Method and apparatus for current limitation in voltage regulators

Description of equivalent WO 2005008353 A2

METHOD AND APPARATUS FOR CURRENT LIMITATION IN VOLTAGE REGULATORS

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates generally to voltage regulators and specifically to Umiting the short circuit current in a voltage regulation circuit. 2. The Prior Art FIG. 1 is a schematic illustrating a prior art voltage regulator circuit. Circuit 10 includes a power-controllingpass device, for example PMOS transistor 15, coupled between supply voltage 20 and output node 25. A stable output voltage Nout over a defined current IL range is produced between output node 25 and ground. The output of amplifier 30 is coupled to the gate of transistor 15, therefore regulating the behavior of transistor 15. Reference resistors 35 and 40 produce a voltage divider input for amplifier 30 and complete a regulation loop created by transistor 15, amplifier 30, and resistors 35 and 40. Capacitor 45 compensates the regulation loop. Amplifier 30 compares the voltage across resistor 40 with reference voltage Vbg. Output voltage Vout is determined by the combination of reference voltage Vbg and resistors 35 and 40.

As current IL increases above its maximum level, amplifier 30 starts to work in a non-liner mode

(i.e. saturation) and as a consequence there is a decline the output voltage Vout. The voltage versus current behavior depends on the characteristics of transistor 15. One problem with circuit 10 is that if transistor 10 is large (for example, in order to have good power supply rejection ratio), then amplifier 30 saturates for high values of current IL in a regulator that features low current load range. This means that the regulator presents a very high short circuit current compared to the typical regulator load current. Such short circuit current primarily depends on characteristics of transistor 15 and is not directly controllable. One solution for the above referenced problem features a switch connected between the gate of transistor 15 and the supply voltage 20, and controlledby the load current value IL. When the current IL is lower than a predetermined threshold the switch is open and the regulator works in normal operation. When IL is higher than the threshold, the switch is closed thus fixing the voltage at the controlling node of transistor 15, and so limiting the short circuit current of the regulator at the selected current threshold. The problem with this approach is that the rapid on-off state sequencing of the switch causes oscillation in circuit behavior. What is needed is a current limitation circuit based on a simple architecture that provides a predictable output response and does not alter the behavior of the regulator in normal operation.

BRIEF DESCRIPTION OF THE INVENTION A circuit for limiting a power current from a power-controlling pass device, the power- controlling pass device being coupled to a supply voltage, comprises the following. A sense device is coupled to the supply voltage with the sense device being configured to draw a sense current that is proportional to the power current. A current mirror is coupled to the sense device and the supply voltage through a low impedance node, for example a resistor, the current rnirror being configured to draw a mirror current through the low impedance node that is relative to the sense current. In one embodiment the mirror current is approximately equal to the sense current, and therefore has approximately the same proportion to the power current. A hrmting device is coupled to the supply voltage, the power-controllingpass device, and the low impedance node, the limiting device being configured to limit the power current according to a voltage difference between the low impedance node and the supply voltage. In one embodiment the limiting device, the power- controlling pass device and the sense device are all MOS transistors.

BRIEF DESCRIPTION OF THE DRAWING FIGURES FIG. 1 is schematic diagram illustrating a prior art voltage regulator circuit. FIG. 2 is schematic diagram illustrating one embodiment of a current limitation circuit implemented with the voltage regulator circuit of FIG. 1. FIG. 3 is a schematic diagram illustrating a circuit equivalent for an amplifier. FIG. 4 is a graph illustrating output voltage versus load current for a voltage regulator with and without current limitation. FIG. 5 is a graph illustrating output voltage versus load current for a voltage regulator with current limitation. FIG. 6 is a graph illustrating control voltage versus load current for a voltage regulatorwith current limitation. FIG. 7 is a block diagram illustrating a method for limiting power current from a power- controlling pass device.

DETAILED DESCRIPTION OF THE INVENTION The following description the invention is not intended to limit the scope of the invention to these embodiments, but rather to enable any person skilled in the art to make and use the invention. FIG. 2 is schematic illustrating one embodiment of a current limitation circuit implemented with the voltage regulator circuit of FIG. 1. Current limitation circuit 100 includes a sense device, for example transistor 110, coupled to supply voltage Vdd, transistor 15, and amplifier 30. In this embodiment transistor 110 is smaller than transistor 15 by a know amount, the sources of both transistors are coupled to supply voltage 20, and both transistors share the same gate voltage from amplifier 30. Transistor 110 couples to current mirror 120, for example transistors 130 and 135 in a current mirror configuration. Current mirror 120 couples to resistor 140 through node 150. Resistor 140 couples to supply voltage 20 and a limiting device, for example transistor 160. Transistor 160 couples to amplifier 30. Node 150 is a low impedance node based on the voltage drop from supply voltage 20 across resistor 140. In another embodiment, transistor 160 is coupled to a low impedance node other than a resistor, for example a PMOS transistor properly biased in the triode region. The sense device should provide a current based on the current of the device it is sensing. In this embodiment, sense device, or transistor 110, is smaller than transistor 15 by a known ratio and therefore provides a current through itself with the known ratio to the current through transistor 15. Current through transistor 110 necessarily passes through current mixror 120 and transistor 135 to ground. Current through node 150 and into current mirror 120 reflects, or approximates, current through transistor 110. Current mirrors may provide whatever ratio of current is desired, but in this embodiment a one-to-one ratio is used. Current through node 150 approximates the current through transistor 15 by the ratio of transistor 110 to transistor 15. If K is the ratio of transistor 110 to transistor 15 and current through transistor 15 is II (neglecting current through resistors 35 and 40), then current through node 150 is K-Il. In one embodiment, resistor 140 couples to supply voltage 20 and converts K-Il into a voltage across the source and gate of transistor 160. Limiting device, or transistor 160, clamps the voltage at the gates of transistors 110 and 15. Transistor 160 is driven through its gate by the voltage across resistor 140 with a resistance of Rim, for a gate voltage of RmrK-Il. In one embodiment transistor 160 is a PMOS transistor. Transistor 160 is driven by a low impedance node and may operate in saturation, so the transition between normal operation and an overcurrent mode is continuous and no stability problems appear since no on-off state sequence of transistor 160 occurs. FIG. 3 is a schematic illustratinga circuit equivalent for amplifier 30 from FIG.2. In one embodiment amplifier 30 is an operational amplifier. A macromodel circuit of amplifier 30 represents the behavior of amplifier 30. The macromodel circuit is composed of ideal voltage controlled voltage source 300 with a voltage of Vopa and resistor 310 with a resistance of Ropa. In this macromodel ^' Vdd- Vs when Av (V₊ -V ) > Vdd-Vs Vopa = ^■ Av (V₊ - V ) Vs < Av ^• (V₊ -V_) < Vdd-Vs Vs when Av ^• (V₊ - V_ ) < Vs, where Vs is the saturation voltage of amplifier 30, Av is the DC differential voltage gain of amplifier 30, Vdd is supply voltage 20, V₊ is the noninverting input to amplifier 30, and V_. is the inverting input to amplifier 30. Vg is the gate voltage of transistors 110 and 15. Vg is determined by amplifier 30 and transistor 160: Vg = Vopa + Ropa ^• Ilm.

Ilm is the drain current of transistor 160 that is, when transistor 160 is on and in saturation: <img file="WO2005008353A2_D0001.tif" /> where Vtop is the threshold voltage and βlm is the gain factor of transistor 160. So Vg = Vopa + FIL, where for K- Rlm - Il &gt; \Vtop\ <img file="WO2005008353A2_D0002.tif" />

Current limitation circuit 100 has three modes of operation: normal, overcurrent and short circuit. In normal operation, load current II increases from zero and the regulation loop (transistor 15 , resistors 35 and 40, and amplifier 30) makes Vout stable by adapting (i.e., by reducing) voltage Vopa. Once II increases to where Rlm^,K^,Il&gt; IVtopl (the threshold voltage of transistor 160), transistor 160 turns on and begins injecting current Ilm into the output of amplifier 30 and so modifying voltage Vg (the gate voltage of transistors 110 and 15). While amplifier 30 is in the linear region, voltage Vopa is adapted to compensate the effect of Ilm and Vout remains stable. In normal operation transistor 15 is in the triode region and amplifier 30 is in the linear region, so: II = βreg (V₈ -Vdd) - ^VθUt -^Vdd -Vtop (Vout-Vdd), where (Vout - R2 \ Vg = Av -Vbg + FIL, R12 = R1 + R2, \, R12 / βreg is the gain factor of transistor 15, Rl is the resistance of resistor 35 and R2 is the resistance of resistor 40. Substituting, the equation for Vg into the equation for II, Av ^■ — - ^-1 ^• Vout² + [ -Av ^• Vbg + E/L - Av ^• -^- ^• Vdd - Vtop ] ^■ Vout + R12 2 R12 Vbg - Vdd-FIL - Vdd+ = 0. <img file="WO2005008353A2_D0003.tif" /><img file="WO2005008353A2_D0004.tif" />

So, solving the quadratic equation for Vout: <img file="WO2005008353A2_D0005.tif" /> C Vbg - Vdd-FIL- <img file="WO2005008353A2_D0006.tif" /><img file="WO2005008353A2_D0007.tif" />

This is valid while amphfier 30 is in the linear region, i.e., Vopa &gt; Vs then <img file="WO2005008353A2_D0008.tif" /> then <img file="WO2005008353A2_D0009.tif" />

As n increases, Vopa decreases until it reaches Vs and amphfier 30 leaves the linear region and current limitation circuit 100 goes into overcurrent operation. The transition from normal to overcurrent operation is continuous and stable because a low impedance node (resistor 140) drives transistor 160 and transistor 160 is in saturation when reaching the saturation voltage of amphfier 30. The regulation loop does not work and voltage Vg becomes vg =vs+FiL.

As II increases, the drain-to-source voltage of transistor 15 increases, and Vout starts to decrease. Due to current hmitation circuit 100, Vg (gate voltage for transistors 110 and 15) is hmited not to Vs (saturation voltage of amphfier 30), which occurs when no current limitation is present, but to a higher value, so the output voltage Vout begins decreasing at a lower level of load current II. During overcurrent operation, the current in transistor 15 is II = βreg ^■ (Vg-Vdd)- ^VθUt-^Vdd -Vtop (Vout -Vdd). Substituting, for Vg yields 1 -- • Vout² + (Vs + FIL - Vtop) ^■ Vout + 2 II +1 -Vs ^■ Vdd- FIL • Vdd+^-+Vtop • Vdd- = 0. βreg Solving for Vout: <img file="WO2005008353A2_D0010.tif" /> A = B = (Vs+FIL-Vtop) <img file="WO2005008353A2_D0011.tif" />

This is valid while transistor 15 is in the triode region, <img file="WO2005008353A2_D0012.tif" />

As H increases, Vout decreases and transistor 15 exits the triode region and enters saturation. Current hmitation circuit 100 now enters short circuit operation. Load current II is, while neglecting the channel modulation in transistor 15, II = ^- ^■ (Vdd -Vg- Vtop)² , where Vg = Vs+FIL.

Substituting for Vg yields: II = ^- ^■ (Vdd -Vs- FIL - Vtop) , and Vout goes to zero.

This value for load current n represents the short circuit current, i.e., the current flowing in transistor 15 when Vout is zero (note that FIL is a function of II, so the equation must be solved numerically). The short circuit current can be programmed by choosing the value of K, Rim, and the size of transistor 160. Without current limitation circuit 100, the short circuit current is // = H . vdd -Vs- Vtop)² , which is higher than the short circuit current with current limitation circuit 100. FIG. 4 is a graph illustrating output voltage Vout versus load current π for a voltage regulator with and without current hmitation. With current limitation, the short circuit current is approximately 3 mA. Without current limitation, the short circuit current is approximately 46 mA. FIG. 5 is a graph illustrating output voltage versus load current for a voltage regulator with current limitation, from normal to overcurrent to short circuit operation. Normal operation, where the regulation loop regulates Vout by reducing Vopa as H increases, is relatively stable at approximately 2.5 V while current increases to approximately 2.9 mA. Overcurrent mode, where amphfier 30 is saturated and Vg is limited, shows current increasing from approximately 2.9 mA to approximately 3.0 mA while Vout decreases from approximately 2.5 V to approximately 2.0 V. Short circuit mode, where transistor 15 is in saturation, shows current reaching a maximum value of approximately 3 mA while Vout drops to approximately 0 V. FIG. 6 is a graph iUustratinggate voltage Vg for transistors 15 and 110 versus load current II for a voltage regulator with current limitation. During normal operation, gate voltage Vg drops from approximately 1.38 V to approximately 1.19 V while current increases from approximately 2.5 mA to approximately2.9 mA. At 2.9 mA of current II, current limitation circuit 100 functions to clamp the Vg at approximately 1.19 volts as current II increases to 3 mA. FIG. 7 is a block diagram illustrating a method for limiting power current from a power- controlling pass device. In block 700, sense the power current with a sense device coupled to the power-controUing pass device. In block 710, draw a sense current with the sense device, the sense current proportional to the power current. In block 720, draw a mirror current with a current mirror coupled to the sense device, the mirror current relative to the sense current. In block 730, draw the mirror current through the low impedance node. In block 740, generate a voltage potential between a supply voltage and a low impedance node. In block 750, limit the power current with a limiting device based on the voltage potential. The preceding equations apply to one exemplary embodiment and are not meant to limit the invention. The equations are presented in order to assist in understanding one embodiment of the invention. Any person skilled in the art will recognize from the previous description and from the figures and claims that modifications and changes can be made to the invention without departing from the scope of the invention defined in the following claims.