1. Field of the Invention
The invention generally relates to a method and a circuit for reducing temperature dependence, in particular, to a method and a circuit for generating a clock signal from a clock integrated circuit for reducing temperature dependence of a clock circuit.
2. Description of Related Art
A RC clock circuit may be applied to various integrated circuits for providing a required clock signal. It can be highly integrated with other on-chip components and generally require no external components.
The rate of change of the voltages on the capacitors C1 and C2 between VDD and Vref is referred to one RC time constant T as shown in
T(ΔV/R)=C·ΔV (1)
T=C·R (2)
The clock period of the RC clock circuit equals to two RC time constant (2T) as shown in
Accordingly, the invention is directed to provide a method and a circuit for generating a clock signal from a clock integrated circuit to reduce temperature dependence of a clock circuit.
The method for generating a clock signal from a clock integrated circuit is described as below. A compensation voltage is calculated according to a temperature coefficient of a resistor of the clock circuit, where the compensation voltage is resistor-corner independent. The temperature dependence of a clock period of the clock circuit is reduced according to the compensation voltage. The clock period of the clock circuit is determined by the resistor and at least one capacitor of the clock circuit.
In an embodiment of the invention, the compensation voltage is calculated according to the following formula.
Vc=ΔV·TC·Δt
wherein ΔV is a voltage drop on the resistor based on a reference temperature, TC is the temperature coefficient of the clock trimming resistor, and Δt is a temperature difference between a temperature and the reference temperature.
In an embodiment of the invention, the method further comprising compensating a voltage change on the at least one capacitor of the clock circuit according to the compensation voltage when the at least one capacitor discharges. For details, the voltage change on the at least one capacitor is subtracted by the compensation voltage when the at least one capacitor discharges, or a threshold voltage is generated by adding the compensation voltage with a reference voltage, where the threshold voltage is used to control the alternating charge or discharge of the at least one capacitor after compensation, and the reference voltage is used to control the alternating charge or discharge of the at least one capacitor before compensation.
The invention also provides a circuit for generating a clock signal from a clock integrated circuit.
The circuit for generating a clock signal from a clock integrated circuit comprises a voltage generator, the voltage generator receives a reference voltage from the RC clock circuit and generates a threshold voltage to control the alternating charge or discharge of at least one capacitor of the clock circuit, where a temperature coefficient of the threshold voltage is substantially the same as a temperature coefficient of a resistor of the RC clock circuit, and the reference voltage is used to control the alternating charge or discharge of the at least one capacitor before compensation.
In an embodiment, the voltage generator comprises a current generating unit and a feedback circuit. The current generating unit is configured to generate a compensation current. The feedback circuit receives the reference voltage from the clock circuit and receives the compensation current from the current generating unit, and generates the threshold voltage according to the reference voltage and the compensation current.
In an embodiment, the feedback circuit comprises an operation amplifier and a third resistor. The operation amplifier receives the reference voltage from the clock circuit. The third resistor is coupled to the operation amplifier and receives the compensation current from the current generating unit, where a temperature coefficient of the third resistor is substantially the same as the temperature coefficient of the clock trimming resistor of the clock circuit, and the operation amplifier and the third resistor construct a negative feedback loop and generate the threshold voltage according to the reference voltage and the compensation current.
In an embodiment of the invention, the circuit for generating a clock signal from a clock integrated circuit includes an operation amplifier, a third resistor, a first current source, and a second current source. The third resistor is coupled between a negative input end and the output end of the operation amplifier so as to construct a negative feedback loop on the operation amplifier. The reference voltage is provided to the positive input end of the operation amplifier. If the temperature coefficient of the clock trimming resistor is positive, the first current source provides a current flow from the output end of the operation amplifier through the third resistor to a ground voltage source, and the second current source provides a current flow from a power voltage source through the third resistor to the output end of the operation amplifier, such that the threshold voltage is generated on the output end of the operation amplifier. If the temperature coefficient of the clock trimming resistor is negative, the first current source and the second current source provide a current flow from the power voltage source through the third resistor to the output end of the operation amplifier, such that the threshold voltage is generated on the output end of the operation amplifier.
In order to make the aforementioned and other features and advantages of the present invention comprehensible, several exemplary embodiments accompanied with figures are described in detail below.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.
Referring to
T=C·R=C·R0(1+TC·Δt) (3)
An example is provided to identify the relationship between the clock period 2T of the RC clock circuit and the temperature as below. For example, a RC clock circuit is proposed to generate a clock period of 30 ns, and the clock trimming resistor R has ±20% resistance variation with positive temperature dependence. After resistance trimming at room temperature (25° C.), the voltage drop ΔV on the clock trimming resistor R and the clock periods are unified. But temperature dependence of the resistance and the clock periods still exist as shown in Table 1-1 and Table 1-2. Table 1-1 shows a temperature distribution of the clock period in three corner cases and Table 1-2 shows the temperature distribution of the resistance in three corner cases, where R_fast, R_slow and R_typ represent the fast corner, the slow corner and the typical corner of the process of the fabrication respectively. In the case of the temperature at 25° C., the clock periods of the three corners can approximate the target period 30 ns by trimming the resistance of R. However, when the temperature increases to 125° C., the clock periods of the tree corners all drift. The same condition also occurs in the case of the temperature −40° C. The variance of the clock period between 125° C. and −40° C. in the fast corner is 4.36 ns which is 14.72% of the clock period at temperature 25° C. (29.62 ns). The same condition also occurs in the slow corner and the typical corner.
In order to reduce temperature dependence of a RC clock circuit, a method is provided in embodiments of the invention. Please referring to formula (1) again, R is temperature dependent, if a compensation voltage Vc with substantially the same temperature coefficient TC of the clock trimming resistor R is subtracted from the voltage change on capacitors C1 and C2 while discharging, the first order temperature term of RC time constant can be eliminated as shown below, wherein Vc is ΔV·TC·Δt:
For the second order temperature term (TC·Δt)2 is smaller than the first order temperature term (TC·Δt) and can be omitted thereby, thus temperature dependence of the RC time constant is reduced and can be approximated as below.
T≈R0·C
According to the formula (1), T(ΔV/R)=C·ΔV, a fixed C with unified T and R implies that ΔV is also unified after clock trimming. In other words, the required compensation voltage, Vc=ΔV·TC·Δt, can be treated as a known value once the design parameters of R, C and TC for a RC clock circuit are determined. If the RC clock circuit has multiple clock periods by changing the resistance value of the RC constant, then the Vc may have multiple values accordingly.
In an embodiment of the invention, the compensation voltage Vc can be embodied by a circuit. As shown in
Vc=ΔV·TC·Δt=Vz−ΔV=ΔV(1+TC·Δt)−ΔV (4)
Regarding the first term of Vc, i.e., Vz or ΔV(1+TC·Δt), it can be, for example but not limited to, embodied by flowing a zero temperature coefficient current Iz through a third resistor Rc, where the temperature coefficient of the resistor Rc is TC, and the voltage drop on the third resistor Rc is ΔV at the room temperature. The second term of Vc, ΔV, can be, for example but not limited to, embodied by scaling a bias voltage independent to temperature and a supply voltage of a bandgap circuit. For example, ΔV is VBG·(Rc/Rs), where VBG is a bias voltage independent to temperature and a supply voltage of the bandgap circuit, Rc is the third resistor, Rs is a fourth resistor with a temperature coefficient of TC. The zero temperature coefficient current Iz mentioned above may also be, for example but not limited to, embodied by a combination of a proportional to absolute temperature (PTAT) current and a complementary to absolute temperature (CTAT) current. The PTAT current may be embodied by the bandgap circuit, for example, ΔBE/R1, where ΔVBE is a voltage drop between a base and an emitter of a bipolar junction transistor (BJT) of the bandgap circuit, and R1 is a first resistor with a temperature coefficient of TC. The CTAT current may be also embodied by the bandgap circuit, for example, VBG/R2, where VBG is the bias voltage independent to temperature and the supply voltage of the bandgap circuit, and R2 is a second resistor with a temperature coefficient of TC. Therefore, the first part of Vc, ΔV(1+TC·Δt), is proportional to Rc·(ΔVBE/R1+VBG/R2).
In summary, Vc can be summarized as below.
If R1, R2, Rs and Rc all have substantially the same temperature coefficient Tc as that of the clock trimming resistor R, Vc depends on resistor ratios and constant voltages. Thus it is also resistor-corner independent.
Referring to
Please refer to
In an embodiment, the feedback circuit comprises an operation amplifier and a third resistor. The operation amplifier receives the reference voltage from the RC clock circuit. The third resistor is coupled to the operation amplifier and receives the compensation current from the current generating unit, where a temperature coefficient of the third resistor is substantially the same as the temperature coefficient of the clock trimming resistor of the RC clock circuit, and the operation amplifier and the third resistor construct a negative feedback loop and generate the threshold voltage according to the reference voltage and the compensation current.
Please refer to
In another embodiment, but not limited thereto, please refer to a circuit 320 of
In further embodiment, but not limited thereto, please refer to a temperature dependence reduction circuit 330 of
Referring to
Table 2-1 shows the temperature distribution of the compensation voltage Vc in three corner cases according to the circuit of
Although the compensation voltage generating circuit is designed for typical corner case, however, the temperature deviation of the clock period is reduced around 10% after resistance trimming. It demonstrates resistance variation independence of the compensation voltage generating circuit. The RC clock circuit 100 of
Please referring to
It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.
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