# Patent

## Title
Area-efficient distributed device structure for integrated voltage regulators

## Abstract

An area efficient distributed device for integrated voltage regulators comprising at least one filler cell connected between a pair of PADS on I/O rail of a chip and at least one additional filler cell having small size replica of said device is coupled to said I/O rails for distributing replicas of said device on the periphery of said chip. The device is connected as small size replica on the lower portion of said second filler cell for distributing said device on the periphery of said chip and providing maximal area utilization.

## Background
PRIORITY CLAIM
This application claims priority from Indian patent application No. 2613Del/2004, filed Dec. 31, 2004, which is incorporated herein by reference.
TECHNICAL FIELD
The invention in general relates to an area-efficient distributed device structure for integrated voltage regulators, and in particular relates to an area-efficient distribution of a large-size device for integrated voltage regulators.
BACKGROUND
In a VLSI chip, both the core and the I/O blocks play an important role. The I/O blocks are arranged in the periphery in a ring-like fashion. To handle various signals like the power signals (high-voltage and low-voltage supplies and grounds), ESD signals, slew control signals, there are various rails passing above all the I/O blocks. However, these I/O blocks are spaced apart by a certain distance, the minimum of which is known as the pitch of the technology used. To ensure the continuity of these rails, some filler cells known as the IO-FILLERS are generally used. These filler cells contain nothing more than metal rails and ensure the continuity of rails in a non-stop ring-like fashion. The rails are generally in the top metal layers. However, the area occupied by these filler cells is not used for the fabrication of any transistor. Thus, the area of the metal rails is underutilized.
A concept of a voltage regulator is such that it contains a driver MOS (also called a pass transistor), whose size depends upon the load-current capability of the voltage regulator and is generally huge to provide current to the entire chip. This MOS needs an input supply VIN and generates an output supply VOUT, controlled by a voltage VCONT generated by a feedback circuit and an error amplifier as illustrated in FIG. 1. The output supply generated needs to be distributed in the entire chip. It is often not feasible to route the output supply to each and every corner of the chip, thus it is preferable to make use of the I/O ring in the periphery of the chip. This I/O ring will automatically route the supply VOUT around the periphery of chip. This is implemented by placing this driver MOS on the periphery of the chip so that it takes the input supply from one rail of the I/O ring and drives the output supply on another rail of the I/O ring, with the controlling voltage VCONT on a third rail.
The rails for VIN, the higher input supply, and VOUT, the lower output supply, are always present in an I/O ring with their corresponding grounds. In addition to these rails, there are certain dedicated rails in an I/O ring to take an external reference signal round the chip. One of these rails can be used to take the VCONT signal round the periphery of the chip to connect the gates of all the pass transistors together. In this way all the three nodes connected to the pass transistor are taken round the I/O ring with great ease.
Conventional voltage regulators have a bypass mode, where VIN is to be bypassed to the output node VOUT by pulling down the VCONT node to ground, and VIN applied is at the level of the VOUT itself. For example, in the bypass mode of a 5V to 1.2V voltage regulator, the voltage VIN, which is otherwise 5V, itself becomes 1.2V, and this voltage is then transferred to the VOUT node via the resistance drop of a switched-on PMOS. So the MOS sizes typically need to be huge to have a low on resistance.
A conventional technique employed for a VLSI chip containing a voltage regulator is illustrated in FIG. 2. Also shown in the figure is the lumped pass transistor, the output transistor of the voltage regulator, along with a number of pads to satisfy electromigration rules. This arrangement may suffer from severe drawbacks such as electromigration problems due to poor power distribution, high IR drops, and difficulty in routing to the core.
Another conventional structure shows distributing the pads and fractions of the pass transistor over the periphery of the chip. If the pass transistor has four such pads, then the transistor can be split into four parts with each part occupying a side of the chip along with a pad. The scheme is shown in FIG. 3. This provides better power distribution and lower IR drops, but a potential problem in this structure is that each such I/O can occupy more area than a standard I/O in order to accommodate the huge-sized pass transistor. The problem may get worse in the bypass mode of the voltage regulators, where much larger sizes of the pass transistor are typically needed as there is no regulation in the bypass mode, and the on-resistance of the pass transistor typically needs to be reduced significantly. Thus, this structure has drawbacks in pad-limited designs.
U.S. Pat. No. 6,594,809 is a prior-art patent pertaining to area utilization within the core of a chip. It relates to low-leakage diode insertion for integrated circuits, particularly to inserting diodes in filler cells in the core of the integrated circuits. A drawback of this patent is that it does not provide a solution for utilization of area on the periphery of chip.
SUMMARY
Thus, a need has arisen for an area-efficient structure that utilizes the area on the periphery of the integrated circuit by ensuring that each I/O occupies the same area as a standard I/O and does not pose area problem to a pad-limited design.
An embodiment of the present invention is an area-efficient distributed device structure for integrated voltage regulators.
Another object of the present invention is a pass transistor that is distributed among a plurality of IOs around the periphery of the chip.
A further embodiment of the present invention minimizes electro migration problems in a chip and minimizes IR drops in the overall chip supplied by integrated voltage regulators.
Yet another embodiment of the present invention distributes the pass transistor so that each I/O occupies the same area as a standard I/O and does not pose a problem to a pad-limited design.
An embodiment of the present invention that provides an area-efficient distributed device structure for integrated voltage regulators comprises:

    
    
        at least one filler cell connected between a pair of PADS on an I/O rail of a chip,
        at least one additional filler cell having a small-size replica of said device is coupled to said I/O rails for distributing replicas of said device on the periphery of said chip.
    
    


Further, an embodiment of the present invention is a method for creating a distributed device structure for integrated voltage regulators comprising steps of:

    
    
        coupling at least one filler cell between a pair of PADS on I/O rail of a chip,
        incorporating replicas of said device in additional filler cells, and
        connecting said additional filler cell to said I/O rails.
    
    





BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a conventional voltage regulator.
FIG. 2 illustrates a conventional technique of placing a pass element in a VLSI chip.
FIG. 3 shows another conventional technique for placing a pass element in a VLSI chip.
FIG. 4 illustrates an area-efficient distributed device structure for integrated voltage regulators in accordance with an embodiment of the invention.
FIG. 5 illustrates a structure within a filler cell in accordance with an embodiment of the instant invention.



DETAILED DESCRIPTION
An embodiment of the invention is a distributed structure for a pass transistor such that each I/O occupies the same area as a standard I/O and does not pose any area problem to a pad-limited design. The remaining pass transistors can be accommodated in the area previously (in the prior art) occupied by the IOFILLERS (FIG. 4) by breaking it into small parts. These FILLER cells can be of multiple sizes (8×, 16×, 32×, 64×) and spread throughout the I/O ring on the periphery of the chip.
Thus, it is possible to use the area occupied by the IOFILLERS to fabricate the pass transistor of a regulator. More specifically, the conventional IOFILLERS can be replaced by new TRANFILLER cells, each containing a small portion of the pass transistor below the I/O rails, besides the various metal layers. The lowest metal layers can be used for the routing purposes in the transistor. Such a structure for the distributed device is illustrated by FIG. 4. Just as there are many IOFILLERS present in a chip, similarly a plurality of TRANFILLERS can replace some or all of these IOFILLERS, depending upon the size of the pass transistor remaining to be placed after placing some of it along with its associated pads; wherein the TRANFILLERS are filler cells comprising the replicas of the device to be distributed. Just as IOFILLERS, the TRANFILLERS can also be made to be available in various sizes of 8×, 16×, 32×, 64×.
As discussed above, this embodiment of the invention is aimed at making a VLSI chip more area-efficient, and has the following advantages over the conventional structures of integrated voltage regulators.
Makes the chip less susceptible to electromigration. Particularly in circuits where the current-carrying capacity of the pass transistor under consideration is expected to be very high, electromigration may become a key factor. By distributing the transistor in the explained fashion, the required current finds several parallel paths instead of a single path as in the case of a lumped transistor. In this way, the fabricated product becomes less susceptible to failures due to current surges.
Reduces the possibility of large IR drops in the chip, which might be there due to the huge size of the chip. In other words, this embodiment improves the uniformity in the distribution of the power supplies.
In a pad-limited design, provides an area-efficient device distribution on the periphery of the chip. The pitch of the pads can be the minimum supported by the technology, even for the special I/Os containing the pass transistor, as the remaining portions of the transistor goes into the FILLERS.
Referring to FIG. 5, along with the pass transistor, other devices such as capacitances and resistances that might be required to compensate the regulator can also be placed in the TRANFILLERS, thereby making the structure more symmetrical and area-efficient. Alternatively, these other devices may be placed in other IOFILLERS, i.e., IOFILLER locations in which portions of a regulator pass transistor are not present.
An integrated circuit such as shown in FIGS. 4-5 may be part of an electronic system such as a computer system.
From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention.

## Claims

1. An area efficient distributed device structure for integrated voltage regulators comprising:
at least one filler cell coupled between a pair of pads on an i/o rail of a chip and having a portion of the device structure disposed therein,
at least one additional filler cell having a smaller-sized portion of the device structure coupled to the i/o rail for distributing signals of the device structure on a periphery of the chip.

2. An area efficient distributed device structure for integrated voltage regulators as claimed in claim 1, wherein the device structure is a small size replication of a transistor.

3. An area efficient distributed device structure for integrated voltage regulators as claimed in claim 2, wherein the transistor is coupled at an output node of a voltage regulator for driving output current.

4. An area efficient distributed device structure as claimed in claim 1, wherein the at least one additional filler cell comprises multiple sized filler cells corresponding to device layout rules.

5. An area efficient distributed device structure for integrated voltage regulators as claimed in claim 1, wherein the additional filler cell is placed on the i/o rail.

6. An area efficient distributed device structure for integrated voltage regulators as claimed in claim 5, wherein the additional filler cell is coupled around a PAD for distributing the signals corresponding to a size of the device structure.

7. An integrated circuit, comprising:
a periphery;
first and second input-output blocks disposed within the periphery;
a first filler cell disposed within the periphery between the first and second input-output blocks; and
a voltage regulator having a component disposed within the first input-output block and the first filler cell.

8. The integrated circuit of claim 7, further comprising:
an input-supply-voltage rail disposed within the periphery;
a regulated-output-voltage rail disposed within the periphery; and
wherein the voltage regulator has an input node coupled to the input-supply-voltage rail and has an output node coupled to the regulated-output-voltage rail.

9. The integrated circuit of claim 7 wherein the component comprises a pass transistor.

10. The integrated circuit of claim 7 wherein the component comprises a capacitor.

11. The integrated circuit of claim 7, further comprising:
a third input-output block within the periphery;
a second filler cell disposed within the periphery between the first and third input-output blocks; and
wherein the component is disposed within the second filler cell.

12. The integrated circuit of claim 7, further comprising:
third and fourth input-output blocks within the periphery;
a second filler cell disposed within the periphery between the third and fourth input-output blocks; and
wherein the component is disposed within the second filler cell.

13. The integrated circuit of claim 7, further comprising:
an input-supply-voltage rail disposed within the periphery;
a regulated-output-voltage rail disposed within the periphery;
a control-signal rail disposed within the periphery; and
wherein the component comprises a transistor having an input node coupled to the input-supply-voltage rail, an output node coupled to the regulated-output-voltage rail, and a control node coupled to the control-signal rail.

14. An integrated circuit, comprising:
a periphery;
first and second input-output blocks disposed within the periphery and having substantially a same size;
a first pad disposed within the periphery and over one of the first and second input-output blocks; and
a voltage regulator having a component disposed within the one of the first and second input-output blocks.

15. The integrated circuit of claim 14, further comprising:
a filler cell disposed within the periphery; and
wherein a first portion of the component is disposed within the one of the first and second input-output blocks and wherein a second portion of the component is disposed within the filler cell.

16. The integrated circuit of claim 14 wherein the component comprises a pass transistor.

17. The integrated circuit of claim 14, further comprising:
a second pad disposed within the periphery and being remote from the first and second input-output blocks;
a filler cell disposed within the periphery between the first and second pads; and
wherein a first portion of the component is disposed within the one of the first and second input-output blocks and wherein a second portion of the component is disposed within the filler cell.

18. An electronic system, comprising:
an integrated circuit, including
a periphery,
first and second input-output blocks disposed within the periphery,
a first filler cell disposed within the periphery between the first and second input-output blocks, and
a voltage regulator having a component disposed within the first input-output block and the first filler cell.

19. An electronic system, comprising:
an integrated circuit including
a periphery,
first and second input-output blocks disposed within the periphery and having substantially a same size,
a first pad disposed within the periphery and over one of the first and second input-output blocks, and
a voltage regulator having a component disposed within the one of the first and second input-output blocks.