Current sensing circuit

A voltage detection unit generates a detection voltage signal representative of a potential difference caused by a current to be detected. A reference current generation unit generates a first reference current and a second reference current having a linear relationship therebetween. In response to the detection voltage signal and the first reference current, a transfer unit determines a first operation voltage. Furthermore, the transfer unit determines a second operation voltage and a transfer current in response to the first operation voltage and the second reference current. The second operation voltage is substantially equal to the first operation voltage. A detection current signal having a linear relationship with the current to be detected is generated through subtracting at least the second reference current from the transfer current.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a current sensing circuit and, more particularly, to a current sensing circuit for detecting a current flowing through a high-voltage/large-current power switch.

2. Description of the Related Art

For a synchronous switching DC/DC voltage regulator, an inductor current needs to be detected in magnitude and variation if a current-mode topology is configured as a mechanism of feedback control. Conventionally, a resistor is connected in series to the inductor and then a potential difference is caused across the resistor by the inductor current, which will provide the appropriate information regarding the magnitude and variation of the inductor current. However, the prior art must utilize the series-connected resistor with a result of the I2R power consumption. In an application of a large inductor current, the series-connected resistor must occupy a large surface area for satisfying the required current flow capacity under limitations determined by the nature of the semiconductor processing and materials, setting up a barrier to the development of a finer semiconductor chip. Moreover, an operational amplifier is necessary for retrieving the potential difference across the series-connected resistor, making the circuitry more complicated and reducing the operation speed.

FIG. 1is a circuit block diagram showing a synchronous switching DC/DC voltage regulator provided with a conventional current sensing circuit. As shown in the figure, a high-side switch HS and a low-side switch LS are coupled in series between an input voltage source Vinand a ground potential. An inductor L has one terminal coupled to a node A between the high-side switch HS and the low-side switch LS, and the other terminal serving as an output terminal for supplying a regulated output voltage Vout. The output terminal may also be provided with an output capacitor Cofor filtering ripples of the output voltage Vout. The high-side switch HS and the low-side switch LS are controlled by a high-side drive signal HD and a low-side drive signal LD, respectively, from a current-mode synchronous-switch control circuit11. In the synchronous switching DC/DC voltage regulator, the high-side switch HS and the low-side switch LS are operated out of phase. When the high-side switch HS is turned ON and the low-side switch is turned OFF, the input voltage source Vinsupplies energy to the inductor L, causing the inductor current ILto gradually increase. On the other hand, when the high-side switch HS is turned OFF and the low-side switch LS is turned ON, the energy stored in the inductor L is delivered to the output terminal as the output voltage Vout, causing the inductor current ILto gradually decrease.

Therefore, in a case that the high-side switch HS is implemented by a PMOS transistor and the low-side switch LS is implemented by an NMOS transistor, the high-side drive signal HD and the low-side drive signal LD are the pulse trains with the same phase. In a case that both of the high-side switch HS and the low-side switch LS are implemented by NMOS transistors, the high-side drive signal HD and the low-side drive signal LD are the pulse trains with 180 degrees out of phase therebetween. In addition, the high-side drive signal HD and the low-side drive signal LD are designed as non-overlapping pulse trains with a turn-on delay for preventing the high-side switch HS and the low-side switch LS from being simultaneously turned ON to erroneously short-circuit the input voltage source Vinand the ground potential.

For the current-mode feedback control mechanism, a resistor Rsis coupled in series to the inductor L for detecting the magnitude and variation of the inductor current IL. An operational amplifier12retrieves a potential difference across the series-connected resistor Rscaused by the inductor current IL, for generating a detection voltage Vsrepresentative of the inductor current IL. Subsequently, the detection voltage Vsgenerated by the operational amplifier12is fed back to the current-mode synchronous-switch control circuit11for performing the current-mode control.

The series-connected resistor Rsis necessary in the prior art for detecting the inductor current IL, resulting in the IL2Rspower consumption. In an application where the inductor current ILshould be made large, the series-connected resistor Rsmust occupy a large surface area for satisfying the required current flow capacity under limitations determined by the nature of the semiconductor processing and materials. Moreover, the operational amplifier12for retrieving the potential difference across the series-connected resistor Rsmakes the circuitry more complicated and reduces the operation speed.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems, an object of the present invention is to provide a current sensing circuit capable of reducing the power consumption for detecting a current.

Another object of the present invention is to provide a current sensing circuit capable of being constructed by circuit components with a fine size.

Still another object of the present invention is to provide a current sensing circuit capable of enhancing the operational speed of the current detection.

The present invention provides a current sensing circuit to replace the conventional series-connected resistor and the operational amplifier. The inventors firstly observe that a current flowing through the high-side switch when the high-side switch is turned ON is identical to the inductor current, and the current flowing through the high-side switch produces a potential difference across a high-side switch-channel resistance, i.e. a drain-source conductive resistance Rds(ON). Therefore, the current sensing circuit according to the present invention directly detects the potential difference across the high-side switch-channel resistance, and then performs inventive voltage/current transformation to obtain a detection current having a linear relationship with the inductor current. The current sensing circuit according to the present invention overcomes the prior art disadvantages regarding the power consumption, size, and operation speed since none of the conventional series-connected resistor and the operational amplifier is needed. Furthermore, the current detection circuit according to the present invention is operated in synchronization with the high-side switch for saving the current-detecting power consumption.

According to one aspect of the present invention, a current sensing circuit includes a voltage detection unit, a reference current generation unit, and a transfer unit. The voltage detection unit generates a detection voltage signal representative of a potential difference caused by a current to be detected. The reference current generation unit generates a first reference current and a second reference current. A first linear relationship is established between the first and the second reference currents. The transfer unit is coupled between the voltage detection unit and the reference current generation unit. In response to the detection voltage signal and the first reference current, the transfer unit determines a first operation voltage. In response to the first operation voltage and the second reference current, the transfer unit determines a second operation voltage and a transfer current. The second operation voltage is substantially equal to the first operation voltage. A detection current signal is generated by subtracting at least the second reference current from the transfer current. A second linear relationship is established between the detection current signal and the current to be detected.

Preferably, the current sensing circuit further includes a voltage feedback control unit, coupled to the transfer unit, for reflecting a variation of the first operation voltage on the second operation voltage.

Preferably, the current sensing circuit further includes a current level shift unit, coupled to the transfer unit or the voltage feedback control unit, for adjusting a direct current level of the detection current signal.

According to another aspect of the present invention, a method of sensing a current includes the following steps. A detection voltage signal is generated to be representative of a potential difference caused by a current to be detected. A first reference current is generated. A second reference current is generated such that a first linear relationship is established between the first and the second reference currents. A first operation voltage is determined in response to the detection voltage signal and the first reference current. A second operation voltage and a transfer current are determined in response to the first operation voltage and the second reference current such that the second operation voltage is substantially equal to the first operation voltage. A detection current signal is generated by subtracting at least the second reference current from the transfer current such that a second linear relationship is established between the detection current signal and the current to be detected.

Preferably, the method of sensing the current further includes a step of reflecting a variation of the first operation voltage on the second operation voltage through a voltage feedback control.

Preferably, the method of sensing the current further includes a step of adjusting a direct current level of the detection current signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments according to the present invention will be described in detail with reference to the drawings.

FIG. 2is a circuit block diagram showing a synchronous switching DC/DC voltage regulator provided with a current sensing circuit13according to the present invention. Referring toFIG. 2, a high-side switch HS is connected between an input voltage source Vin and a node A while a low-side switch LS is connected between the node A and a ground potential. An inductor L is connected between the node A and an output terminal. The inventors firstly observe that a channel current IHS flowing through the high-side switch HS when the high-side switch HS is turned ON is identical to an inductor current IL, and the high-side switch-channel current IHSproduces a potential difference across the high-side switch-channel resistance RHS:
Vin−Vsen=IHS·RHS

Therefore, the current sensing circuit13according to the present invention directly detects the potential difference (Vin−Vsen) across the high-side switch-channel resistance RHS, and then performs inventive voltage/current transformation to obtain a detection current signal Isenhaving a linear relationship with the inductor current IL. The current sensing circuit13according to the present invention overcomes the prior art disadvantages regarding the power consumption, size, and operation speed since none of the series-connected resistor Rsand the operational amplifier12is needed. Furthermore, the current detection circuit13according to the present invention activates to detect the current when the high-side switch HS is turned ON and stops detecting when the high-side switch HS is turned OFF, for saving the current-detecting power consumption.

FIG. 3is a detailed circuit diagram showing a current sensing circuit13-1of a first embodiment according to the present invention. The current sensing circuit13-1includes a voltage detection unit (P1, P2), a reference current generation unit (Ibias, N1, N2, N3), and a transfer unit (P3, P4, P5, P6).

More specifically, the voltage detection unit is used for detecting the potential difference across the high-side switch-channel resistance RHS. Assumed that the high-side switch-channel resistance is RHSand the channel current flowing through the high-side switch HS is IHS, the potential difference Vdsbetween the drain and source of the high-side switch HS may be expressed as:
Vds=Vin−Vsen=IHS·RHS

In the embodiment shown inFIG. 3, the voltage detection unit is implemented by PMOS transistors P1and P2. The transistor P1has a source connected to the input voltage source Vin, a gate connected to the ground potential, and a drain connected to a source (i.e. node B) of the transistor P2. The transistor P2has a gate connected to a gate of the high-side switch HS, and a drain connected to a drain of the high-side switch HS. When a high-side drive signal HD turns ON the high-side switch HS, both of the transistors P1and P2are operated in the triode region and therefore become equivalent to channel resistances. Assumed that the transistor P1has a channel resistance RP1and the transistor P2has a channel resistance RP2, the voltage VBat the node B may be expressed as a division of the potential difference since the series-coupled transistors P1and P2form a resistive voltage divider:

For preventing the current sensing circuit13-1according to the present invention from influencing the original characteristics of the circuit to be detected, the voltage detection unit is designed to have a high impedance. Consequently, the channel resistances RP1and RP2of the transistors P1and P2are designed to be extremely larger than the channel resistance RHSof the high-side switch HS:
RP1+RP2>>RHS

In this case, the current flowing through the transistors P1and P2can be neglected in comparison with the high-side switch-channel current IHS. As a result, during the ON period of the high-side switch HS, the high-side switch-channel current IHSappropriately indicates the inductor current ILeven under the detection of the current detection circuit13-1:
IL≈IHS

In other words, although the current sensing circuit13-1according to the present invention detects in practice the high-side switch-channel current IHS, it may be said in circuit application that the inductor current ILis detected.

The reference current generation unit is used for supplying a first reference current Ir1and a second reference current Ir2such that a linear relationship is established between the first reference current Ir1and the second reference current Ir2:
Ir1=K·Ir2

where K is a proportional constant larger than or equal to 1. In the embodiment shown inFIG. 3, the reference current generation unit includes a bias current source Ibiasand three NMOS transistors N1, N2, and N3. The transistor N1has a drain connected to the bias current source Ibias, a gate connected to its own drain, and a source connected to the ground potential. The transistor N2has a gate connected to the gate of the transistor N1, a source connected to the ground potential, and a drain for allowing the first reference current Ir1to sink or flow. The transistor N3has a gate connected to the gate of the transistor N1, a source connected to the ground potential, and a drain for allowing the second reference current Ir2to sink or flow. The transistors N1, N2, and N3together form a multiple-output-stage current mirror having the transistors N2and N3as independent current output stages. If the transistors N2and N3are identically manufactured except the width-to-length ratio of the gate is designed under the following condition:
(W/L)N2=K·(W/L)N3
then the first reference current Ir1and the second reference current Ir2can effectively establish the desired linear relationship:

The transfer unit is coupled between the voltage detection unit and the reference current generation unit for transferring the detection voltage signal VBgenerated from the voltage detection unit into the desired detection current signal Isenin accordance with the first and second reference currents Ir1and Ir2generated from the reference current generation unit. In the embodiment shown inFIG. 3, the transfer unit includes four PMOS transistors P3, P4, P5, and P6. The transistor P3has a source connected to the node B, a gate connected to the ground potential, and a drain connected to a node C. Consequently, the transistor P3is operated in the triode region as an equivalent channel resistance RP3. The transistor P4has a source connected to the input voltage source Vin, a gate connected to the ground potential, and a drain connected to a node D. Consequently, the transistor P4is operated in the triode region as an equivalent channel resistance RP4. Moreover, the transistor P5has a source connected to the node C while the transistor P6has a source connected to the node D. The transistors P5and P6have their gates connected together and the gate of the transistor P6is further connected to its own drain. Therefore, the transistors P5and P6form a current mirror. The transistor P5has a drain connected to the drain of the transistor N2for allowing the first reference current Ir1to flow through the transistors P3and P5. The transistor P6has a drain connected to the drain of the transistor N3for allowing the second reference current Ir2to flow through the transistor P6.

Since the linear relationship with the proportional constant K is established between the first and second reference currents Ir1and Ir2, the width-to-length ratios of the transistors P5and P6must be designed to satisfy the following condition:
(W/L)P5=K·(W/L)P6

for allowing the first and second reference currents Ir1and Ir2to smoothly flow through the transistors P5and P6, respectively, given that the transistors P5and P6are otherwise identically manufactured.

Because the first reference current Ir1also flows through the transistor P3, a voltage VCat the node C may be expressed as:

VC=VB-Ir1·RP3=Vin-(Vin-VB)-Ir1·RP3=Vin-RP1RP1+RP2·(Vin-Vsen)-Ir1·RP3=Vin-RP1RP1+RP2·IHS·RHS-Ir1·RP3
Now assumed that a transfer current Itflows though the transistor P4, a voltage VDat the node D may be expressed as:
VD=Vin−It·RP4

As described above, because the transistors P5and P6are coupled as the current mirror and the first and second reference currents Ir1and Ir2correspondingly follow the width-to-length ratios (W/L)P5and (W/L)P6, the gate-source voltage VGS(P5)of the transistor P5is operated equal to the gate-source voltage VGS(P6)of the transistor P6. In this case, since the gates of the transistors P5and P6are coupled together, the voltage at the source of the transistor P5(i.e. the voltage VCat the node C) is equal to the voltage at the source of the transistor P5(i.e. the voltage VDat the node D):

Isen=It-Ir2=Ω·IHS+(Φ-1)·Ir2
Since the proportional constants Ω and Φ and the second reference current Ir2are predetermined parameters and characteristic during the circuit design procedure, the current sensing circuit13-1according to the present invention effectively outputs the detection current signal Isenhaving the listed-above linear relationship with the high-side switch-channel current IHS. Since the high-side switch-channel current IHSis substantially equal to the inductor current IL, the current sensing circuit13-1according to the present invention achieves a precise measurement of the inductor current IL.

In one embodiment of the present invention, the channel resistances RP3and RP4of the transistors P3and P4may be designed with the same value, and the transistors P5and P6are also designed with the same width-to-length ratio such that the proportional constant K becomes equal to 1, thereby making the value of the proportional constant φ equal to 1. In this case, the detection current signal Isenis further reduced to be directly in proportion to the high-side switch-channel current IHS:
Isen=Ω·IHS

FIG. 4is a detailed circuit diagram showing a current sensing circuit13-2of a second embodiment according to the present invention. As seen by comparing withFIGS. 3 and 4, the second embodiment is different from the first embodiment in that the current sensing circuit13-2of the second embodiment is further provided with a voltage feedback control unit (P7) for rapidly reflecting the variation of the detection voltage signal VBin order to ensure a stable operation of the current sensing circuit13-2and a precise detection current signal Isen.

In the second embodiment shown inFIG. 4, the voltage feedback control unit includes a PMOS transistor P7having a gate connected to the drain of the transistor P5, a source connected to the source of the transistor P6, and a drain for outputting the desired detection current signal Isen. When the high-side switch-channel current IHSincreases (or decreases), the voltage Vsenat the node A decreases (or increases) such that a corresponding fall (or rise) happens to the detection voltage signal VBat the node B. As a result, the voltage at the source of the transistor P5(i.e. the voltage VCat the node C) and the voltage at the drain of the transistor P5simultaneously decrease (or increase) with the same magnitude. Through the feedback control provided by the transistor P7, the variation of the voltage at the drain of the transistor P5rapidly causes the same magnitude of variation to the voltage at the source of the transistor P6(i.e. the voltage VDat the node D). Consequently, the voltage VDat the node D rapidly reflects the variation of the voltage VCat the node C, thereby maintaining the equality therebetween to ensure the stable operation of the current sensing circuit13-2and the precise detection current signal Isen.

FIG. 5is a detailed circuit diagram showing a current sensing circuit13-3of a third embodiment according to the present invention. As seen by comparing withFIGS. 4 and 5, the third embodiment is different from the second embodiment in that the current sensing circuit13-3of the third embodiment is further provided with a current level shift unit (N4) for adjusting a direct current level of the detection current signal Isenso as to produce a predetermined current offset thereon for facilitating the circuit application or design.

In the third embodiment shown inFIG. 5, the current level shift unit includes an NMOS transistor N4having a gate connected to the gate of the transistor N1, a source connected to the ground potential and a drain connected to the drain of the transistor P7(i.e. node E) for allowing a shift current Ia1to sink or flow. Therefore, the detection current signal Isenoutput from the node E has a direct current level adjusted in accordance with the shift current Ia1:

Isen=It-Ir2-Ia1=Ω·IHS+(Φ-1)·Ir2-Ia1
If the shift current Ia1is preset equal to (Φ−1)Ir2, the detection current signal Isenis reduced to be directly in proportion to the high-side switch-channel current IHS:
sen=Ω·IHS

FIG. 6is a detailed circuit diagram showing a current sensing circuit13-4of a fourth embodiment according to the present invention. As seen by comparing withFIGS. 4 and 6, the fourth embodiment is different from the second embodiment in that the current sensing circuit13-4of the fourth embodiment is further provided with a current level shift unit (N5) for adjusting a direct current level of the detection current signal Isenso as to produce a predetermined current offset for facilitating the circuit application or design.

In the fourth embodiment shown inFIG. 6, the current level shift unit includes an NMOS transistor N5having a gate connected to the gate of the transistor N1, a source connected to the ground potential, and a drain connected to the source of the transistor P7(i.e. node D) for allowing a shift current Ia2to sink or flow. Therefore, the detection current signal Isenoutput from the drain of the transistor P7has a direct current level adjusted in accordance with the shift current Ia2:

Isen=It-Ir2-Ia2=Ω·IHS+(Φ-1)·Ir2-Ia2
If the shift current Ia2is preset equal to (Φ−1)Ir2, the detection current signal Isenis reduced to be directly in proportion to the high-side switch-channel current IHS:
Isen=Ω·IHS

To sum up, the current sensing circuit according to the present invention directly detects the potential difference across the high-side switch-channel resistance, and then performs the inventive voltage/current transformation to obtain the detection current signal having the linear relationship with the inductor current. The current sensing circuit according to the present invention overcomes the prior art disadvantages regarding the power consumption, size, and operation speed since none of the conventional series-connected resistor and the operational amplifier is needed. Furthermore, the current detection circuit according to the present invention is operated in synchronization with the high-side switch for saving the current-detecting power consumption.

While the invention has been described by way of examples and in terms of preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications.