# Patent

## Title
Method and apparatus for measuring charge pump output current

## Abstract

A method for measuring output current of a charge pump, the method including providing a charge pump including a plurality of n charge pump stages, wherein an output of stage n−1(Von−1) is output to stage n, an output voltage of stage n being referred to as charge pump voltage output Vout, connecting an additional output pass device to the output of stage n−1, an output voltage of the additional output pass device being referred to as Voutm, forcing Voutm to be at least approximately equal to Vout, drawing at least one of output voltage (Voutm) and output current (Ioutm) from the additional output pass device, measuring Ioutm (e.g., comparing Ioutm with a reference current), and correlating Iout with Ioutm.

## Background
FIELD OF THE INVENTION
The present invention relates generally to charge pumps for boosting voltages in microelectronic circuitry, and particularly to a method for measuring the output current of the charge pump.
BACKGROUND OF THE INVENTION
Non-volatile memory arrays, such as erasable, programmable read only memory (EPROM) or flash memory arrays, or electrically erasable, programmable read only memory (EEPROM) arrays, require high positive or negative voltages to program and erase memory cells of the array. Typically, these voltages are higher than the voltage supplied (Vdd). Charge pumps are generally used to boost on-chip voltages above the supply voltage Vdd to reach the voltages required for programming or erasing.
A charge pump typically comprises cascaded stages that progressively boost the voltage to higher levels. The charge pump functions by progressively storing more charge on a capacitor which is part of a capacitor-diode combination, with several such stages being placed together in a network to obtain the desired increase in voltage. The diode functions to prevent discharge of the capacitor prior to placing the additional charge thereon.
Many applications require measuring the output current of the charge pump, for example, for measuring the program or erase current for flash cell applications. In the prior art, measuring the output current of the charge pump is typically accomplished by sensing or measuring internal signals of the charge pump. For example, for an on/off charge pump, the internal signal “oscillator_on” include reference information about the charge pump load—the ratio of the actual output current to the maximum output current (Iout/Iout_max). For a supply charge pump, the internal signal “supply clamp” may provide that reference information.
However, using these internal signals has disadvantages. The charge pump maximal output current Iout_max is a function of several parameters, such as but not limited to, Vdd, temperature and corner technology process. Since the ratio of the actual output current to the maximum output current is dependent upon these parameters, it is not possible to measure the absolute value of the output current (Iout).
Other methods have been used in the art to measure the output current, but they have disadvantages, such as requiring significant circuit board area and/or having relatively high power dissipation.
SUMMARY OF THE INVENTION
The present invention provides a novel method for measuring the output current of a charge pump or charge pump stage, as is described more in detail hereinbelow. In the present invention, an additional output stage may be connected to the existing charge pump output stage. The additional output stage may use the same phase signals as the existing charge pump output stage. The additional output stage can be scaled down to minimize size. The included stage output current may be measured by comparing with a reference current. If the output voltages of the additional and existing output stages are equal, then the ratio of their currents is equal to their size ratio.
In accordance with a non-limiting embodiment of the invention, instead of adding an entire additional output stage, an additional output pass device (transistor or diode) of stage #n can be added, which connects the previous stage output (Von−1) to the charge pump output (Vout). The transistor/diode may be scaled down to a minimal size to reduce power dissipation and/or board size.
The simple circuitry implementation of the present invention may significantly reduce the chip/board area needed for the circuitry, as well as significantly reducing power dissipation. The invention is fast, accurate and insensitive to charge pump architecture and regulation.
There is thus provided in accordance with an embodiment of the present invention a method for measuring output current of a charge pump, the method including providing a charge pump including a plurality of n charge pump stages, wherein an output of stage n−1 (Von−1) is output to stage n, an output voltage of stage n being referred to as charge pump voltage output Vout, connecting an additional output pass device to the output of stage n−1, an output voltage of the additional output pass device being referred to as Voutm, forcing Voutm to be at least approximately equal to Vout, drawing at least one of output voltage (Voutm) and output current (Ioutm) from the additional output pass device, measuring Ioutm (e.g., comparing Ioutm with a reference current), and correlating Iout with Ioutm.
In accordance with an embodiment of the present invention if output voltages of the additional output pass device and the charge pump stage n are equal, then a current ratio (Ioutm/Iout) is equal to a ratio of sizes of the additional output pass device and the charge pump stage n.
Further in accordance with an embodiment of the present invention the additional output pass device may include an additional output stage. The additional output stage may use identical phase signals as stage n. The additional output pass device may be scaled down to minimize its size.
In accordance with an embodiment of the present invention the additional output pass device may include a p-n diode; a n-p diode; an NMOS transistor, whose source is connected to the output of stage n−1 and whose drain is connected to an input of stage n, and whose gate and source are connected to one another; a PMOS transistor, whose drain is connected to the output of stage n−1 and whose source is connected to an input of stage n, and whose gate and drain are connected to one another; an NMOS transistor, whose source is connected to the output of stage n−1 and whose drain is connected to an input of stage n, and whose gate is connected to a control voltage; or alternatively, a PMOS transistor, whose drain is connected to the output of stage n−1 and whose source is connected to an input of stage n, and whose gate is connected to a control voltage.
Further in accordance with an embodiment of the present invention comparing Ioutm with the reference current may include inputting the charge pump output Vout and the additional output device output Voutm into a current mirror, and measuring and comparing currents flowing through sides of the current mirror.
There is also provided in accordance with an embodiment of the present invention circuit architecture for measuring output current of a charge pump, including a charge pump including a plurality of n charge pump stages, wherein an output of stage n−1 (Von−1) is output to stage n, an output voltage of stage n being referred to as charge pump voltage output Vout, an additional output pass device connected to the output of stage n−1, an output voltage of the additional output pass device being referred to as Voutm, a first circuit portion for forcing Voutm to be at least approximately equal to Vout and for drawing at least one of output voltage (Voutm) and output current (Ioutm) from the additional output pass device, and a second circuit portion for measuring current Ioutm and correlating Iout with Ioutm.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:
FIG. 1 is a simplified circuit diagram of a charge pump architecture, constructed and operative in accordance with an embodiment of the present invention, with an additional output stage connected to the existing charge pump output stage;
FIGS. 2A–2F are simplified circuit diagrams of different types of output pass devices (e.g., transistors or diodes) that can be used to connect the previous stage output to the charge pump output, in accordance with different embodiments of the present invention; and
FIGS. 3 and 4 are simplified circuit diagrams of circuitry, wherein output current of the additional output stage is measured by comparing with a reference current, in accordance with different embodiments of the present invention.



DETAILED DESCRIPTION OF EMBODIMENTS
Reference is now made to FIG. 1, which illustrates a charge pump architecture, constricted and operative in accordance with an embodiment of the present invention.
The charge pump architecture includes a plurality of n charge pump stages (stage 1, stage 2, . . . stage n−1, stage n). As is well known in the art, the charge pump stages may include charge transfer transistors that may use, but are not limited to, CMOS (complementary metal oxide semiconductor) technology, being either n-channel or p-channel (NMOS or PMOS) field effect transistors (FETs). NMOS is generally used to pump positive voltages, whereas PMOS is generally used to pump negative voltages.
Each stage outputs a voltage to the next successive stage. Accordingly, the output of stage 1 (Vo1) is output to stage 2, the output of stage 2 (Vo2) is output to stage 3, and so forth, wherein the output of stage n−1 (Von−1) is output to stage n. The voltage output of stage n is the charge pump output Vout.
In accordance with an embodiment of the present invention an additional output stage may be connected to the existing charge pump output stage. Specifically an additional output stage n may be connected to the output of stage n−1. The additional output stage may use the same phase signals as the existing charge pump output stage. The additional output stage can be scaled down to minimize size. The additional output stage output current is referred to as Ioutm.
In simplified terms, which will be described more in detail hereinbelow with reference to FIGS. 3 and 4, a first circuit portion is provided that forces the output voltage of the additional output stage (Voutm) to be equal or approximately equal to the charge pump output voltage (Vout). The first circuit portion also draws output current or voltage from the additional output stage and sends it to a second circuit portion. The second circuit portion measures the output current or voltage from the additional output stage (wherein Voutm may be converted to Ioutm by suitable application of Ohm's law, for example). The current Ioutm may be measured in a number of ways. For example, it may be measured by comparing with a reference current Iref (as is described more in detail hereinbelow). This comparison may be used to produce a binary result. For example, if Iref >Ioutm then the binary result is 0. If Iref ≦Ioutm then the binary result is 1.
Reference is now made to FIGS. 2A–2C. In accordance with a non-limiting embodiment of the invention, instead of adding an entire additional output stage, an additional output pass device (e.g., transistor or diode) of stage n can be added, which connects the previous stage output (Von−1) to the charge pump output Vout. In all embodiments, the additional output pass device may be scaled down to a minimal size to reduce power dissipation and/or board size.
For example, in the charge pump stages of FIG. 2A, the previous stage output (Von−1) is connected to the charge pump output Vout with a p-n diode 10. Accordingly, in the present invention, an identical output pass device 12 (i.e., p-n diode 12) may be connected to the previous stage output (Von−1). The current output Ioutm of this additional p-n diode 12 may be measured (by comparing with a reference current) to obtain the charge pump output current.
In the charge pump stages of FIG. 2B, the previous stage output (Von−1) is connected to the charge pump output Vout with an NMOS transistor 14 in diode connection. By “diode connection” it is meant that the source of NMOS transistor 14 is connected to the output of the previous stage output (Von−1) and its drain is connected to the input of stage n (Vout). The gate of NMOS transistor 14 is connected to its source. Accordingly, in the present invention, an identical output pass device 16 (i.e., NMOS transistor 16) may be connected to the previous stage output (Von−1) in diode connection. The current output Ioutm of this additional NMOS transistor 16 may be measured (by comparing with a reference current) to obtain the charge pump output current.
In the charge pump stages of FIG. 2C, the previous stage output (Von−1) is connected to the charge pump output Vout with an NMOS transistor 18 with gate control signal. In this connection, the source of NMOS transistor 18 is connected to the output of the previous stage output (Von−1) and its drain is connected to the input of stage n (Vout). The gate of NMOS transistor 18 is connected to a control voltage Vgate-control (or briefly, Vgc). Accordingly, in the present invention, an identical output pass device 20 (i.e., NMOS transistor 20) may be connected to the previous stage output (Von−1). The same gate control voltage Vgc may be connected to the gate of NMOS transistor 20. The current output. Ioutm of this additional NMOS transistor 20 may be measured (by comparing with a reference current) to obtain the charge pump output current.
The circuitries shown in FIGS. 2A–2C are non-limiting examples of circuitry which may be used in positive charge pumps. Reference is now made to FIGS. 2D–2F, which are non-limiting examples of circuitry which may be used in negative charge pumps.
In the charge pump stages of FIG. 2D, the previous stage output (Von−1) is connected to the charge pump output Vout with an n-p diode 22. Accordingly, in the present invention, an identical output pass device 24 (i.e., n-p diode 24) may be connected to the previous stage output (Von−1). The current output Ioutm of this additional n-p diode 24 may be measured (by comparing with a reference current) to obtain the charge pump output current.
In the charge pump stages of FIG. 2E, the previous stage output (Von−1) is connected to the charge pump output Vout with a PMOS transistor 26 in diode connection. The drain of PMOS transistor 26 is connected to the output of the previous stage output (Von−1) and its source is connected to the input of stage n (Vout). The gate of PMOS transistor 26 is connected to its drain. Accordingly, in the present invention, an identical output pass device 28 (i.e., PMOS transistor 28) may be connected to the previous stage output (Von−1) in diode connection. The current output Ioutm of this additional PMOS transistor 28 may be measured (by comparing with a reference current) to obtain the charge pump output current.
In the charge pump stages of FIG. 2F, the previous stage output (Von−1) is connected to the charge pump output Vout with a PMOS transistor 30 with gate control signal. In this connection, the drain of PMOS transistor 30 is connected to the output of the previous stage output (Von−1) and its source is connected to the input of stage n (Vout). The gate of PMOS transistor 30 is connected to a control voltage Vgc. Accordingly, in the present invention, an identical output pass device 32 (i.e., PMOS transistor 32) may be connected to the previous stage output (Von−1). The same gate control voltage Vgc may be connected to the gate of PMOS transistor 32. The current output Ioutm of this additional PMOS transistor 32 may be measured (by comparing with a reference current) to obtain the charge pump output current.
Reference is now made to FIG. 3, which illustrates current comparison circuitry for measuring the output current of the additional output stage/output pass device, in accordance with an embodiment of the present invention. The output current Ioutm is measured by comparing with a reference current.
In the non-limiting embodiment of FIG. 3, the charge pump output Vout and the additional output device output Voutm are input into a current mirror. Specifically, in this non-limiting example, the current mirror includes a pair of PMOS transistors 34 and 36, whose gates are connected to one another. Vout is input to the source of PMOS transistor 34. The gate of PMOS transistor 34 is connected to its bulk and drain Voutm is input to the source of PMOS transistor 36. The source of PMOS transistor 34 is connected to a resistor R1 The drain of PMOS transistor 36 is connected to a node n which outputs Vout and which is connected to a resistor R2. The current flowing through resistor R1 is I1 and current flowing through resistor R2 is I2. Current I1 is the current from stage n and current I2 is the current from the additional stage.
The voltage exiting PMOS transistor 34 and entering the gate of PMOS transistor 36 (Vg36) is equal to Vout−Vgs34 (i.e., the gate-source voltage of PMOS transistor 34). In addition, Voutm=Vg36+Vgs36 (i.e., the sum of the gate voltage and gate-source voltage of PMOS transistor 36). Since Vg36=Vout−Vgs34, Voutm=Vout−Vgs34+Vgs36,
The current mirror of PMOS transistors 34 and 36 is preferably designed such that Vgs34≈Vgs36. This results in Voutm≈Vout. Accordingly, the current mirror of PMOS transistors 34 and 36 and the resistor R1 form the first circuit portion, which forces Voutm to be equal or approximately equal to the charge pump output voltage (Vout). The first circuit portion also draws output current or voltage from the additional output stage (e.g., I2) and sends it to a second circuit portion, which comprises node n and resistor R2. In the second circuit portion, I2 can easily be measured. For example, since the charge pump output voltage Vout and the additional output device output voltage Voutm are equal or approximately equal, the ratio of the currents I1 and I2 is equal or approximately equal to their size ratio. For example, in the non-limiting case of R1=R2, I1=I2.
Reference is now made to FIG. 4, which illustrates current comparison circuitry for measuring the output current of the additional output stage/output pass device, in accordance with another embodiment of the present invention.
In the non-limiting embodiment of FIG. 4, the charge pump output Vout and the additional output device output Voutm are input into a current mirror comprising PMOS transistors 34 and 36, as similarly described for the embodiment of FIG. 3. In the embodiment of FIG. 4, instead of resistors R1 and R2, the PMOS current mirror is connected to an NMOS current mirror. Specifically, the drain of PMOS transistor 34 is connected to the drain of an NMOS transistor 38 and the drain of PMOS transistor 36 is connected to the drain of an NMOS transistor 40. The gates of NMOS transistors 38 and 40 are connected to each other. The sources of NMOS transistors 38 and 40 are connected to ground. The gate of NMOS transistor 38 is connected to the gate of another NMOS transistor 42, whose source is grounded. The drain of NMOS transistor 42 is connected to a current source Iref. The gate of NMOS transistor 42 is connected to its drain.
The current from stage n flows through the left side of the current mirrors, and the current from the additional stage flows through the right side of the current mirrors.
As similarly described above for FIG. 3, the current mirror of PMOS transistors 34 and 36 is preferably designed such that Vgs34≈Vgs36, which means Voutm≈Vout. Accordingly, the current mirror of PMOS transistors 34 and 36 and the NMOS transistor 38 form the first circuit portion, which forces Voutm to be equal or approximately equal to the charge pump output voltage (Vout). The first circuit portion also draws output current or voltage from the additional output stage (e.g., I2) and sends it to a second circuit portion, which comprises node n and NMOS transistor 40. In the second circuit portion, I2 can easily be measured. For example, since the charge pump output voltage Vout and the additional output device output voltage Voutm are equal or approximately equal, the ratio of the currents I1 and I2 is equal or approximately equal to their size ratio. By comparing with Iref, the current from stage n (the charge pump current) may be obtained.
It will be appreciated by person skilled in the art, that the present invention is not limited by what has been particularly shown and described herein above. Rather the scope of the present invention is defined only by the claims which follow:

## Claims

1. A method for measuring output current (Iout) of a charge pump, the method comprising:
providing a charge pump comprising a plurality of n charge pump stages, wherein an output of stage n−1(Von−1) is output to stage n, an output voltage of stage n being referred to as charge pump voltage output Vout;
connecting an additional output pass device to the output of stage n−1, an output voltage of said additional output pass device being referred to as Voutm;
forcing Voutm to be at least approximately equal to Vout;
drawing at least one of output voltage (Voutm) and output current (Ioutm) from the additional output pass device;
measuring Ioutm; and
correlating Iout with Ioutm.

2. The method according to claim 1, wherein measuring Ioutm comprises comparing Ioutm with a reference current.

3. The method according to claim 1, wherein if output voltages of said additional output pass device and said charge pump stage n are equal, then a current ratio (Ioutm/Iout) is equal to a ratio of sizes of said additional output pass device and said charge pump stage n.

4. The method according to claim 1, wherein said additional output pass device comprises an additional output stage.

5. The method according to claim 4, wherein said additional output stage uses identical phase signals as stage n.

6. The method according to claim 1, comprising scaling said additional output pass device down to minimize its size.

7. The method according to claim 1, wherein said additional output pass device comprises a p-n diode.

8. The method according to claim 1, wherein said additional output pass device comprises an n-p diode.

9. The method according to claim 1, wherein said additional output pass device comprises an NMOS transistor, whose source is connected to the output of stage n−1 and whose drain is connected to an input of stage n, and whose gate and source are connected to one another.

10. The method according to claim 1, wherein said additional output pass device comprises a PMOS transistor, whose drain is connected to the output of stage n−1 and whose source is connected to an input of stage n, and whose gate and drain are connected to one another.

11. The method according to claim 1, wherein said additional output pass device comprises an NMOS transistor, whose source is connected to the output of stage n−1 and whose drain is connected to an input of stage n, and whose gate is connected to a control voltage.

12. The method according to claim 1, wherein said additional output pass device comprises a PMOS transistor, whose drain is connected to the output of stage n−1 and whose source is connected to an input of stage n, and whose gate is connected to a control voltage.

13. Circuit architecture for measuring output current (Iout) of a charge pump, comprising:
a charge pump comprising a plurality of n charge pump stages, wherein an output of stage n−1 (Von−1) is output to stage n, an output voltage of stage n being referred to as charge pump voltage output Vout;
an additional output pass device connected to the output of stage n−1, an output voltage of said additional output pass device being referred to as Voutm;
a first circuit portion for forcing Voutm to be at least approximately equal to Vout and for drawing at least one of output voltage (Voutm) and output current (Ioutm) from the additional output pass device; and
a second circuit portion for measuring Ioutm and correlating Iout with Ioutm.

14. The circuit architecture according to claim 13, wherein if output voltages of said additional output pass device and said charge pump stage n are equal, then a current ratio (Ioutm/Iout) is equal to a ratio of sizes of said additional output pass device and said charge pump stage n.

15. The circuit architecture according to claim 13, wherein said additional output pass device comprises an additional output stage.

16. The circuit architecture according to claim 15, wherein said additional output stage uses identical phase signals as stage n.

17. The circuit architecture according to claim 13, wherein said additional output pass device comprises a p-n diode.

18. The circuit architecture according to claim 13, wherein said additional output pass device comprises an n-p diode.

19. The circuit architecture according to claim 13, wherein said additional output pass device comprises an NMOS transistor, whose source is connected to the output of stage n−1 and whose drain is connected to an input of stage n, and whose gate and source are connected to one another.

20. The circuit architecture according to claim 13, wherein said additional output pass device comprises a PMOS transistor, whose drain is connected to the output of stage n−1 and whose source is connected to an input of stage n, and whose gate and drain are connected to one another.

21. The circuit architecture according to claim 13, wherein said additional output pass device comprises an NMOS transistor, whose source is connected to the output of stage n−1 and whose drain is connected to an input of stage n, and whose gate is connected to a control voltage.

22. The circuit architecture according to claim 13, wherein said additional output pass device comprises a PMOS transistor, whose drain is connected to the output of stage n−1 and whose source is connected to an input of stage n, and whose gate is connected to a control voltage.