INTEGRATED CIRCUITS FOR PROVIDING CLOCK PERIODS AND OPERATING METHODS THEREOF

The present disclosure relates generally to the field of semiconductor circuits, and more particularly, to integrated circuits for providing clock periods and operating methods thereof.

Memory circuits have been used in various applications. Conventionally, a dynamic random access memory (DRAM) circuit includes a plurality of memory cells. For a conventional DRAM circuit in which arrays of capacitive storage memory cells are provided, each memory cell has an access transistor. Data stored in such memory cells is actually a charge stored on a small capacitor. When the data are to be output, the access transistor is activated by a word line (WL) that is electrically coupled to the gate or control terminal of the transistor. The access transistor can couple the capacitor to a bit line (BL) coupled to a sense amplifier for sensing the voltage of the capacitor.

Data stored in memory cells of the DRAM circuit are vulnerable because of charge leakages of the memory cells. To retain the data stored in the DRAM circuit, a refresh operation is periodically applied to the DRAM circuit to recharge capacitors of the memory cells. Generally, a temperature-controlled oscillator (TCO) has been used to provide a refresh period or refresh frequency for the refresh operation.

The TCO has a capacitor electrically coupled with a switch. A fixed current is provided to charge the capacitor. A comparator compares a voltage level on a top plate of the capacitor and a reference voltage that is inversely proportional to absolute temperature. If the voltage level is higher than the reference voltage, the switch is closed such that charges stored in the capacitor can be discharged and the voltage level on the top plate of the capacitor declines. If the voltage level is lower than the reference voltage, the switch is opened such that the capacitor can be charged by the fixed current and the voltage level on the top plate of the capacitor increases. By detecting the open-close state of the switch, the refresh period or the refresh frequency is provided.

It is found that the environmental temperature around the DRAM circuit affects the charge leakages. If the environmental temperature increases, the charge leakages of the memory cell capacitors also increase. The refresh period should be shortened and the refresh frequency should be increased to refresh the memory cells. If the environmental temperature decreases, the charge leakages of the memory cell capacitors are decreased. The refresh frequency can be slowed down. It is proposed that the refresh frequency at 125° C. should be about six times the refresh frequency at 25° C.

It is understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Moreover, the formation of a feature on, connected to, and/or coupled with or to another feature in the present disclosure that follows may include embodiments in which the features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the features, such that the features may not be in direct contact. In addition, spatially relative terms, for example, “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top,” “bottom,” etc. as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) are used for ease of the present disclosure of one features relationship to another feature. The spatially relative terms are intended to cover different orientations of the device including the features.

FIG. 1A is a schematic drawing illustrating an exemplary integrated circuit for providing a clock period. In FIG. 1A, an integrated circuit 100 for providing a clock period can include a capacitor 110, a switch 120, a comparator 130, a transistor M₁, and a circuit 140. In some embodiments, the integrated circuit 100 can be configured to provide a clock period for refreshing memory cells (not shown), such as dynamic random access memory (DRAM) cells or embedded DRAM cells. The clock period can be converted to a clock frequency for refreshing memory cells. In other embodiments, the integrated circuit 100 can be referred to as a temperature-controlled oscillator (TCO) or a temperature-tracking clock generator.

Referring to FIG. 1A, the switch 120 can be electrically coupled with the capacitor 110 in a parallel fashion. In some embodiments, the capacitor 110 and the switch 120 can be electrically coupled with a power line that can provide a power voltage, e.g., a power voltage V_SSor ground. The comparator 130 can include input nodes 130a, 130b and an output node 130c. The input node 130a can be configured to receive a reference voltage V_ref. In some embodiments, the reference voltage V_refcan be temperature independent. The input node 130b can be electrically coupled with a plate of the capacitor 110 and the switch 120. The output node 130c can be electrically coupled with the switch 120. In some embodiments, the comparator 130 can be configured to compare the reference voltage V_refand a voltage level V_Con a node N_Athat is electrically coupled with the plate of the capacitor 110. The comparator 130 can output a comparison result on the output node 130c so as to close or open the switch 120.

Referring to FIG. 1A, the transistor M₁can be electrically coupled with a plate of the capacitor 110. In some embodiments, the transistor M₁can be referred to as a tail transistor. In some embodiments using an N-type transistor, a source of the transistor M₁can be electrically coupled with the capacitor 110 and the power voltage V_SSor ground. A gate and a drain of the transistor M₁can be electrically coupled with the circuit 140. The circuit 140 can be configured to control the transistor M₁. In some embodiments, the circuit 140 can be configured to provide a bias voltage V_biasto the gate of the transistor M₁so as to control a current I_Cthat is supplied to charge the capacitor 110. In some embodiments, the transistor M₁can be a P-type transistor. Voltages applied to the P-type transistor can be opposite to those applied to the N-type transistor.

As noted, the reference voltage V_refcan be temperature independent. The term “temperature independent” here means that the reference voltage V_refis substantially free from being affected by the environmental temperature. The circuit 140 can be configured to provide a bias voltage V_biasto the gate of the transistor M₁so as to control the current I_C. In some embodiments, the bias voltage V_biasis programmable and temperature dependent. The term “programmable” here means that the bias voltage V_biascan be adjusted by adjusting at least one parameter, such as one or more impedances and/or resistances of at least one component within the circuit 140, and/or a proportional factor. By adjusting the bias voltage V_bias, the current I_Ccan be adjusted in correspondence to a change of environmental temperature around the circuit 100. In some embodiments, if the environmental temperature goes up, the bias voltage V_biasis raised to increase the current I_Cfor charging the capacitor 110. Because a large current is provided to charge the capacitor 110, the clock period for refreshing memory cells can be thus reduced. In other embodiments, if the environmental temperature goes down, the bias voltage V_biasis lowered to decrease the current I_Cfor charging the capacitor 110. Because a small current is provided to charge the capacitor 110, the clock period for refreshing memory cells can be increased. In still other embodiments, the circuit 100 can provide a refresh frequency at 125° C. that can be about six times the refresh frequency at 25° C.

FIG. 1B is a schematic drawing illustrating another exemplary integrated circuit. Items of FIG. 1B that are the same or similar items in FIG. 1A are indicated by the same reference numerals. In FIG. 1B, the circuit 140 can include a transconductance amplifier 141 that can be electrically coupled with the transistor M₁and the capacitor 110. In some embodiments, the transconductance amplifier 141 can include input nodes 141a, 141b and an output node 141c. The input node 141a can be electrically coupled with the output node 141c, which can be electrically coupled with the capacitor 110. In some embodiments, the transconductance amplifier 141 can be configured to convert a voltage drop between the input nodes 141a and 141b to the current I_C.

In some embodiments, the transconductance amplifier 141 can include a current mirror 201 and a pair of transistors 203 and 205 as shown in FIG. 2. The transistors 203 and 205 can be electrically coupled in a parallel fashion between the current mirror 201 and the transistor M₁. Gates of the transistors 203 and 205 can be electrically coupled with the input nodes 141a and 141b, respectively. The output node 141c can be electrically coupled with a node (not labeled) that is between the current mirror 201 and the transistor 203. It is noted that the structure of the transconductance amplifier 141 shown in FIG. 2 is merely exemplary. Any structure of the transconductance amplifier that includes a current mirror may be used.

Referring again to FIG. 1B, the circuit 140 can include an amplifier 145. The amplifier 145 can include input nodes 145a, 145b and an output node 145c. The input node 145a can be electrically coupled with the input node 141b of the transconductance amplifier 141. The output node 145c can be electrically coupled with the gate of the transistor M₁. A resistor R₁can be electrically coupled with the input node 145b. Another resistor R₂can be electrically coupled between the input node 145b and the output node 145c. The resistor R₁has a resistance R_Aand the resistor R₂has a resistance R_B. In some embodiments, the resistors R₁and R₂can each be a fixed resistor or an adjustable resistor. The resistances R_Aand R_Bcan each be a fixed resistance or an adjustable resistance.

In some embodiments, the input node 145a can be configured to receive the reference voltage V_ref. The resistor R₁can be electrically coupled with a temperature-dependent voltage V_EB. In some embodiments, the temperature-dependent voltage V_EBcan be inversely proportional to absolute temperature. In other embodiments, the temperature-dependent voltage V_EBcan be provided from a bandgap reference circuit (not shown). The bandgap reference circuit can include a bipolar transistor (not shown). The temperature-dependent voltage V_EBcan be substantially equal to a voltage drop across an emitter and a base of the bipolar transistor.

FIG. 3 is a flow chart illustrating an exemplary method of generating a clock period in view of the integrated circuit 100 shown in FIG. 1B. In FIG. 3, a method 300 of generating a clock period for refreshing memory cells can include providing a bias voltage V_biasto a gate of a transistor so as to control a current that is supplied to charge a capacitor that is electrically coupled with the transistor (Step 310). In some embodiments using the integrated circuit 100 shown in FIG. 1B, the Step 310 can include providing the bias voltage V_biasto the gate of the transistor M₁so as to control a current I_Cthat is supplied to charge the capacitor 110.

As noted, the bias voltage V_biascan be programmable. In some embodiments, the bias voltage V_biascan be adjusted by adjusting the resistances R_Aand R_B. The resistances R_Aand/or R_Bcan be adjustable to achieve a desired bias voltage V_bias. In some embodiments, the bias voltage V_biascan be represented below in Equation (1)

Vbias=Vref+(Vref-VEB)·RBRA(1)

wherein the reference voltage V_refis temperature independent, V_EBis temperature dependent, and the resistances R_Aand R_Bcan each be fixed or adjustable. From Equation (1), the bias voltage V_biascan be adjusted by adjusting the ratio R_B/R_A.

In some embodiments, the environmental temperature around the integrated circuit 100 rises. If the environmental temperature increases, charge leakages of memory cells are increased. To retain data stored in the memory cells, the clock period should be shortened or the clock frequency should be increased. In this embodiment, the Step 310 can include increasing the bias voltage V_biasto turn up the transistor M₁so as to induce increasing the current I_C. For example, the increased bias voltage V_biascan turn up the transistor M₁as shown in FIG. 2. The turned-up transistor M₁can increase a current I_Mflowing through the transistor M₁. The increased current I_Mcan induce increasing the current I_Cthat is provided to charge the capacitor 110 (shown in FIG. 1B).

The method 300 can include comparing the temperature-independent reference voltage V_refand the voltage level on the node N_Aof the capacitor 110 for controlling an open/close state of the switch 120 so as to generate the clock period (Step 320). If the voltage level on the node N_Ais higher than the reference voltage V_ref, the comparator 130 can output a signal, closing the switch 120, such that charges stored in the capacitor 110 can be discharged. If the voltage level on the node N_Ais lower than the reference voltage V_ref, the comparator 130 can output a signal, opening the switch 120, such that the current I_Ccan be supplied to charge the capacitor 110. By detecting the close/open state of the switch 120, a clock period that is provided for refreshing memory cells can be generated.

In some embodiments, a clock period (T) for refreshing memory cells can be represented below in Equation (2)

T=C_xV_ref/I_C  (2)

wherein, C_xrepresents the capacitance of the capacitor 110. As noted, the clock period T is inversely proportional to the current I_C. The reference voltage V_refis temperature independent. In some embodiments, the reference voltage V_refand the capacitance C_xcan be fixed factors. If the environmental temperature rises such that the current I_Cis increased, the clock period T can be reduced and the clock frequency can be increased.

In some embodiments, the environmental temperature around the integrated circuit 100 goes down. If the environmental temperature falls, charge leakages of memory cells may become small. To retain data stored in the memory cells, the clock period can be extended or the clock frequency can be slowed down. In this embodiment, the Step 310 can include decreasing the bias voltage V_biasto turn down the transistor M₁so as to decrease the current I_C. For example, the decreased bias voltage V_biascan turn down the transistor M₁as shown in FIG. 2. The turned-down transistor M₁can decrease the current I_Mflowing through the transistor M₁. The decreased current I_Mcan decrease the current I_Cthat is provided to charge the capacitor 110 (shown in FIG. 1B). As noted, the clock period T is inversely proportional to the current I_C. By decreasing the current I_C, the clock period T can be proportionally increased and the clock frequency is decreased.

FIG. 4 is a schematic drawing illustrating another exemplary integrated circuit for providing a clock period. In FIG. 4, an integrated circuit 400 can include a capacitor 410, a switch 420, a comparator 430, and circuits 440 and 450. In some embodiments, the integrated circuit 400 can be configured to provide a clock period for refreshing memory cells, such as dynamic random access memory (DRAM) cells or embedded DRAM cells. In other embodiments, the integrated circuit 400 can be referred to as a temperature-controlled oscillator (TCO) or a temperature-tracking clock generator.

Referring to FIG. 4, the switch 420 can be electrically coupled with the capacitor 410 in a parallel fashion. In some embodiments, the capacitor 410 and the switch 420 can be electrically coupled with a power line that can provide a power voltage, e.g., a power voltage V_SSor ground. The comparator 430 can include input nodes 430a, 430b and an output node 430c. The input node 430b can be electrically coupled with a plate of the capacitor 410 and the switch 420. The output node 430c can be electrically coupled with the switch 420.

Referring to FIG. 4, the circuit 440 can be electrically coupled with the input node 430a of the comparator 430. The circuit 440 can be configured to provide a temperature-dependent reference voltage V_ref′ on the input node 430a. In some embodiments, the temperature-dependent reference voltage V_ref′ can be inversely proportional to absolute temperature. The circuit 450 can be electrically coupled with the capacitor 410. The circuit 450 can be configured to provide a temperature-dependent current I_C′ that is supplied to charge the capacitor 410. The comparator 430 can compare the temperature-dependent reference voltage V_ref′ and the voltage level V_C′ on the node N_A′. The comparison result output on the output end 430c is directed to control the open/close state of the switch 420. By detecting the open/close state of the switch 420, a clock period T′ that is provided for refreshing memory cells can be generated.

In some embodiments, the temperature-dependent reference voltage V_ref′ is programmable. The term “programmable” here means that the reference voltage V_ref′ can be adjusted by adjusting one or more proportional factors and/or one or more impedances and/or resistances of at least one component of the circuit 440. In other embodiments, the temperature-dependent current I_C′ is programmable. The term “programmable” here means that the temperature-dependent current I_C′ can be adjusted by adjusting one or more proportional factors and/or one or more impedances and/or resistances of at least one component of the circuit 450. By adjusting the temperature-dependent reference voltage V_ref′ and/or the temperature-dependent current I_C′, the clock period T′ can be modified.

As noted, the temperature-dependent reference voltage V_ref′ and/or the temperature-dependent current I_C′ can be adjusted in correspondence to a change of environmental temperature around the circuit 400. In some embodiments, if the environmental temperature rises, the temperature-dependent reference voltage V_ref′ can be reduced and/or the temperature-dependent current I_C′ can be increased. By adjusting the temperature-dependent reference voltage V_ref′ and/or the temperature-dependent current I_C′, the clock frequency for refreshing memory cells can be increased. In other embodiments, if the environmental temperature falls, the temperature-dependent reference voltage V_ref′ can be increased and/or the temperature-dependent current I_C′ can be reduced. By adjusting the temperature-dependent reference voltage V_ref′ and/or the temperature-dependent current I_C′, the clock frequency for refreshing memory cells can be reduced.

FIG. 5 is a schematic drawing illustrating an exemplary integrated circuit. Items of FIG. 5 that are the same or similar items in FIG. 4 are indicated by the same reference numerals. In FIG. 5, the circuit 440 can include resistors R₁′ and R₂′ and at least one current mirror, e.g., current minors 441, 443, 445, and 447, and an amplifier 459. The resistor R₁′ can be electrically coupled with the input node 430a of the comparator 430. In some embodiments, the resistor R₁′ can be electrically coupled between the input node 430a and a power line that is configured to provide a power voltage, e.g., a power voltage V_SSor ground. In other embodiments, the resistor R₁′ can be an adjustable resistor that can provide an adjustable resistance R_A′. In still other embodiments, the resistor R₁′ can have a fixed resistance.

Referring to FIG. 5, the current minor 441 can be electrically coupled with the input node 430a of the comparator 430. The current mirror 443 can be electrically coupled with the current mirrors 441, 445, and 447. In some embodiments, the current minor 443 can be electrically coupled with the current mirrors 441, 445, 447, and a power line that is configured to provide a power voltage, e.g., a power voltage V_SSor ground.

Referring to FIG. 5, the resistor R₂′ can be electrically coupled with the current mirror 447. In some embodiments, the resistor R₂′ can be electrically coupled between the current minor 447 and a power line that is configured to provide a power voltage, e.g., a power voltage V_SSor ground. In other embodiments, the resistor R₂′ can be an adjustable resistor that can provide an adjustable resistance R_B′. In still other embodiments, the resistor R₂′ can have a fixed resistance.

Referring again to FIG. 5, the amplifier 449 can include input nodes 449a, 449b and an output node 449c. The input node 449a of the amplifier 449 can be configured to receive a temperature-dependent voltage KV_EB′. In some embodiments, V_EB′ represents a temperature-dependent voltage and K represents a proportionality constant. In some embodiments, the temperature-dependent voltage KV_EB′ can be provided from a bandgap reference circuit 460. In other embodiments, the bandgap reference circuit 460 can include a bipolar transistor 730 (shown in FIG. 7). The temperature-dependent voltage V_EB′ can be substantially equal to a voltage drop across an emitter and a base of the bipolar transistor 730. The input node 449b can be electrically coupled with a node N_B′ between the current mirror 447 and the resistor R₂′. The output node 449c can be electrically coupled with the current mirror 447.

Referring again to FIG. 5, the circuit 450 can be electrically coupled with the input end 430b of the comparator 430. In some embodiments, the circuit 450 can include a resistor R₃′, at least one current mirror, e.g., current mirrors 451, 453, and 455, and an amplifier 457.

In some embodiments, the current mirror 451 can be electrically coupled with the input node 430b of the comparator 430. The current mirror 453 can be electrically coupled between the current mirrors 451 and 455. In some embodiments, the current mirror 453 can be electrically coupled with the current mirrors 451, 455 and a power line that is configured to provide a power voltage, e.g., a power voltage V_SSor ground.

Referring to FIG. 5, the resistor R₃′ can be electrically coupled with the current mirror 455. In some embodiments, the resistor R₃′ can be electrically coupled between the current minor 455 and a power line that is configured to provide a power voltage, e.g., a power voltage V_SSor ground. In other embodiments, the resistor R₃′ can be an adjustable resistor that can provide an adjustable resistance R_C′. In still other embodiments, the resistor R₃′ can have a fixed resistance.

Referring again to FIG. 5, the amplifier 457 can include input nodes 457a, 457b and an output node 457c. The input node 457a of the amplifier 457 can be configured to receive the temperature-dependent voltage V_EB′. The temperature-dependent voltage V_EB′ can be provided from the bandgap reference circuit 460. The input node 457b can be electrically coupled with a node N_C′ between the current mirror 455 and the resistor R₃′. The output node 457c can be electrically coupled with the current mirror 455.

FIG. 6 is a flow chart illustrating another exemplary method of generating a clock period in view of the integrated circuit 400 shown in FIG. 5. In FIG. 6, a method 600 of generating a clock period can include providing a temperature-dependent reference voltage and a temperature-dependent current that is supplied to charge a capacitor (Step 610). In some embodiments using the integrated circuit 400 shown in FIG. 5, the Step 610 can include providing the temperature-dependent reference voltage V_ref′ and the temperature-dependent current I_C′ that is supplied to charge the capacitor 410 (shown in FIG. 5).

In some embodiments for providing the temperature-dependent reference voltage V_ref′, the current minor 445 (shown in FIG. 5) can provide a temperature-independent current I_bg. The temperature-independent current I_bgcan be provided or mirrored, for example, from the bandgap reference circuit 460.

In some embodiments, a circuit 700 of the bandgap reference circuit 460 that is configured to provide the temperature-independent current I_bgcan include a current minor 710, a resistor 720, and a bipolar transistor 730 as shown in FIG. 7. The resistor 720 can be disposed between nodes X and Y and electrically coupled with the current minor 710. An emitter (E) of the bipolar transistor 730 can be electrically coupled with the node Y. A base (B) of the bipolar transistor 730 can be electrically coupled with the node X. A collector (C) of the bipolar transistor 730 can be electrically coupled with a power line that can provide a power voltage, e.g., a power voltage V_SSor ground.

In some embodiments, the current mirror 710 can provide temperature-dependent currents I_aand I_btoward the nodes X and Y, respectively. The temperature-dependent currents I_acan be substantially equal to or proportional to the temperature-dependent current I_b. In other embodiments, the temperature-dependent currents I_aand I_bboth are proportional to absolute temperature (PTAT) currents. In still other embodiments, the temperature-independent current I_bgcan be represented as below in Equation (3)

Ibg=Ia+VEB′RX(3)

wherein, R_Xrepresents a resistance of the resistor 720, and V_EB′ represents a voltage across the emitter and base of the bipolar transistor 730. The temperature-dependent voltage V_EB′ can be inversely proportional to absolute temperature. Since the temperature-dependent current I_ais proportional to absolute temperature, the temperature effects of the temperature-dependent voltage V_EB′ and the temperature-dependent current I_acan be substantially canceled by each other. The current I_bgis thus temperature independent.

Referring again to FIG. 5, the current mirror 445 can minor the temperature-independent current I_bgto temperature-independent currents I_C1′ and I_C2′. The temperature-independent current I_C1′ can be provided to the current source 443. In some embodiments, the temperature-independent currents I_C1′ and I_C2′ can each be substantially equal or proportional to the temperature-independent current I_bg. In other embodiments, the temperature-independent current I_C1′ and/or I_C2′ can be proportional to the temperature-independent current I_bgwith the same or different proportional factors.

Referring to FIG. 5, the current minor 447 can provide the current minor 445 a current

KVEB′RB′,

wherein R_B′ is the resistance of the resistor R₂′ and KV_EB′ is inversely proportional to absolute temperature. The current

KVEB′RB′

is temperature dependent. Since the current

KVEB′RB′

and the temperature-independent current I_C1′ are provided to the current minor 443, the current mirror 441 can provide the resistor R₁′ a current I_ref′ that is temperature dependent and can be represented below as in Equation (4).

Iref′=KVEB′RB′-IC1′(4)

Since the current I_ref′ flows through the resistor R₁′, the temperature-dependent voltage V_ref′ on the input node 430a can be represented below as in Equation (5).

Vref′=(KVEB′RB′-IC1′)·RA′(5)

As noted, the temperature-dependent voltage V_ref′ can be programmable. In some embodiments, the temperature-dependent voltage V_ref′ can be programmed by adjusting at least one of the resistances R_A′, R_B′, the proportional factor K, and the temperature-independent current I_C1′.

In some embodiments for providing the temperature-dependent current I_C′, the current mirror 455 (shown in FIG. 5) can provide the resistor R₃′ a temperature-dependent current

VEB′RC′,

wherein R_C′ represents the resistance of the resistor R₃′. The current mirror 455 can minor the temperature-dependent current

VEB′RC′

to the current mirror 453.

In some embodiments, the current minor 453 can receive a temperature-independent current I_C2′. In other embodiments, the temperature-independent current I_C2′ can be provided from the circuit 440 or the current minor 445. Since the temperature-dependent current

VEB′RC′

and the temperature-independent current I_C2′ are provided to the current minor 453, the current mirror 451 can provide the temperature-dependent current I_C′ that can be represented below in Equation (6).

IC′=IC2′-VEB′RC′(6)

As noted, the temperature-dependent current I_C′ can be programmable. In some embodiments, the temperature-dependent current I_C′ can be programmed by adjusting the resistance R_C′ and/or the temperature-independent current I_C2′.

In some embodiments, the environmental temperature around the integrated circuit 400 rises. If the environmental temperature increases, charge leakages of memory cells are increased. To retain data stored in the memory cells, the clock period should be shortened or the clock frequency should be increased. In this embodiment, the method 600 can include increasing the temperature-dependent current I_C′ and/or decreasing the temperature-dependent reference voltage V_ref′.

The method 600 can include comparing the temperature-dependent reference voltage V_ref′ and the voltage level on the node N_A′ of the capacitor 410 (FIG. 5) for controlling an open/close state of the switch 420 so as to generate a clock period (Step 620). If the voltage level on the node N_A′ is higher than the temperature-dependent reference voltage V_ref′, the comparator 430 can output a signal, closing the switch 420, such that charges stored in the capacitor 410 can be discharged. If the voltage level on the node N_A′ is lower than the voltage-dependent reference voltage V_ref′, the comparator 430 can output a signal, opening the switch 420, such that the current I_C′ can be supplied to charge the capacitor 410. By detecting the open/close state of the switch 420, a clock period that is provided for refreshing memory cells can be generated.

In some embodiments, a clock period (T′) for refreshing memory cells can be represented below in Equation (7)

T′=C_x′V_ref′/I_C′  (7)

wherein, C_x′ represents the capacitance of the capacitor 410 and can be fixed. As noted, the clock period T′ is inversely proportional to the current I_C′. If the environment temperature rises, the temperature-dependent current I_C′ can be increased and/or the temperature-dependent reference voltage V_ref′ can be decreased. Based on Equation (7), the clock period T′ can be reduced and the refresh frequency can be increased.

In some embodiments, the environmental temperature around the integrated circuit 400 goes down. If the environmental temperature falls, charge leakages of memory cells become small. To retain data stored in the memory cells, the clock period can be longer or the clock frequency can be slowed down. In this embodiment, the method 600 can include decreasing the temperature-dependent current I_C′ and/or increasing the temperature-dependent reference voltage V_ref′. Based on Equation (7), the clock period T′ can be proportionally increased and the clock frequency is decreased.

FIG. 8 is a schematic drawing showing a system including an exemplary memory circuit. In FIG. 8, a system 800 can include a processor 810 coupled with the memory circuit 801. The memory circuit 801 can be similar to a memory circuit that includes the integrated circuit 100 or 400 described above in conjunction with FIGS. 1A, 1B, 4, and 5. The processor 810 is capable of accessing the datum stored in the memory cell of the memory circuit 801. In some embodiments, the processor 810 can be a processing unit, central processing unit, digital signal processor, or other processor that is suitable for accessing data of memory circuit.

In some embodiments, the processor 810 and the memory circuit 801 can be formed within a system that can be physically and electrically coupled with a printed wiring board or printed circuit board (PCB) to form an electronic assembly. The electronic assembly can be part of an electronic system such as computers, wireless communication devices, computer-related peripherals, entertainment devices, or the like.

In some embodiments, the system 800 including the memory circuit 801 can provide an entire system in one IC, so-called system on a chip (SOC) or system on integrated circuit (SOIC) devices. These SOC devices may provide, for example, all of the circuitry needed to implement a cell phone, personal data assistant (PDA), digital VCR, digital camcorder, digital camera, MP3 player, or the like in a single integrated circuit.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.