JP2011009291A - Method for designing semiconductor integrated circuit, semiconductor integrated circuit, and countermeasure method against electromagnetic interference - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor integrated circuit that allows the EMI countermeasure to be taken after being assembled into a product without using information outside the LSI.SOLUTION: The semiconductor integrated circuit 100 is mounted with functional block circuits 130, 140 and a switching capacitor 150. The switching capacitor 150 has a switch 151 provided between a power wire 110 and a grounding wire 120, and a decoupling capacitor 152. A frequency band in which interference should be avoided is set by using an arbitrary frequency ft and an arbitrary frequency bandwidth Δft. A capacitance value Cs of the decoupling capacitor 152 is designed such that a resonance frequency fr with external factors is shifted by the frequency bandwidth Δft or above when the switch 151 is turned ON or OFF.

Description

  The present invention relates to a method for designing a semiconductor integrated circuit, a semiconductor integrated circuit, and a countermeasure method for electromagnetic interference.

In recent years, EMI (Electro Magnetic Interference) has become a problem due to the increase in the speed of LSIs and the increase in electronic devices with wireless communication systems.
FIG. 8 shows a case where the first device 11 and the second device 12 which are communication devices communicate with each other and the third device 13 is in the vicinity thereof.
Examples of communication devices include PCs, mobile phones, and game machines.
The third device 13 has an LSI chip 15 and a power / GND plane 16 mounted on a printed circuit board (PCB) 14.
Here, if a current flows when the LSI chip 15 of the third device 13 operates, the power / GND plane 16 may function like an antenna to generate electromagnetic radiation.

FIG. 9 is an output example of electromagnetic radiation (such as a near electromagnetic field) synchronized with the LSI operation clock period (T).
It has been elucidated that the electromagnetic radiation attributed to LSI is mainly related to the internal operating clock frequency and is a phenomenon caused by power supply noise generated at the rising / falling of the clock. Here, a phenomenon is shown in which even-order harmonics are predominantly output around the fundamental frequency (f = 1 / T). For example, when the dominant operating frequency inside the LSI is 50 MHz, the second harmonic (100 MHz) tends to attenuate to the peak. In this case, it is expected that the level of electromagnetic radiation is considerably reduced in the band of 1 GHz or higher.
Actually, the case of an LSI in which power supply noise that does not necessarily occur at the rising / falling of the operation clock is dominant is also considered, so the electromagnetic radiation pattern is not limited to this.
Here, when resonance occurs due to an external factor of the LSI chip and the LSI, inadvertent electromagnetic radiation due to a resonance phenomenon as shown inside the dotted line (near the resonance frequency fr) in the figure occurs.
If this electromagnetic radiation overlaps the communication band between the first device 11 and the second device 12, the electromagnetic radiation from the third device 13 causes electromagnetic interference in the communication between the first device 11 and the second device 12. Will cause.
As a result, phenomena such as communication errors and device failures may occur during electromagnetic interference.

FIG. 10 is a diagram showing a case where the communication system 17, the LSI chip 15, and the power / GND plane 16 are mounted in one system.
Examples of the communication system include Bluetooth, WiFi, and other wireless LAN communication systems. Even in such a case, electromagnetic interference may be caused in its communication system.

  In airplanes and medical settings, equipment malfunctions caused by electromagnetic interference (EMI) are a serious problem. For this reason, in order to prevent electromagnetic interference (EMI) problems in advance, there are EMI regulatory standards for electronic devices in each country. In particular, in developed countries such as the US, EU and Japan, shipment of products is prohibited if this regulatory standard cannot be satisfied.

As a countermeasure against the EMI problem caused by the resonance phenomenon described above, the user who mounts the LSI can suppress the resonance phenomenon by redesigning the inductor and the capacitance component.
However, changing the inductor component of the PCB will reverse the design of the device, increasing the number of development efforts for the device designer.
In addition, since the analysis error of the inductor estimated at the time of design is large, it is difficult to perform redesign that finely adjusts the resonance frequency by the inductor component. The same applies to the capacitance component outside the LSI. Therefore, LSI users (such as set manufacturers) request LSI vendors to take EMI countermeasures in order to take EMI countermeasures at the earliest possible stage before manufacturing the equipment.

However, in reality, there are many cases where EMI problems caused by LSI occur after the manufacture of a product equipped with LSI. If you try to take countermeasures against EMI problems during product design, the design will go back and the man-hours and costs will increase, so the product may be manufactured without detailed EMI countermeasures caused by LSI. This is probably because there are many.
Here, even if LSI vendors take EMI countermeasures, LSI vendors cannot obtain detailed information on PCBs and chassis from set manufacturers.
Against this background, there is a demand for a technology that can easily take EMI countermeasures even after the manufacture of devices equipped with LSIs.

Here, systems for dynamically varying the decoupling capacitance inside the LSI have been proposed as countermeasures against power supply noise after the manufacture of the device on which the LSI is mounted (Patent Document 1, Patent Document 2).
In general, it is widely known that noise is reduced by decoupling capacitance inside an LSI.
Patent Document 1 proposes an LSI system in which power supply noise is measured by a measurement circuit installed inside the LSI, and the decoupling capacitance is varied by a control circuit in accordance with the noise state to suppress noise.
Patent Document 2 discloses an amplifier circuit that changes a resonance frequency by controlling a capacitance value with a switch in accordance with frequency hopping of an input signal.

JP 2008-85321 A JP 2005-33596 A

If noise can be constantly measured and the decoupling capacitance value can be adjusted as in Patent Document 1, a measure can be taken when an unexpected EMI problem occurs due to a resonance phenomenon in the power supply circuit.
However, the realization of this system has problems of an increase in chip area and power consumption.
The reason for increasing the chip area is that the decoupling capacity at the time of design is overestimated.
To constantly monitor noise and increase / decrease the capacitance value means to secure a decoupling capacitance having an area larger than necessary in advance. Therefore, the design target for decoupling is not clarified, which causes an increase in chip area.
Further, it is conceivable that leakage current increases due to excessive mounting of decoupling capacitance, leading to excessive power consumption.
In addition, mounting a complex measurement group used for noise measurement system and analog noise measurement system such as an AD converter on the LSI also increases the chip area.
One of the reasons is that it is difficult to reduce the size of the analog circuit using the same process technology as that of the core logic unit.
The constant operation of these analog circuits also places a power load on parts other than the original LSI function.

In recent years, there has been a demand for suppression of increase in chip area for downsizing of electronic devices and system integration with other functions.
In addition, based on the guidelines for energy saving promoted by the Ministry of Economy, Trade and Industry, there are many demands for low power consumption of LSIs. Therefore, an increase in chip area and power consumption is regarded as a problem in LSI development. Because of these problems, it is desirable to realize a technology that allows decoupling capacitors to be designed and mounted with as small a capacitance value as possible, eliminates the need for peripheral systems such as analog circuits, and can take EMI countermeasures even after manufacturing electronic equipment. .

  In addition, Patent Document 2 discloses a method for reducing noise generated in its own circuit, but does not disclose EMI countermeasures when resonance with external factors occurs. It cannot be applied at all to EMI countermeasures.

A method for designing a semiconductor integrated circuit according to the present invention includes:
A plurality of functional block circuits;
A switching capacitor unit having a switch and a decoupling capacitor provided between a power line and a ground line, and a method for designing a semiconductor integrated circuit including
Set a frequency band to avoid interference using an arbitrary frequency ft and an arbitrary frequency bandwidth Δft,
The capacitance value of the decoupling capacitor is designed to shift a resonance frequency fr with an external factor by the frequency bandwidth Δft or more when the switch is turned on or off.

The semiconductor integrated circuit of the present invention is
A plurality of functional block circuits;
A switching capacitor unit having a switch and a decoupling capacitor provided between a power supply line and a ground line, and a semiconductor integrated circuit comprising:
The capacitance value Cs of the decoupling capacitor is
Any frequency ft in the band you want to avoid, and
The width Δft from the ft to the upper limit or lower limit of the band to avoid interference,
Based on a fixed capacitance value Cf by other elements excluding the decoupling capacitor in the semiconductor integrated circuit,
When the switch is turned on or off, the resonance frequency fr with an external factor is designed to be shifted by the frequency bandwidth Δft or more.

The electromagnetic interference countermeasure method of the present invention includes:
In a product incorporating the semiconductor integrated circuit,
When the resonance frequency fr between the semiconductor integrated circuit and the external factor occurs in the frequency band where the interference should be avoided,
The resonance frequency fr is shifted by turning on or off the switch of the switching capacitor unit.

According to the present invention as described above, EMI countermeasures such as electromagnetic radiation reduction can be performed by realizing an LSI system using a switching capacitor capable of shifting the resonance frequency.
In designing a switching capacitor that shifts the resonance frequency, a target of a frequency band that should avoid interference is clearly set in advance. Furthermore, a design target is set that electromagnetic interference can be avoided if the resonance frequency fr is shifted by at least the frequency bandwidth. Accordingly, the switching capacitor unit can be designed using a decoupling capacitor having a necessary minimum capacitance value.
At this time, information outside the LSI is not necessary.
When an EMI problem occurs in a product incorporating such a semiconductor integrated circuit, the electromagnetic interference problem is solved by shifting the resonance frequency fr by Δfr by turning the switch of the switching capacitor section ON or OFF. It can be solved after manufacturing.

1 is a diagram showing an example of a configuration of a semiconductor integrated circuit (LSI) provided with EMI countermeasures according to the present invention. The figure which shows the case where the switching capacitor is distributedly arranged. Diagram for explaining EMI countermeasures. Diagram for explaining EMI countermeasures. Diagram for explaining EMI countermeasures. Diagram explaining the flow from LSI design to EMI countermeasures. The figure which shows the Example of an EMI countermeasure. The figure which shows the case where electromagnetic interference is produced in another communication apparatus as background art. The figure which shows the output example of the electromagnetic radiation which synchronizes with the operation clock period (T) of LSI. The figure which shows the case where an electromagnetic interference is produced in own communication system.

Embodiments of the present invention will be described below with reference to the drawings.
(First embodiment)
FIG. 1 is a diagram showing an example of the configuration of a semiconductor integrated circuit (LSI) provided with EMI countermeasures according to the present invention.
In FIG. 1, a semiconductor integrated circuit (LSI) 100 includes a plurality of functional block circuits 130 and 140 and a switching capacitor unit 150 between a power supply line 110 and a ground line 120.
The functional block circuit is a circuit for demonstrating the function of the semiconductor integrated circuit 100, such as the core logic circuit 130 and the memory circuit 140, and is appropriately determined according to design requirements.

The switching capacitor unit 150 includes a switch 151 and a decoupling capacitor 152.
In FIG. 1, a switch 151 and a decoupling capacitor 152 are connected in series between the power supply line 110 and the ground line 120.

  Here, the switching capacitor unit 150 may have a simple structure including the switch 151 and the decoupling capacitor 152. The switch 151 and the decoupling capacitor 152 may be connected not only in series but also in parallel.

1 illustrates the case where the switching capacitor unit 150 includes one switch and one decoupling capacitor, the arrangement position and number of the switching capacitor units are not limited.
For example, since it is desirable to install a decoupling capacitor near a PKG (package part) having a relatively large inductor component, it is desirable to provide a switching capacitor part close to a terminal such as a power supply pad. It is not limited to.

Further, the switching capacitors may be distributed and divided into two as shown in FIG. 2, for example. That is, in FIG. 2, the first switching capacitor 160 is provided near the power supply terminal, and the second switching capacitor 170 is provided between the memory circuit 140 and the core logic circuit 130.
Thereby, the gap in the LSI can be effectively used.
In this case, the combined capacity of the capacitor 162 of the first switching capacitor 160 and the capacitor 172 of the second switching capacitor 170 is defined as the capacity of the decoupling capacitor.

Further, the structure and arrangement of the control terminal of the switch 151 are not particularly limited.
The control terminal may be drawn out of the LSI and controlled by an external control circuit, or a circuit for controlling the control terminal may be provided inside the LSI.
The switch 151 may be normally turned on and turned off when taking EMI countermeasures. Conversely, the switch 151 may be turned off and turned on normally when taking EMI countermeasures, and these switches are appropriately selected according to the LSI design specifications. It is what is done.

Next, a method for designing the capacitance value Cs of the decoupling capacitor will be described as an EMI countermeasure.
3, 4, and 5 are diagrams for explaining EMI countermeasures.
FIG. 3 is a diagram showing a band used for wireless communication.
In wireless communication, a communication band to be used is generally determined by various standards. Therefore, the frequency band of a channel often used for communication is set as a “band to avoid interference” (reference numeral 200 in FIG. 3).
At this time, the band 200 in which interference is to be avoided is set to have a bandwidth Δft centered on the frequency ft.
The frequency center ft and the bandwidth Δft may be arbitrarily determined based on the communication device standard or the like.

FIG. 4 is a diagram illustrating a state in which the resonance frequency fr between the LSI 100 and an external factor such as a PCB overlaps the band 200 where interference is to be avoided.
When current flows through the LSI 100, electromagnetic radiation is generated by resonance with the surrounding PCB, and the resonance frequency of the electromagnetic radiation is fr.
If the resonance frequency fr of electromagnetic radiation overlaps the band “ft ± Δft” where interference is to be avoided as shown in FIG. 4, communication will be affected.

Therefore, as shown in FIG. 5, the switch 151 of the switching capacitor 150 is turned ON / OFF, and the resonance frequency fr is shifted by the decoupling capacitor 152.
At this time, if the shift amount Δfr of the resonance frequency fr is an appropriate magnitude and the resonance frequency is out of the band 200 where interference is to be avoided, EMI countermeasures can be realized.

  Therefore, a method for determining the capacitance value Cs of the decoupling capacitor for realizing the shift amount Δfr of the resonance frequency fr will be described.

(Method of determining the capacitance value Cs of the decoupling capacitor)
A method for determining the capacitance value Cs of the decoupling capacitor will be described.
A fixed capacitance value by other elements in the LSI chip 100 excluding the decoupling capacitor is defined as Cf.
Further, let L be the inductor outside the LSI chip 100.
At this time, the resonance frequency fr is expressed by the following (Equation 1).

  Although the inductor component is also present inside the LSI 100, it is relatively small with respect to the external inductor and has little influence on the resonance frequency, and is not incorporated in (Equation 1).

  The shift Δfr of the resonance frequency fr using the switching capacitor 150 is expressed by the following (Equation 2).

  By modifying (Equation 2) above, the external inductor L can be eliminated as follows.

  Further formula transformation is as follows.

  Here, the relationship between the resonance frequency fr and the center frequency ft of the use band is assumed as in the following equation.

  Further, as described with reference to FIG. 5, in order to shift the resonance frequency fr out of the use band, the following equation needs to be satisfied.

In (Equation 7), it is assumed that the resonance frequency fr is close to the center frequency ft of the use band in order to estimate the minimum value of the capacitance Cs of the decoupling capacitor 152.
In actual EMI countermeasures, it is not limited to the case where the resonance frequency fr is equal to the center frequency ft of the used band.

  From the above (Formula 6) and (Formula 7), the capacitance Cs of the decoupling capacitor 152 is estimated as follows using the center frequency ft of the used band, the bandwidth Δft of the used band, and the fixed capacitance value Cf of the LSI. be able to.

According to (Equation 10), the capacitance Cs of the decoupling capacitor 152 is obtained using the center frequency ft of the used band, the bandwidth Δft of the used band, and the fixed capacitance value Cf of the LSI 100. Information about PKG is not necessary.
Then, by incorporating a decoupling capacitor 152 having the capacitance value Cs determined by (Equation 9) into the LSI 100, EMI countermeasures caused by the LSI are taken after product manufacture to avoid electromagnetic interference in the band where interference is to be avoided. Can do.

  When the minimum value is used as the capacitance value of the decoupling capacitor, the case where the equal sign is established in (Equation 9) may be taken.

In the case of a design with a margin for the frequency shift amount Δfr, a capacitance value that is 10% to 20% higher than the capacitance value Cs obtained by (Equation 10) may be employed.
This eliminates the need to use an unnecessarily large decoupling capacitor, so that it does not affect the miniaturization of the LSI.

Next, the flow from LSI design to EMI countermeasures will be described with reference to the flowchart of FIG.
First, an LSI that meets the required specifications is designed (ST100). That is, the necessary arrangement of the core logic circuit 130, the memory circuit 140, and the like are determined.
Next, the information of the frequency band 200 that should avoid interference is grasped (ST110).
By referring to the communication standard or the like, the center ft and the bandwidth Δft of the frequency band used for communication are grasped, and the frequency band to avoid interference is determined as described with reference to FIG.

  Next, the capacitance value Cs of the decoupling capacitor is determined (ST120). That is, the capacitance value Cs of the decoupling capacitor 152 is determined by the above (Equation 9) using the center frequency ft of the use band, the bandwidth Δft of the use band, and the fixed capacitance value Cf of the LSI 100. Then, the decoupling capacitor 152 having the obtained capacitance value Cs and its switch 151 are designed to be mounted on the LSI 100 (ST130).

  The LSI 100 designed in this way is manufactured (ST140) and shipped. The LSI user then combines this with other parts to produce a product (ST150).

Here, when electromagnetic interference caused by LSI occurs (ST160: YES), EMI countermeasures are executed. That is, a control signal is given to the control terminal of the switch 151 to switch the switching capacitor 150 to ON or OFF (ST170).
As a result, the resonance frequency fr can be shifted to avoid electromagnetic interference in the communication use band.

(Example 1)
Next, Example 1 to which specific numerical examples are applied will be described.
In the first embodiment, a wireless LAN standard communication system using a 26 MHz bandwidth (2.471 GHz to 2.497 GHz), which is a system using the 2.4 GHz band, is taken as an example.
Here, the use frequency (arbitrary frequency value in the band) ft is set to 2.48 GHz.
From information on these frequencies, Cs / Cf≈0.021 from (Equation 8).
This ratio should be designed so that the capacitance value (Cs) of the decoupling capacitor is at least 2% of the fixed capacitance component (Cf) of the elements in the LSI chip excluding the decoupling capacitor. Indicates that it should be.

Specifically, it is assumed that the fixed capacitance value Cf inside the LSI chip for which logic design or layout design has been completed is 10 nF.
As for the fixed capacitance value Cf inside the LSI chip, there is an existing technique for estimating from a static capacitance of a transistor, a capacitance between wirings, a well capacitance, etc., and a certain degree of prediction can be made at the LSI design stage. Then, in the first embodiment, the required capacitance value Cs of the decoupling capacitor is determined to be 0.21 nF.
The switching capacitor 150 may be mounted inside the LSI with the capacitance value Cs = 0.21F as a minimum value.

FIG. 7 shows a countermeasure implementation example when interference to the communication band occurs due to a resonance phenomenon caused by LSI.
In a device equipped with LSI 100, when a resonance phenomenon occurs in the vicinity of 2.49 GHz (an example of a value near the assumed operating frequency ft), switching capacitor 150 is turned off. That is, by changing the capacitance value inside the LSI from 10.21 nF to 10 nF, the resonance frequency fr is shifted to 2.51 GHz.
As a result, the peak of electromagnetic radiation can be moved out of band.
However, since the phenomenon of shifting to the low frequency side is conceivable, the operation pattern of the switching capacitor is not limited to this.

  The first embodiment is an embodiment in which the capacitance value Cs of the decoupling capacitor is estimated to the minimum, and the design is made with a margin for the frequency shift amount (the decoupling capacitance Cs is set to 3% (0.3 nF)). If it is designed, etc., safer EMI countermeasures can be taken.

As described above, according to this embodiment, it is possible to avoid an inadvertent EMI problem caused by a resonance phenomenon caused by an LSI and its external factors even after the product is manufactured. In designing the capacitance value Cs of the decoupling capacitor, design information outside the LSI (information on the PCB and the housing) is not necessary.
Further, it is not necessary to mount a complicated circuit such as noise measurement or capacity adjustment. These can prevent an increase in chip area and LSI power consumption.

Note that the present invention is not limited to the above-described embodiment, and can be changed as appropriate without departing from the spirit of the present invention.
In the above description, the band 200 in which interference is to be avoided is set to have the bandwidth Δft with the frequency ft as the center. However, how to obtain ft and Δft can be appropriately changed within a range in which the band in which interference is to be avoided can be specified.
For example, if it is assumed that the resonance frequency is always shifted in the direction of high frequency, an arbitrary low frequency is selected as ft within a band in which interference is to be avoided, and Δft goes out of the high frequency side from ft. The width may be set as follows.

Further, the determination of the capacitance value Cs of the decoupling capacitor by the above design method and the design in which the switching capacitor unit having the decoupling capacitor is incorporated in the LSI may be automatically executed by a computer.
In this case, the input of frequency band information (ft, Δft) that should avoid interference is received, the capacitance value Cs of the decoupling capacitor is calculated by (Equation 9), and the switching capacitor unit is mounted in the LSI. What is necessary is just to add the program to design to a known LSI design program.

11 ... 1st device, 12 ... 2nd device, 13 ... 3rd device, 15 ... LSI chip, 16 ... Power supply / GND plane, 17 ... Communication system, 100 ... Semiconductor integrated circuit (LSI), 110 ... Power supply line, 120 DESCRIPTION OF SYMBOLS ... Ground line, 130 ... Core logic circuit, 140 ... Memory circuit, 150 ... Switching capacitor, 151 ... Switch, 152 ... Decoupling capacitor, 160, 170 ... Switching capacitor.

Claims (8)

A plurality of functional block circuits;
A switching capacitor unit having a switch and a decoupling capacitor provided between a power line and a ground line, and a method for designing a semiconductor integrated circuit including
Set a frequency band to avoid interference using an arbitrary frequency ft and an arbitrary frequency bandwidth Δft,
The semiconductor device is characterized in that the capacitance value of the decoupling capacitor is designed so as to shift a resonance frequency fr with an external factor by the frequency bandwidth Δft or more when the switch is turned on or off. Integrated circuit design method. In the method for designing a semiconductor integrated circuit according to claim 1,
The lower limit of the capacitance value Cs of the decoupling capacitor,
Any frequency ft in the band you want to avoid, and
The width Δft from the ft to the upper limit or lower limit of the band to avoid interference,
A method for designing a semiconductor integrated circuit, comprising: determining a fixed capacitance value Cf based on other elements excluding the decoupling capacitor in the semiconductor integrated circuit. In the method of designing a semiconductor integrated circuit according to claim 2,
The method of designing a semiconductor integrated circuit, wherein the capacitance value Cs of the decoupling capacitor is determined by the following equation.

In the method of designing a semiconductor integrated circuit according to claim 2,
The method of designing a semiconductor integrated circuit, wherein the capacitance value Cs of the decoupling capacitor is determined by the following equation.

A plurality of functional block circuits;
A switching capacitor unit having a switch and a decoupling capacitor provided between a power supply line and a ground line, and a semiconductor integrated circuit comprising:
The capacitance value Cs of the decoupling capacitor is
Any frequency ft in the band you want to avoid, and
The width Δft from the ft to the upper limit or lower limit of the band to avoid interference,
Based on a fixed capacitance value Cf by other elements excluding the decoupling capacitor in the semiconductor integrated circuit,
When the switch is turned on or off, the semiconductor integrated circuit is designed to shift a resonance frequency fr with an external factor by the frequency bandwidth Δft or more. In the semiconductor integrated circuit according to claim 5,
The capacitance value Cs of the decoupling capacitor is expressed by the following equation.

In the semiconductor integrated circuit according to claim 5,
The capacitance value Cs of the decoupling capacitor is expressed by the following equation.

In a product incorporating the semiconductor integrated circuit according to any one of claims 5 to 7, when the resonant frequency fr of the semiconductor integrated circuit and an external factor occurs in a frequency band in which the interference should be avoided,
An electromagnetic interference countermeasure method characterized by shifting the resonance frequency fr by turning on or off the switch of the switching capacitor unit.

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