# Patent

## Title
Delay generator

## Abstract

A delay generator comprises: a current source for supplying a current; a first delay portion, connected to the current source, comprising at least a plurality of inverters and a first capacitor having a first capacitance; and a second delay portion, connected to the current source, comprising at least a plurality of inverters and a second capacitor having a second capacitance, wherein the first capacitance is the same as the second capacitance, wherein the first delay portion generates a first delay by discharging of the first capacitor, wherein the second delay portion generates a second delay by charging of the second capacitor, and wherein the total delay generated by the delay generator is obtained by summation of the first delay and the second delay.

## Background
BACKGROUND OF THE INVENTION
The present invention relates to a delay generator. More particularly, it relates to a process-insensitive current-controlled delay generator with threshold voltage compensation.
DESCRIPTION OF THE RELATED ART
Sampled-data systems incorporating data conversion and switched-capacitor filters are indispensable in state-of-art IC design, and are crucial for applications such as telecommunication, consumer electronics and medical imaging. In such discrete-time systems, the clock generator is extremely important, and the accuracy of the clock signal is determinant in the overall design since it often affects the overall resolution. However, the variation of the clock pulse width exists inevitably and is normally associated with process or temperature variations in the delay paths. Usually, large design margins should be adopted in the transistor implementation to overcome such process variations. Unfortunately, this would imply extra power consumption with the subsequent degradation of system performance.
Therefore, process-insensitive delay generators are highly demanded and effective solutions have been proposed either off-chip or on-chip.
Traditionally, the delay generator is implemented by the inverter-chain, also referred to as gm/C circuit that accumulates the time delay of the inverters and provides the time delay for the system. Although its architecture is quite simple, it suffers from a significant process variation sensitivity that can lead to a significant ±15% variation in time delay.
An alternative solution, the current-controlled delay generator was previously proposed to achieve higher process-insensitivity with the utilization of less process-sensitive circuit elements. FIG. 1a schematically illustrates a simplified delay generator which contains basically a current source, a capacitor, switches, and output buffers; and FIG. 1b illustrates input and output waveforms of the delay generator of FIG. 1a. The top-plate of the capacitor C is firstly charged to the voltage supply VDD and Φout remains at HIGH level. Then, the capacitor C is linearly discharged by a constant current Ib which is controlled by the current source, and thus a delay td is generated.
To calculate the delay td, firstly refer to the following equation:




  
    
      
        i
        =
        
          C
          ⁢
          
            
              d
              
                V
                C
              
            
            
              d
              t
            
          
        
      
    
    
      
        (
        1
        )
      
    
  




Under a linear situation, dt≈Δt and dVC≈ΔVC,
thus when i=Ib; we obtain




  
    
      
        
          Δ
          ⁢
          
            
          
          ⁢
          t
        
        =
        
          
            C
            ⁢
            
              
                Δ
                ⁢
                
                  
                
                ⁢
                
                  V
                  C
                
              
              
                I
                b
              
            
          
          =
          
            t
            d
          
        
      
    
    
      
        (
        2
        )
      
    
  




where ΔVC is equal to VDD−Vth (Vth is the threshold voltage of the inverter connected to Vc).
The current Ib is provided by the current source and its accuracy is mainly related to the precision of the current mirror and the reference current. This is usually accurate and the current mirror is relatively easy to design with good matching. Therefore, the current will not be significantly affected by process variations. Besides, a MOS-capacitor is adopted to ensure less sensitivity to process variations, when compared with other type of implementations of the capacitors. Normally, the MOS-capacitance varies around ±5% with process.
However, the delay generator of FIG. 1a is still sensitive to process variations, mainly because of threshold voltage variation of the inverter connected to the capacitor. When VC decreases, the inverter will be triggered to generate the delay td until VC passes its threshold voltage Vth that depends on the robustness of the N/PMOS transistors and is highly process-sensitive.
SUMMARY OF INVENTION
In view of the above, it is an object of the present invention to provide an advanced current-controlled delay generator using process-insensitive components such as current mirrors and MOS-capacitors, thereby avoiding the complexity of a delay-locked loop (DLL). By applying the threshold voltage compensation, the delay generator of the present invention reduces the deviation induced by the internal inverter buffer, and thus becomes more robust to process variations than prior art.
According to an aspect of the present invention, a delay generator comprises: a current source for supplying a current; a first delay portion, connected to the current source, comprising at least a plurality of inverters and a first capacitor having a first capacitance; and a second delay portion, connected to the current source, comprising at least a plurality of inverters and a second capacitor having a second capacitance, wherein the first capacitance is the same as the second capacitance, wherein the first delay portion generates a first delay by discharging of the first capacitor, wherein the second delay portion generates a second delay by charging of the second capacitor, and wherein the total delay generated by the delay generator is obtained by summation of the first delay and the second delay.
According to the above aspect of the present invention, the total delay is determined by the current and the first capacitance.
According to the above aspect of the present invention, the first delay portion further comprises two switches that will be tamed on by opposite input clocks.
According to the above aspect of the present invention, the second delay portion further comprises two switches that will be turned on by opposite input clocks.



BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.
FIG. 1a shows a simplified diagram of a delay generator according to prior art.
FIG. 1b illustrates input and output waveforms of the delay generator of FIG. 1a. 
FIG. 2 shows a block diagram of a delay generator according to the present invention.
FIG. 3 illustrates input and output waveforms of the delay generator according to the present invention.
FIG. 4 shows the circuit implementation of the delay generator according to the present invention.
FIG. 5a shows the characteristic waveforms for a first delay portion of the delay generator according to the present invention.
FIG. 5b shows the characteristic waveforms for a second delay portion of the delay generator according to the present invention.
FIGS. 6a and 6b show characteristic waveforms of the delay generator according to the present invention in a typical case and two extreme cases.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
A preferred embodiment for a delay generator according to the invention is described with reference to the drawings as follows.
Refer firstly to FIG. 2, which illustrates an embodiment of a current-controlled delay generator 200 according to the invention. In this embodiment, the delay generator 200 comprises a current source 210, a first delay portion 220 and a second delay portion 230. The current source 210 provides a current Ib to both the first delay portion 220 and the second delay portion 230. An input clock Φin in is fed into the first delay portion 220 and an output clock Φmiddle having a delay td1 with respect to the clock Φin is generated. Next, the clock Φmiddle is fed into the second delay portion 230 and an output clock Φout having a delay td2 with respect to the clock Φmiddle is generated. As a result, the total delay tdtotal generated by the delay generator 200 is obtained as td1+td2. A schematic diagram showing the total delay tdtotal=td1+td2 is shown in FIG. 3. The total delay tdtotal generated by the delay generator 200 is insensitive to the process variation because the process variation incurred when td1 is generated and the process variation incurred when td2 is generated will be compensated by each other. A further detail for achieving the compensation effect of the process variation according to the present invention is described as follows.
The circuit of FIG. 4 represents a preferred embodiment of the delay generator 200. As shown in the figure, the delay generator 200 is comprised of the first delay portion 220 and the second delay portion 230. Although, in FIG. 4, the current source 210 is shown as included in the first delay portion 220, the current Ib is also provided to the second delay portion 230 through the bias point Vbn.
The operation of the first delay portion 220 can be divided into two phases. At the first phase, the clock Φin is going from LOW to HIGH, the switch S1 is open (turned off) and the switch S2 is closed (turned on). Accordingly, a transient current will charge a capacitor C1 until the top-plate voltage VC1 of the capacitor C1 reaches VDD. Next, at the second phase, the clock Φin is going from HIGH to LOW, the switch S1 is closed (turned on) and the switch S2 is open (turned off). Because VC1 has been charged to VDD, the voltage Vtr1 (which is obtained from VC1 after two inverters) will be VDD, too. Thus, M3 connects and a discharging current begins to flow. The current mirror comprised of M1 and M2 renders the discharging current equal to Ib. The charge stored on the top plate of the capacitor C1 flows to the ground through S1, M3 and M2, and thus the voltage VC1 starts dropping. When VC1 reaches the threshold voltage Vth (triggering point) of the inverter, Vtr1 becomes digital ‘0’ and thus shuts off M3. At this moment, the discharging current stops flowing from C1 and VC1 remains the same. The above discharging operation generates a delay td1, and thus the clack Φmiddle is output, as clearly illustrated in FIG. 5a. Subsequently, the operation returns to the first phase, VC1 will be charged to VDD again and Vtr also goes to VDD by rapid pull-up of the inverter.
As shown in FIG. 5a, VC1 is dropping linearly. Accordingly, by referring to the above-mentioned equation (2), td1 can be expressed as:




  
    
      
        
          t
          
            d
            ⁢
            
              
            
            ⁢
            1
          
        
        =
        
          C
          ⁢
          
            
          
          ⁢
          1
          ⁢
          
            
              
                V
                th
              
              -
              VDD
            
            
              -
              
                I
                b
              
            
          
        
      
    
    
      
        (
        3
        )
      
    
  




Similarly, the operation of the second delay portion 230 can be divided into two phases. For the second delay portion 230, the clock Φmiddle is inverted and used as an input clock. At the first phase, the clock Φmiddle is going from HIGH to LOW, the switch S3 is open (turned off) and the switch S4 is closed (turned on). Accordingly, C2, which has a capacitance as the same as that of C1, will be discharged through the switch S4 and reset to ground. Next, at the second phase, the clock Φmiddle is going from LOW to HIGH, the switch S3 is closed (turned on) and the switch S4 is open (turned off). Accordingly, C2 is charged by a constant current from a p-type current mirror comprised of M6 and M7, and the voltage VC2 starts raising from 0 (ground). When VC2 reaches the threshold voltage Vth (triggering point) of the inverter, Vtr2 becomes digital ‘0’ and thus shuts off M8. At this moment, the charging current stops flowing to C2 and VC2 remains the same. The above charging operation generates a delay td2, and thus the clock Φout is output, as clearly illustrated in FIG. 5b. 
As shown in FIG. 5b, VC2 is raising linearly. Again, by referring to the above-mentioned equation (2), ttd2 can be expressed as:




  
    
      
        
          t
          
            d
            ⁢
            
              
            
            ⁢
            2
          
        
        =
        
          C
          ⁢
          
            
          
          ⁢
          2
          ⁢
          
            
              
                V
                th
              
              -
              0
            
            
              I
              b
            
          
        
      
    
    
      
        (
        4
        )
      
    
  




As mentioned above, the total delay tdtotal generated by the delay generator 200 is obtained by summation of td1 and td2, that is, td1+td2. From equations (3) and (4), we obtain:




  
    
      
        
          t
          dtotal
        
        =
        
          
            
              
                (
                
                  
                    C
                    ⁢
                    
                      
                    
                    ⁢
                    1
                  
                  -
                  
                    C
                    ⁢
                    
                      
                    
                    ⁢
                    2
                  
                
                )
              
              ⁢
              
                V
                th
              
            
            +
            
              C
              ⁢
              
                
              
              ⁢
              1
              ×
              VDD
            
          
          
            I
            b
          
        
      
    
    
      
        (
        5
        )
      
    
  




As C1=C2, it leads to,




  
    
      
        
          t
          dtotal
        
        =
        
          
            C
            ⁢
            
              
            
            ⁢
            1
            ×
            VDD
          
          
            I
            b
          
        
      
    
    
      
        (
        6
        )
      
    
  




Since C1, VDD and Ib are all preset values, tdtotal will be a constant. In other words, the total delay tdtotal is not affected by the threshold voltage Vth, which is highly process-sensitive.
To further explain the threshold voltage compensation applied by the present invention, refer now to FIGS. 6a and 6b, wherein characteristic waveforms of the delay generator according to the present invention are illustrated in a typical case (‘tt’) and two extreme cases (‘fs’: fast NMOS and slow PMOS, ‘sf’: slow NMOS and fast PMOS). The two extreme cases are provided as examples of process corners which will produce most significant drifts in the threshold voltage Vth. As shown in the figures, waveforms similar to those of FIG. 5a and 5b can be observed for the typical case (process corner ‘tt’). For the ‘fs’ process corner, a shorter td1,fs and a longer td2,fs can be observed For the ‘sf’ process corner, a longer td1,sf and a shorter td2,sf can be observed. However, both the summation of td1,fs and td2,fs and the summation of td1,sf and td2,sf will lead to the same total delay tdtotal as that of the typical process corner ‘tt’. In other words, the shorter td1,fs is “compensated” by the longer td2,fs, and the longer td1,sf is “compensated” by the shorter td2,sf. This is not difficult to derive because, according to the above equation (6), the same total delay tdtotal will be obtained for either the ‘fs’ process corner or the ‘sf’ process corner.
While the present invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. Therefore, it is intended that the invention will include all embodiments falling within the scope of the appended claims.

## Claims

1. A delay generator, comprising:
a first delay portion comprising at least a plurality of inverters and a first capacitor having a first capacitance; and
a second delay portion comprising at least a plurality of inverters and a second capacitor having a second capacitance;
a current source for providing a current to the first delay portion and the second delay portion,
wherein the first capacitance is the same as the second capacitance,
wherein the first delay portion generates a first delay by discharging of the first capacitor,
wherein the second delay portion generates a second delay by charging of the second capacitor, and
wherein the total delay generated by the delay generator is obtained by summation of the first delay and the second delay.

2. The delay generator according to claim 1, wherein the total delay is determined by the current and the first capacitance.

3. The delay generator according to claim 1, wherein the first delay portion further comprises two switches that will be turned on by opposite input clocks.

4. The delay generator according to claim 1, wherein the second delay portion further comprises two switches that will be turned on by opposite input clocks.