# Patent

## Title
Diode stack high voltage regulator

## Abstract

A high voltage regulator including a current mirror including a pair of transistors, one of the transistors being connected to a node that outputs an output voltage Vout, a diode stack that includes a plurality of serially connected transistors T0, T1, T2, . . . Tn, wherein the transistor T1 is connected to a node n0, to which is connected another transistor T0 that receives an input bias voltage Vbias, and wherein a feedback voltage fb from node n0 is fed to an input of the differential amplifier, the differential amplifier receiving an input reference voltage Vref at one of its other inputs, and is also connected to positive voltage supply Vdd, the differential amplifier outputting to an NMOS transistor M, and wherein the high voltage regulator has a large diode stack gain and lower GDA*GNMOS*m, resulting in a generally constant feedback (loop) gain Gloop, wherein the loop gain is given by:

Loop Gain=Gloop=Gstack*GDA*GNMOS*m 

    
    
        wherein m is the ratio of the two currents I1 and I2, that is, I2=mI1, Gstack is the gain of the diode stack, GDA is the gain of the differential amplifier and GNMOS is the gain of the NMOS transistor M.

## Background
FIELD OF THE INVENTION
The present invention relates generally to voltage regulators, and particularly to a high voltage regulator with a diode stack instead of a divider, e.g., a resistor or capacitor divider.
BACKGROUND OF THE INVENTION
Non-volatile memory (NVM) 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.
Read and write operations are typically carried out with voltages that are regulated above a positive voltage supply Vdd. The circuitry that supplies and controls the programming and verification voltages generally comprises a high voltage regulator or high voltage pump (the terms being used herein interchangeably). A typical high voltage regulator architecture is shown in FIG. 1.
A current mirror including a pair of PMOS (p-channel metal oxide semiconductor) transistors 4 and 5 have their gates connected to each other and their sources connected to a high voltage supply Vhv—supply. The gate of transistor 4 is connected to its drain. The current through transistor 4 is I1 and the current through transistor 5 is I2. The drain of transistor 5 is connected via a node n to Vout and to a divider 6 comprising a pair of serially connected circuit elements B1 and B2, e.g., resistors, diodes or capacitors. Divider 6 passes a feedback voltage fb to one of the inputs of a voltage amplifier (also called a differential stage or differential amplifier) 7. Differential amplifier 7 receives an input reference voltage Vref at one of its other inputs, and is also connected to positive voltage supply Vdd. The output of differential amplifier 7 may be connected to the gate of an NMOS (n-channel metal oxide semiconductor) transistor M. The drain of transistor M is connected to the drain of transistor 4, and the source of transistor M is connected to ground.
The open loop gain (Gloop) of the high voltage regulator of FIG. 1 (i.e., the ratio of the output voltage to the differential input voltage without any external feedback) is given by:

Loop Gain=Gloop=Gdivider*GDA*GNMOS*m 

wherein m is the ratio of the two currents I1 and I2, that is, I2=mI1 
The feedback voltage Vfb is approximately equal to the reference voltage Vref (Vfb≈Vref)
In the case of divider 6 comprising a pair of serially connected resistors, the following relations hold:

ΔVfb=Gdivider*ΔVout=(RB1+RB2)/RB2*ΔVout 

Vout=(RB1+RB2)/RB2*Vfb≈(RB1+RB2)/RB2*Vref 

There is an inherent stability problem with the prior art voltage regulator of FIG. 1, because a high loop gain (although having a fast recovery time) leads to instability of the regulator. On the other hand, a low loop gain results in a slow recovery time. In the case of a resistor divider, there may be a problem of parasitic capacitance to ground of the resistors, leading to another stability/recovery time problem. An additional capacitor divider problem is that of parasitic capacitance to ground which adversely affects the accuracy of Vout. An additional diode divider problem is that it is not possible to have an arbitrary Vout without significantly changing I2.
SUMMARY OF THE INVENTION
The present invention seeks to provide a novel high voltage regulator with a diode stack, as is described more in detail hereinbelow. The present invention may have a large diode stack gain (=1) but lower GDA*GNMOS*m, resulting in a generally constant feedback (loop) gain Gloop. The invention has lower feedback delay, better stability and faster recovery time than the prior art.
There is thus provided in accordance with an embodiment of the present invention circuitry including a voltage regulator including a current mirror including a pair of transistors, one of the transistors being connected to a node that outputs an output voltage Vout, a diode stack that includes a plurality of serially connected transistors T0, T1, T2, . . . Tn, wherein the transistor T1 is connected to a node n0, to which is connected another transistor T0 that receives an input bias voltage Vbias, and wherein a feedback voltage fb from node n0 is fed to an input of the differential amplifier, the differential amplifier receiving an input reference voltage Vref at one of its other inputs, and is also connected to positive voltage supply Vdd, the differential amplifier outputting to an NMOS transistor M, and wherein the high voltage regulator has a large diode stack gain and lower GDA*GNMOS*m, resulting in a generally constant feedback (loop) gain Gloop, wherein the loop gain is given by:

Loop Gain=Gloop=Gstack*GDA*GNMOS*m 

wherein m is the ratio of the two currents I1 and I2, that is, I2=mI1, Gstack is the gain of the diode stack, GDA is the gain of the differential amplifier and GNMOS is the gain of the NMOS transistor M.
In accordance with an embodiment of the present invention ΔVfb=Gstack*ΔVout.
Further in accordance with an embodiment of the present invention Vout=Vfb+n*Vbias≈Vref+n*Vbias, and Gstack≈1. The gates of the transistors of the current mirror may be connected to each other and their sources may be connected to a high voltage supply The serially connected transistors may include NMOS transistors. The transistors of the current mirror may include PMOS transistors.
There is also provided in accordance with an embodiment of the present invention a high voltage regulator including a current mirror including a pair of PMOS transistors that have their gates connected to each other and their sources connected to a high voltage supply, wherein current through one of the PMOS transistors is I1 and the current through the other PMOS transistor is I2, wherein the current I1 flows to a drain of an NMOS transistor M whose gate is connected to an output of a differential amplifier, wherein gates of the PMOS transistors of the current mirror are connected to each other and their sources are connected to a high voltage supply, and wherein the current I2 flows to a diode stack that includes a plurality of serially connected NMOS transistors T0, T1, T2, . . . Tn, wherein a drain of transistor Tn is connected to a drain of the PMOS transistor through which flows current I2, and wherein a gate of transistor Tn is connected to its drain and a source of transistor Tn is connected to its bulk and to a drain of adjacent NMOS transistor Tn-1 and wherein a source of NMOS transistor T0 is connected to a node n0, which is connected to a drain of NMOS transistor T0, wherein a gate of NMOS transistor T0 receives an input bias voltage Vbias and a source of NMOS transistor T0 is connected to its bulk and to ground, and wherein a feedback voltage from node n0 is fed to an input of the differential amplifier, the differential amplifier receiving an input reference voltage Vref at one of its other inputs, and is also connected to positive voltage supply Vdd, wherein the feedback voltage is approximately equal to the reference voltage Vref and a gate-source voltage of the diode stack is approximately equal to the bias voltage, and wherein the high voltage regulator may has a large diode stack gain but lower GDA*GNMOS*m, resulting in a generally constant feedback (loop) gain Gloop, wherein the loop gain is given by:

Loop Gain=Gloop=Gstack*GDA*GNMOS*m 

wherein m is the ratio of the two currents I1 and I2, that is, I2=mI1 

ΔVfb=Gstack*ΔVout and

Vout=Vfb+n*Vbias≈Vref+n*Vbias. (Gstack may be approximately equal to 1)




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 block diagram of a typical prior art high voltage regulator architecture; and
FIG. 2 is a simplified block diagram of a high voltage regulator architecture, in accordance with an embodiment of the present invention.



DETAILED DESCRIPTION OF EMBODIMENTS
Reference is now made to FIG. 2, which illustrates a simplified block diagram of a high voltage regulator, in accordance with an embodiment of the present invention. Components of the circuitry of FIG. 2 that are similar to that of FIG. 1 are designated with the same reference labels, and the description is not repeated for the sake of brevity.
The divider 6 of the architecture of FIG. 1 is replaced in the non-limiting embodiment of FIG. 2 with a diode stack 10. Diode stack 10 may include a plurality of serially connected NMOS transistors T0, T1, T2, . . . Tn. The drain of transistor Tn is connected to the drain of PMOS transistor 5. The gate of transistor Tn is connected to its drain. The source of transistor Tn is connected to its bulk and to the drain of the next NMOS transistor Tn-1. The source of transistor T1 is connected to node n0. The drain of another NMOS transistor T0 is connected to node n0. The gate of transistor T0 receives an input Vbias. The source of transistor T0 is connected to its bulk and to ground.
In the high voltage regulator of the present invention, as with the prior art, the open loop gain (Gloop) is again given by:

Loop Gain=Gloop=Gstack*GDA*GNMOS*m 

wherein m is the ratio of the two currents I1 and I2, that is, I2=mI1 
The gate-source voltage of the diode stack 10 (Vgs) is approximately equal to the bias voltage Vbias (Vgs≈Vbias). As with the prior art, The feedback voltage Vfb is approximately equal to the reference voltage Vref (Vfb≈Vref).
The high voltage regulator may have a large diode stack gain (=1) but lower GDA*GNMOS*m, resulting in a generally constant feedback (loop) gain Gloop.

ΔVfb=Gstack*ΔVout(wherein Gstack=1)

Vout=Vfb+n*Vbias≈Vref+n*Vbias 

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 that follow:

## Claims

1. A high voltage regulator comprising:
a current mirror comprising a pair of transistors, one of the transistors being connected to a node that outputs an output voltage Vout;
a diode stack that comprises a plurality of serially connected transistors T0, T1, T2, . . . Tn, wherein said transistor T1 is connected to a node n0, to which is connected another transistor T0 that receives an input bias voltage Vbias,
and wherein a feedback voltage fb from node n0 is fed to an input of the differential amplifier, said differential amplifier receiving an input reference voltage Vref at one of its other inputs, and is also connected to positive voltage supply Vdd, said differential amplifier outputting to an NMOS transistor M,
and wherein said high voltage regulator has a large diode stack gain and lower GDA*GNMOS*m, resulting in a generally constant feedback (loop) gain Gloop, wherein said loop gain is given by:


Loop Gain=Gloop=Gstack*GDA*GNMOS*m 


wherein m is the ratio of the two currents I1 and I2, that is, I2=mI1, Gstack is the gain of said diode stack, GDA is the gain of said differential amplifier and GNMOS is the gain of said NMOS transistor M.

2. The high voltage regulator according to claim 1, wherein ΔVfb=Gstack*ΔVout.

3. The high voltage regulator according to claim 1, wherein Vout=Vfb+n*Vbias≈Vref+n*Vbias.

4. The high voltage regulator according to claim 1, wherein Gstack≈1.

5. The high voltage regulator according to claim 1, wherein gates of the transistors of the current mirror are connected to each other and their sources are connected to a high voltage supply.

6. The high voltage regulator according to claim 1, wherein the serially connected transistors comprise NMOS transistors.

7. The high voltage regulator according to claim 1, wherein the transistors of said current mirror comprise PMOS transistors.

8. A high voltage regulator comprising:
a current mirror comprising a pair of PMOS transistors that have their gates connected to each other and their sources connected to a high voltage supply, wherein current through one of the PMOS transistors is I1 and the current through the other PMOS transistor is I2, wherein the current I1 flows to a drain of an NMOS transistor M whose gate is connected to an output of a differential amplifier;
wherein gates of the PMOS transistors of the current mirror are connected to each other and their sources are connected to a high voltage supply;
and wherein the current I2 flows to a diode stack that comprises a plurality of serially connected NMOS transistors T0, T1, T2, . . . Tn, wherein a drain of transistor Tn is connected to a drain of the PMOS transistor through which flows current I2, and wherein a gate of transistor Tn is connected to its drain and a source of transistor Tn is connected to its bulk and to a drain of adjacent NMOS transistor Tn-1, and wherein a source of NMOS transistor T1 is connected to a node n0, which is connected to a drain of NMOS transistor T0, wherein a gate of NMOS transistor T0 receives an input bias voltage Vbias and a source of NMOS transistor T0 is connected to its bulk and to ground,
and wherein a feedback voltage from node n0 is fed to an input of the differential amplifier, said differential amplifier receiving an input reference voltage Vref at one of its other inputs, and is also connected to positive voltage supply Vdd,
wherein said feedback voltage is approximately equal to the reference voltage Vref and a gate-source voltage of said diode stack is approximately equal to said bias voltage,
and wherein said high voltage regulator may has a large diode stack gain but lower GDA*GNMOS*m, resulting in a generally constant feedback (loop) gain Gloop, wherein said loop gain is given by:


Loop Gain=Gloop=Gstack*GDA*GNMOS*m 


wherein m is the ratio of the two currents I1 and I2, that is, I2=mI1 


ΔVfb=Gstack*ΔVout and



Vout=Vfb+n*Vbias≈Vref+n*Vbias.

9. The high voltage regulator according to claim 8, wherein Gstack≈1.