Proportional regulation for optimized current sensor performance

An integrated circuit device comprises a first transistor having a gate coupled to an output of a first operational amplifier, a second transistor having a threshold voltage proportional to a threshold voltage of the first transistor, the second transistor having a gate coupled to an inverting input of a second operational amplifier, an output of the second operational amplifier coupled to an inverting input of the first operational amplifier, a first resistor coupled between the second transistor gate and the inverting input of the second operational amplifier, and a second resistor coupled between the output of the second operational amplifier and the inverting input of the second operational amplifier, a ratio of the second resistor to the first resistor selected based upon a ratio of a production distribution of a transistor source voltage offset to a production distribution of a transistor threshold voltage mismatch.

TECHNICAL FIELD

The present invention relates to current sensing in power MOSFET integrated circuits.

BACKGROUND

In Smart Power integrated circuits (ICs), the current through a load transistor is measured using a sense transistor. The current in the load MOSFET is measured using a second MOSFET that has much smaller area. Ideally, the sense transistor and the load transistor are operated at the same operating point in which the transistors' source, gate, and drain voltages are identical. The ratio of the currents in the load and sense transistors is called KILIS (“K”):

The accuracy of K is an important characteristic of Smart Power ICs. However, scattering causes K to vary in each production due to several factors. At low drain currents, the two most important factors are the threshold voltage mismatch between the transistors and the voltage difference between the load and sense source voltages (source voltage offset).

The problem of minimizing the scatter of K at low currents is not solved by known techniques. Currently, two operation modes are typically used—normal operation and gate-back regulation. In normal operation, the load and sense transistors are operated at a constant gate voltage. In the low current regime, the drain voltage is very small and the source voltage offset leads to linear increase of the scatter in K with decreasing drain current. In gate-back regulation, the transistors are operated at constant drain voltage. In the low current regime, the gate overdrive (the gate-threshold voltage difference ) is very small and the threshold voltage mismatch between the transistors leads to a linear increase of the scatter in K with decreasing drain current.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention in which a power MOSFET transistor is biased using a controller circuit that is configured based upon a ratio between a transistor source voltage offset production distribution and a transistor threshold voltage mismatch production distribution.

In accordance with a preferred embodiment of the present invention, an integrated circuit device comprises a first transistor having a gate coupled to an output of a first operational amplifier, a second transistor having a threshold voltage proportional to a threshold voltage of the first transistor, the second transistor having a gate coupled to an inverting input of a second operational amplifier, an output of the second operational amplifier coupled to an inverting input of the first operational amplifier, a first resistor coupled between the second transistor gate and the inverting input of the second operational amplifier, and a second resistor coupled between the output of the second operational amplifier and the inverting input of the second operational amplifier, a ratio of the second resistor to the first resistor selected based upon a ratio of a production distribution of a transistor source voltage offset to a production distribution of a transistor threshold voltage mismatch.

The integrated circuit may further comprise a sense transistor having a gate coupled to the gate of the first transistor, a third operational amplifier having a first input coupled to a source of the first transistor and a second input coupled to a source of the sense transistor, and a current source controlled by an output of the third operational amplifier.

In accordance with another preferred embodiment of the present invention, a controller for a power MOSFET transistor comprises a reference MOSFET transistor having a gate, the reference MOSFET transistor biased to create a voltage at the gate, the gate voltage corresponding to a threshold voltage of the reference MOSFET transistor, a first amplifier circuit having an input coupled to the reference MOSFET transistor gate via a voltage divider circuit, the voltage divider circuit scaled in proportion to a ratio between a production distribution of a transistor source voltage offset and a production distribution of a transistor threshold voltage mismatch, and a second amplifier circuit coupled to an output of the first amplifier circuit, the second amplifier circuit having an output adapted to drive a gate of the power MOSFET transistor.

The voltage divider circuit may comprise a first resistor coupled between the gate of the reference MOSFET transistor and an inverting input of the first amplifier circuit, and a second resistor coupled between the output of the first amplifier circuit and the inverting input. The ratio of the second resistor to the first resistor may be equivalent to the ratio between a production distribution of a transistor source voltage offset and a production distribution of a transistor threshold voltage mismatch.

The mismatch of the threshold voltages, the difference in source voltage, and the operating point of the transistors strongly affects KILIS accuracy. The analytic model presented herein is used to quantify their influence. The threshold voltage mismatch is most important during gate-back-regulation. The source voltage offset is most important during normal operation. In both operation modes the accuracy of KILIS increases linearly with increasing drain current. Gate-back-regulation gives better KILIS accuracy than normal operation if the drain voltage is small. However, for large drain voltages, gate-back-regulation results in worse KILIS accuracy than normal operation. A new operation mode, proportional regulation, is described herein that optimizes KILIS accuracy at low current densities.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

FIG. 1illustrates a generic current measurement circuit100in which load MOSFET transistor101is measured using sense MOSFET transistor102. To provide a highly accurate K value at low currents, the operating conditions of transistors101,102need to be optimized such that the influence on K from the threshold voltage mismatch and the source voltage offset are minimized simultaneously. As illustrated further below, a quantitative model may be used to model how these two factors affect K. In a preferred embodiment, the operating conditions of transistors101,102are optimized such that their influence on K is minimized. The present invention is directed to a circuit for obtaining optimal operation conditions in a Smart Power IC.

The accuracy of K may be specified as a function of drain current. The same drain current can be achieved with several combinations of gate voltage (“Vgs”) and drain voltage (“Vds”). One of these combinations provides an optimal operating condition for the best accuracy of K.

In an ideal world, K, the ratio of the drain currents of the load and sense transistors, would equal the geometric ratio (“Kgeo”) given by the areas (“A”) or channel width (“W”) of the two transistors:

In reality, K deviates from the ideal value, particularly at low currents. Transistors101,102have slightly different operating points due to a threshold voltage mismatch and source voltage offset. The threshold voltage mismatch (“ΔVth”) and source voltage offset (“Voff”) are defined as follows:
ΔVth=Vth(load)−Vth(sense)  (Eq. 3)
Voff=Vs(load)−Vs(sense)  (Eq. 4)

FIG. 1illustrates these error sources. The parameters ΔVthand Voffresult in different operating conditions in load transistor101and sense transistor102and, therefore, cause the actual K to deviate from Kgeoin circuit100. Because of production variations, there is a statistical scatter in ΔVthand in Voff. This scatter may be characterized by the standard deviations σ(Voff) and σ(ΔVth), respectively. The production variations result in a distribution of K that may be characterized by σ(K). It has been observed that these deviations are largest at low drain current densities (“J”). This is expected because the gate-threshold voltage difference (i.e. gate overdrive, “Vgs−Vth”) and the drain voltage (“Vds”) are small at low drain current densities J and, therefore, the effects of ΔVthand Voffare largest for low values of J.

Circuit100illustrates the components of the KILIS measurement. As described above, due to ΔVthand Voff, load transistor101and sense transistor102operate at slightly different operating points. The current in the sense path (“I(sense)”) is set by current source103, which is in turn controlled by operational amplifier104. The voltage offset Voffis the input to operation amplifier104. The threshold voltage mismatch ΔVthand the input voltage offset Voffcause slightly different operating points for the load and sense transistors101,102.

Equations 1 to 4 can be combined as show in Equation 5 to express J as a function of the gate overdrive Vgs−Vthand drain voltage Vds:

Equation 5 is the ratio of ideal KILIS value, K, to the actual value of KILIS, KgeoUsing first order Taylor expansion, Equation 5 can be simplified to:

Typically, Voffis at least a factor of 10 smaller than ΔVth. Therefore, Voffcan be neglected in the second term on the right side of Equation 6. The current ratio in Equation 6 can be simplified to:

KKgeo=1-1J·∂J∂(Vgs-Vth)·Δ⁢⁢Vth-1J·∂J∂Vds·Voff(Eq.⁢7)
where J is the current density and Kgeois the ratio of channel widths of load and sense transistors. Equation 7 equates the ratio between the ideal and real values of KILIS to the three terms on the right side of the equation. The second term on the right side of equation 7 is associated with the threshold voltage mismatch, ΔVth. The third term on the right side of Equation 7 is associated with the source voltage offset, Voff. In this approximation, K is a linear function of ΔVthand Voff. ΔVthand Voffvary statistically over production. Their distributions are characterized by their mean values and their standard deviations σ(ΔVth) and σ(Voff). Due to these variations, K also varies over production. K is characterized by its mean value and standard deviation σ(K). Based on Equation 7, the standard deviations are related in the following way:

Equation 8 relates the operating points of the transistors101,102and the statistical spreads in ΔVthand Voffto the distribution of K. Accordingly, Equation 8 may be used to model the accuracy of K as it varies over production. To obtain a high accuracy of K, the distribution of K needs to be narrow—i.e. σ(K) must be minimized for each value of J. Because the same J can be achieved with different combinations Vgsand Vds, there are optimal operating conditions for the best accuracy of K. The accuracy of K is most critical when J depends linearly on Vdsand the gate-overdrive, Vgs−Vth. Equation 8 may be further simplified by assuming a simple relationship for the term J(Vgs−Vth, Vds). In the linear region, the current density of a MOSFET may be expressed as:
J=C*·(Vgs−Vth)·Vds(Eq. 9)

Note that the term C* is a MOSFET-specific constant. The constant C* is usually labeled “K,” but C* is used here instead of the usual variable in order to avoid confusion between this parameter and the current ratio parameter K (KILIS). If J follows Equation 9, then Equation 8 has a minimum value if the ratio of (Vgs−Vth) and Vdsequals the ratio of σ(ΔVth) and σ(Voff):

If load and sense transistors101,102are operated such that Equation 10 is fulfilled for each current, then σ(K) is minimized and the accuracy of K is maximized for each J. Therefore, in a Smart Power IC, the drain current needs to be measured and a control loop needs to generate an appropriate combination of Vgsand Vdsso that Equation 10 if satisfied for the measurement current. Preferably, σ(ΔVth) and σ(Voff) are determined experimentally beforehand. Because (Vgs−Vth) and Vdsare proportional to each other, this operating mode is referred to herein as proportional regulation. The optimal ratio (Vgs−Vth) to Vdsis given by the ratio of σ(ΔVth) and σ(Voff). If the load and sense transistor are operated at such bias conditions in the proportional regulation mode, then the resulting σ(K) is given by Equation 11:

As shown in Equation 11, σ(K) increases with the inverse of the square-root of the current density J. This is in contrast to known approaches in which σ(K) increases with the inverse of J. The drain current dependence is less severe than for normal operation and gate-back regulation. Accordingly, the proportional regulation mode, σ(K) is minimized for all current densities J in the low current regime. Additionally, σ(K) increases more slowly with decreasing J than in other operating modes.

Embodiments of the present invention operate the load and source transistors101,102for a given current density at a combination of gate-overdrive and drain-source voltage that is given by the standard deviation of the threshold voltage mismatch and the standard deviation of the source offset voltage. This operating condition minimizes the standard deviation of K for a give current density. As a result, the accuracy of K is maximized over production variations.

FIG. 2illustrates one embodiment of the invention for a low side power switch. Transistor M1201is a power MOS device that provides switching functionality in circuit200. In order to simplify the schematic shown inFIG. 2, the circuitry for switching transistor M1201ON and OFF with the proper timing is not shown, but would be known to one of ordinary skill in the art. For purposes of the circuit illustrated inFIG. 2, transistor M1201is assumed to be in the ON state.

Transistor M2202is a power MOS device having a smaller area than transistor M1201. Transistor M2202is used together with operational amplifier1(OA1)204for sensing the current flowing in transistor M1201and external load205. σ(ΔVoff) is the one sigma offset at OA1204. Current sensing works effectively for Vds>>σ(ΔVoff) at transistor M1201. In the low current regime, the condition would be violated, so Vdsat transistor M1201needs to be limited. Otherwise, the accuracy in current sensing would be lost. Operation amplifier2(OA2)206is introduced for this purpose. OA2206sets the minimum value to Vds=VREF.

In typical Smart Power IC circuits, VREFis a constant value in the range of 10-50 mV. However, for embodiments of the present invention, this value is not optimum. The value of VREFcan be varied according to Equation 10 to minimize the overall error in output current sensing error σ(ΔVoff) at OA1204and error σ(ΔVth) at transistors M1-M2.

Circuit207provides the regulation required for Equation 10, which can be rewritten as:

Transistor M3203may be small in proportion to transistor M1201. For purposes ofFIG. 2, it is assumed that the threshold voltage, Vth-M1, of transistor M1201is equal to the threshold voltage, Vth-M3, of transistor M3203. Transistor M3203is used to extract the power MOS threshold Vthby means of a very small biasing current IREF208. Operational amplifier3(OA3)209provides a reference voltage to OA2206. The value of this reference voltage, VREF, is shown in Equation 13:

VREF=-(VIN-VG)·R2R1=(Vgs-Vth)·R2R1(Eq.⁢13)
where R1and R2are the values of resistors R1210and R2211, respectively, that are used to bias OA3209.

Comparing equations 12 and 13, it follows that if VREF=Vds, then

σ⁡(Voff)σ⁡(Vth)=R2R1(Eq.⁢14)
Accordingly, if Equation 14 is satisfied, then OA2206will drive the gate of transistor M1201to set the optimal Vdsfor transistor M1201.