Reducing level shifter standby power consumption

[0002] This is a continuation of prior application Ser. No. 09/811,025 filed Mar. 16 2001 now U.S. Pat. No. 6,385,099.

[0003] This invention relates generally to level shifters that are utilized for example in shifting voltage levels in memories such as flash memories.

[0004] Flash memories may need a plurality of different voltage levels to perform read, programming and erase functions. Generally these voltages are produced by on chip charge pumps. A charge pump is a DC-to-DC voltage converter. The charge pump generates a desired voltage level by selectively clocking capacitors and transistors in a prescribed sequence.

[0005] Charge pumps may include a number of different stages. The amount of input current to the charge pump is generally proportional to the number of those stages. The voltages generated by the charge pump are eventually coupled to the gate, drain and source terminals of flash memory cells in a flash memory array by way of decoder and switch circuits. The decoder and switch circuits may be constructed using complementary metal oxide semiconductor (CMOS) transistors. As a result, they may have a predetermined amount of associated capacitance.

[0006] The capacitance of the decoder and switch circuits may be significant. As a result, it may be necessary to keep the decoder and switch circuits in a ready state for memory read operations to achieve fast read access times. Thus, this capacitance is normally charged to prescribed read voltage levels during read mode. The read mode may include sequences when memory reads occur and intervening time periods between memory operations. During the-intervening periods, the chip may assume a low power or standby state. Thus, this charging of capacitance, even in the low power or standby state may result in substantial leakage.

[0007] Subthreshold leakage occurs in these decoder and switch circuits. The leakage is significant because the decoder and switch capacitance must be fully charged even during standby. The leakage results in input current and therefore adds to the average current consumed during standby.

[0008] The widespread use of portable systems with on-board flash memories increases the need to limit power consumption. Even relatively small amounts of standby power consumption in devices with a large number of transistors may result in considerable drain in battery powered systems. Limited battery life means that reducing overall power consumption is a critical design goal.

[0009] Leakage power consumption occurs in the absence of a switching event. Leakage power consumption may be due to non-zero reverse bias leakage and subthreshold current. With a large number of transistors, leakage power consumption can significantly contribute to overall power consumption. Subthreshold leakage is due to carrier diffusion between the source and drain regions of transistors in weak inversion. An MOS transistor in the subthreshold operating region may behave like a bipolar device, exhibiting exponential dependence on the gate-to-source voltage. Particularly when the gate-to-source voltage is smaller than, but very close to, the threshold voltage of the device, subthreshold leakage current may become significant.

[0010] Level shifters may be used in connection with the decoder and switch circuits. Steering logic may be utilized to select the appropriate voltage for a given memory function such as read, program or erase. Level shifters may experience significant standby power consumption.

[0011] Thus, there is a need to reduce level shifter standby power consumption.

[0012] FIG. 1 is a block depiction of a flash memory in accordance with one embodiment of the present invention;

[0013] FIG. 2 is a circuit diagram for a level shifter in accordance with the prior art; and

[0014] FIG. 3 is a circuit diagram for a level shifter in accordance with one embodiment of the present invention.

[0015] Referring to FIG. 1, a flash memory 10 may receive a supply voltage that may be boosted using one or more boosters or charge pumps 12 to one or more voltages indicated as V_ext. For example the boosted voltage V_extamay be a read voltage (e.g. 5v.) and the boosted voltage V_extbmay be the program voltage (e.g. 11.5 v.). Each boosted voltage (V_ext) may then provide a supply voltage to a switch 13 including P-channel transistors 15. The external voltage V_extmay than be supplied to a plurality of level shifters 14 that turn the P-channel transistors 15 on and off via the level shifter 14 output voltage (V_out). The control logic 11 supplies an input voltage (V_int) to each level shifter 15.

[0016] The output voltage from the switch 13 may be applied to an address decoder 16. The address decoder 16 may receive off chip address information 20, buffered by the address buffer 18. The address decoder 16 then supplies the appropriate voltage level signals to memory cells in the memory array 22.

[0017] Referring to FIG. 2, a level shifter 14a, in accordance with the prior art, may experience significant standby subthreshold leakage current. The level shifter 14areceives the input voltage V_intvia the gate of an N-channel MOS input transistor 28. The input transistor 28 has its drain coupled to an output voltage (V_out2) node and its source coupled to ground. The input voltage V_intmay also be coupled through an inverter 32 to another N-channel MOS transistor 30 that also has its source grounded. The drain of the N-channel transistor 30 may be coupled to a output voltage (V_out1) node.

[0018] The drain of each N-channel transistor 28 or 30 is coupled to P-channel cross-coupled transistors 24 and 26. The sources of the P-channel transistors 24 and 26 may be coupled to an external voltage V_ext.

[0019] In operation, when the input voltage V_intis low (such as zero volts), the input transistor 28 does not turn on. As a result, the output voltage V_out2may rise to the level of the voltage V_ext. This is because the drain of the input transistor 28 is at a high potential, turning off the transistor 26. At the same time, the N-channel transistor 30 receives a high, inverted signal on its gate from the inverter 32. This gate voltage enables the transistor 30 to couple the V_out1node to a ground or low potential. As a result, V_out, goes to ground and V_out2goes to the voltage V_ext.

[0020] Even with a low level output voltage V_int, significant subthreshold leakage may be experienced by the N-channel transistor 28. Subthreshold leakage current is an exponential function of V_gs, the gate-to-source voltage, minus the threshold voltage (V_t) of the input transistor 28. When the input transistor 28 has a zero gate voltage and a grounded source bias, V_gsis effectively zero volts. Thus, the subthreshold leakage is as follows:

[0021] I_sub=I₀(^{(vgs−Δvt)/s}−1)

[0022] where I₀is the leakage when V_gs−V_t=0 and s is the subthreshold slope which is the rate of change of current through the transistor as a function of V_gs. ΔV_tis V_to−V_tbswhere V_tois the threshold voltage when V_bsis equal to zero and V_tbsis the threshold voltage when V_bsis not equal to zero.

[0023] Thus, when V_gs−ΔV_tis negative, the subthreshold leakage current may be equal to the subthreshold leakage current at V_gs−ΔV_t=0 times a significantly small value. This may be a substantial reduction in the amount of leakage current.

[0024] During standby conditions, input transistor 28 operating or performance characteristics may not be very critical. There is no need to have a relatively high gate-to-source voltage in order to have effective input transistor 28 performance. Thus, during standby, the gate-to-source voltage of the input transistor 28 may be set relatively low. Conversely, during level shifting operations, a more substantial gate-to-source voltage may be advantageous.

[0025] Referring to FIG. 3, a level shifter 14 may be similar to the level shifter 14ain FIG. 2. The source of the N-channel input transistor 28 may be coupled to the output of the inverter 32. Thus, in a standby mode, the source of the input transistor 28 is at a relatively higher voltage. During normal level shifting operations, the source of the input transistor 28 may be at a relatively lower voltage. As a result, during standby operations, the voltage V_gsof the transistor 28 may be a larger negative voltage, related to a supply voltage. Conversely, during regular mode, when level shifting is occurring, V_gsmay be a positive voltage to provide desirable transistor operating characteristics.

[0026] As a result, the leakage current, in standby, may be reduced as much as one hundred times or more. This may result in current savings of a microamp or more depending on memory architecture. Coupling the source of transistor 28 to a positive voltage not only makes V_gsnegative, it also increases the threshold voltage of the transistor 28, further reducing subthreshold current.

[0027] In some embodiments, the subthreshold leakage current may be reduced for positive current pumps, allowing for lower standby current. Also, lower supply voltages may be used to achieve standby currents close to those that are offered on similar chips with higher supply voltages. Since the leakage current is reduced, faster flash memory read access times may be achieved, in some embodiments, because the internal capacitance may be charged to the read voltage while still meeting the desired standby current consumption requirements.

[0028] More than two voltages (read and program) may be handled in other embodiments of the present invention. Also, a different inverter may be used to drive the source high. The same approach may also be and with the transistor 30.

[0029] While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.