and drain connected to a voltage source V=hd dd=l . Transistor 16 charges capacitor 15 to V_dd with an N-channel threshold (V_tn) of V_dd upon startup.

A first pair of transistors formed of N-channel FET 17 and P-channel FET 18 are connected with their source-drain circuits in series between the junction of transistor 16 and capacitor 15 and V_dd, the source of transistor 13 being connected with its substrate to the junction of transistor 16 and capacitor 15. That junction forms the output 19 of the circuit, where the voltage V_pp, the word line supply, is provided.

A second pair of transistors, one being P-channel FET 20 and one being N-channel FET 21 have their source-drain circuits connected in series between the voltage supply V_dd and ground. The source of transistor 20 is connected to voltage supply V_dd with its substrate. A second capacitor 22 is connected between the junctions of the two pairs of transistors.

While the above-described circuit would operate in a manner to be described below to generate a voltage 2V_dd at the output 19, it provides only a half wave boosting function, and should significant current be drawn, the voltage could drop. In order to provide a full wave boosting function, an additional circuit is included as follows.

A third pair of transistors comprising N-channel FET 23 and P-channel FET 24 have their source-drain circuits connected in series between V_dd and the output terminal 19, the source of transistor 24 being connected to the output terminal with its substrate. A fourth pair of FETs comprised of P-channel FET 24 and N-channel FET 25 have their source-drain circuits connected in series between V_dd and ground, the source of transistor 24 being connected to V_dd with its substrate. A third capacitor 27 is connected between the junctions of the third and fourth pairs of transistors.

Clock sources are applied to the gates of the various transistors as follows: φ₁ to the gate of transistor 25, /φ₁ to the gate of transistor 20, φ₂ to the gate of transistor 21, and /φ₂ to the gate of transistor 26.

Boosted clock signals are applied to thy gates of the various transistors as follows: φ₁+ to the gate of transistor 23, /φ₁ to the gate of transistor 18, φ₂+ to the gate of transistor 17 and /φ₂+ to the gate of transistor 24.

A schematic of a clock generator is shown in FIG. 5. P-channel transistors 51 and 52 are cross-coupled to form a bistable flip-flop, the sources and substrates of the transistors being connected to the V_pp output 19, the gate of transistor 52 being connected to the drain of transistor 51 and the gate of transistor 51 being connected to the drain of transistor 52. N-channel transistor 53 has its source-drain circuit connected between the drain of transistor 51 and ground and N-channel transistor 54 has its source-drain circuit connected between the drain of transistor 52 and ground. The clock φ₁ is applied to the gate of transistor 54 and the clock /φ₁ is-applied to the gate of transistor 53.

When the clock φ_dd goes high, transistor 54 is enabled and the junction of transistors 52 and 54 is pulled to ground, enabling transistor 51 which passes V_pp to the junction of transistors 51 and 53. This is the clock φ₁+, boosted to V_pp. When the clock φ₁ goes low, and /φ₁ goes high, transistor 54 is inhibited and transistor 53 is enabled and the junction of transistors 51 and 53 (φ₁+) is pulled to ground. This enables transistor 52 which passes V_pp to the junction of transistors 52 and 54, the clock /φ₁+ output.

A similar circuit (not shown) provides boosted clocks φ₂+ and /φ₂+.

FIG. 4 illustrates the clock signal logic levels and timing which are applied to the various gates, and reference is made thereto for the explanation below.

In operation, at initialization, capacitor 15 is charged through the N-channel FET diode 16 from V_dd, charging it up to V_dd=31 V_tn. The circuit then goes through a number of cycles to charge up reservoir capacitor 15 to the required level. The following discussion describes the voltages and charge transfers occurring in the pump circuit once the V_pp level has almost reached the desired level, and is sufficient to fully turn on an N-channel transistor with its source at V_dd.

Now considering the switching circuit for capacitor 27 to the left of diode 16, and the waveforms of FIG. 4, φ₁ and /φ₁+ go high, enabling transistors 23 and 25. Capacitor 27 charges to the level of V_dd. Transistors 23 and 25 are then inhibited, ceasing conduction at the end of the φ₁ pulse.

After a discrete period of time, /φ₂ and /φ₂+ go low and transistors 24 and 26 are enabled. The capacitor terminal which was connected to V_dd becomes connected to output terminal 19 and the other, negative terminal of capacitor 27 becomes connected to V_dd. If capacitance C_R (15) was equal to 0, the voltage from the positive terminal of capacitor 27, at terminal 19 to ground would be equal to the initial voltage on capacitor 27 plus the voltage V_dd to ground, i.e. 2V_dd. However, reservoir capacitor C_R (15) typically has a large value so that the voltage step at node 19 will be attenuated to (C_S/(C_S+C_R))*(2V_dd−V_pp), where C_R and C_S are the values of capacitors 15 and 22 or 27 respectively. Thus the pump can attain a maximum level of 2V_dd.

The voltage pulses /φ₂ and /φ₂+ then go high, inhibiting transistors 23 and 25, and after a discrete period of time φ₁ and /φ₁+ go high again, reconnecting capacitor 27 between V_dd and ground. Again it charges, and as capacitor 27 is alternately switched between V_dd and ground and output terminal 19 and V_dd, the voltage between terminal 19 and ground rises to 2V_dd.

A similar function occurs with capacitor 22. When the clock voltage /φ₁ and /φ₁+ go low, capacitor 27 is connected between terminal 19 and V_dd through transistors 20 and 18. When the clock voltages φ₂ and φ₂+ go high, capacitor 22 is connected between V_dd and ground via transistors 17 and 21, charging capacitor 22 to the voltage V_dd. Thus, while capacitor 27 is being charged between V_dd and ground, capacitor 22 is connected between output terminal 19 and V_dd through FETs 20 and 18, due to the phase and polarity of the clock signals /φ₁. The two capacitors 27 and 22 thus alternately charge and boost the voltage on capacitor 15.

The clock signals φ₁, φ₂, /φ₁ and /φ₂ have similar amplitudes, and vary between V_dd, a logic 1, and a V_ss, a logic zero.

The clock signals φ₁+, φ₂+, /φ₁+ and /φ₂+ have similar amplitudes, and vary between V_pp, a logic 1, and V_ss (ground), and logic 0.

It should be noted that the capacitors 15, 22 and 27 charge from the main voltage supply V_dd, and not from the clock sources. This allows the clock sources to have reduced power supply requirements, since they drive only the gates of the FETs which have minimal capacitance. This is in contrast to the prior art boosting circuit in which the clock sources supply the charge required for capacitors 9 and 11 (FIG. 1), and thus supply the current required to boost the voltage, indirectly supplying part of the word line current.

In addition, since the voltage boosting current is not routed through an FET configured as a diode, as in the prior art circuit, there is no reduction of the boosting voltage by a threshold of conduction voltage V_tn as in the prior art.

Since non-overlapping clocks are used, the boosting current will not flow between the output terminal 19 and V_dd. This also prevents charge from leaking away from the capacitor 15 during switching.

It is preferred that the N-channel transistor substrates should all be connected to a voltage V_ss or V_bb which is below V_ss (ground) in this embodiment. The connection of the substrates of the P-channel transistors 24 and 18 to V_pp avoids forward biasing of the P-channel tubs.

Turning now to FIG. 6, a word line supply is shown. A word line voltage source such as provided on lead 29 is connected through a word line decoder 30 to a word line 31. A memory cell access transistor 32 has its gate connected to the word line, and its source-drain circuit connected to a bit line 33 and to a memory cell bit storage capacitor 34. The capacitor is referenced to the cell plate reference voltage V_ref.

In operation of the above well-known circuit, if a voltage V_pp on lead 29 is supplied through a word line decoder 30 to a word line 31, which voltage is applied to the gate of transistor 32, the bit storage charge capacitor 34 is connected to bit line 33 through transistor 32. The charge stored on capacitor 34 is thereby transferred to bat line 33.

The circuit of FIG. 6 provides a word line voltage regulator. A sample transistor 35 is fabricated identical to word line access transistor 32. It thus exhibits the same characteristics, including the same thresholds of conduction.

The source or transistor 35 is connected to the voltage supply V_dd and the drain is connected through a P-channel transistor 36 to the word line voltage source lead 29. The gate of transistor 36 is connected to its drain.

A P-channel transistor 37 mirrors the current in transistor 36 having its gate connected to the gate and drain of transistor 36, its source connected to the word line voltage source lead 29 and the drain connected to the drain of N-channel transistor 38, which has its other source connected to ground (V_ss), and its gate connected to V_dd, to operate in the linear region as a resistor.

Transistors 36 and 37 form a current mirror of current passing through transistor 36. When V_pp rises to the point at which transistor 35 begins to conduct, a similar current is conducted through transistor 38. A positive voltage appears between the junction of transistors 37 and 38 and ground. This voltage is used as a feedback voltage to inhibit the generation of additional increase in voltage of V_pp on lead 29.

Since transistor 35 is identical to transistor 32, the exactly correct V_pp sufficient to turn on transistor 32 is set.

The voltage V_pp at lead 29 can be provided by means or a pump in accordance with the prior art, or preferably the voltage pump 39 described with reference to FIGS. 3 and 4 above. Either the prior art pump or the pump in accordance with the present invention is driven by an oscillator 40, which provides the clock signals, e.g. φ₁, φ₂, /φ₁ and /φ₂. Oscillator 44 has an inhibit input, which stops its operation upon receipt of an inhibit signal.

The feedback voltage from the current mirror is applied via a pair of serially connected inverters 41 and 42 to the inhibit input of oscillator 44. Actually, any even number of inverters could be used. Therefore when transistor 35 begins conduction, signifying that the correct word line (and transistor 32) driving voltage V_pp has been reached, the feedback voltage to the inhibit input of oscillator 44 shuts oscillator 44 down, causing cessation of the charging of the capacitors in the voltage boosting circuits, and cessation of increasing of the voltage V_pp.

The voltage regulator described above thus eliminates the boosting of V_pp if it is not required, and only allows the voltage boosting circuit to boost the voltage to the level required by the word line, i.e. cell access transistors. This saves power and provides protection to the cell access transistors, increasing reliability of the memory. The dangerous double boot-strap circuits boosting voltage to about 2V_dd which were previously found on the chip are thus eliminated, and voltage stress is minimized.

Narrow channel transistors can have higher than expected threshold voltages under back-bias conditions, and the present regulator which actually measures the memory cell access transistor turn-on voltage provides the exact word line supply voltage, neither too low nor too high. The combined embodiments of FIGS. 3 and 5 thus provide a substantially more reliable word line voltage, resulting in a more reliable memory, with reduced power requirements.

A person understanding this invention may now conceive of alternative structures and embodiments or variations of the above. All of those which fall within the scope of the claims appended hereto are considered to be part of the present invention.

Claims

1. A boosted voltage supply comprising: DC voltage supply providing plural voltage levels; a boosting capacitor having first and second terminals; and a switching circuit including a first transistor between one level of the voltage supply and the first terminal of the boosting capacitor and a second transistor between the first terminal of the boosting capacitor and a capacitive load, the first transistor and the second transistor being driven by clock signals, the switching circuit alternately connecting the first terminal of the boosting capacitor to the voltage supply and to the capacitive load while alternating the level of the voltage supply connected to the second terminal of the boosting capacitor to pump the voltage on the capacitive load to a boosted voltage level greater than and of the same polarity as the DC voltage supply, the second transistor being fully switched to substantially eliminate a threshold voltage reduction of boosted voltage.

2. A method of supplying a boosted voltage comprising: providing plural voltage levels and a boosting capacitor having first and second terminals; and with clock signals applied to switches, alternately switching the first terminal of the boosting capacitor to the voltage supply and to a capacitive load while alternating the level of the voltage supply connected to the second terminal of the boosting capacitor to pump the capacitive load to a boosted voltage level greater than and of the same polarity as the DC voltage supply, the switch between the first terminal of the boosting capacitor and the capacitive load being =p1 fully switched to substantially eliminate a threshold voltage reduction of boosted voltage.