Improving the accuracy of an exponential current digital-to-analog converter (IDAC) using a binary-weighted MSB

An apparatus of an exponential current digital-to-analog converter (IDAC) using a binary-weighted MSB to efficiently drive current controlled light emitting diode (LED) devices. The apparatus comprises of an exponential current digital-to-analog converter (IDAC) current source, a voltage buffer to create an active cascode at the output stage, and an error amplifier that by means of a DC-DC converter voltage loop imposes an appropriate voltage at the output of the IDAC, depending on the current load set to drive the LEDs. The definition of the apparatus involves defining an exponential LSB and exponential MSB current mirrors according to a defined methodology.

RELATED PATENT APPLICATION

This application is related to Ser. No. 14/166,035, filed on Jan. 28, 2014, owned by a common assignee, and which is herein incorporated by reference in its entirety.

BACKGROUND

Field

The disclosure relates generally to a current digital-to-analog converter circuit, and, more particularly, an apparatus of an exponential current digital-to-analog converter (IDAC) using a binary-weighted MSB to efficiently drive current controlled light emitting diode (LED) devices. The definition of the apparatus involves defining an exponential LSB and exponential MSB current mirrors according to a defined methodology for an exponential current digital-to-analog converter (IDAC) having improved accuracy using a binary-weighted most significant bit (MSB) and a method thereof.

Description of the Related Art

Light emitting diode (LED) device brightness control is achieved by controlling the current that passes through the LED device. In order to dim LED device with less power dissipation than current control, a method of power control is used known as pulse width modulation (PWM). By varying the average current across the LED device, the device can be made to appear dimmer, or brighter. The brightness of current controlled light emitting sources is substantially proportional to the current flowing in them. Because of this characteristic, a digital-to-analog current converter (IDAC) is commonly used to control the brightness of visual displays.

Using a sole linear current digital-to-analog solution has the disadvantage of being perceived by human visual perception as a non-linear dimming process. Therefore a current source that is exponentially related to digital inputs, such as an exponential IDAC, is needed to maintain uniformity in changes to the level of the display brightness.

It is a common practice, the IDAC exponential characteristic is separated into a least significant bit (LSB) and most significant bit (MSB) parts as necessary to contain the circuit complexity and reduce the silicon area. The LSB generates an exponential current with a DNLLSBand so does the MSB with a DNLMSB

As discussed in published U.S. Pat. No. 7,132,966 to Adler et al., a circuit to convert a floating point number into an analog current is discussed. The conversion is performed directly by using an exponential current digital-to-analog converter (e.g. exponential IDAC) and a cascaded linear current digital-to-analog converter (e.g. linear DAC).

As discussed in published U.S. Pat. No. 7,038,402 to Adler et al., a circuit to achieve linear and exponential control over current to drive color LEDs is shown. A method for convert floating point number into an analog current is discussed. The conversion is performed directly by using an exponential current digital-to-analog converter (e.g. exponential IDAC) and a cascaded linear current digital-to-analog converter (e.g. linear DAC) to drive color LEDS, and preferably red, green, blue (RGB) LEDs.

As discussed in published U.S. Pat. No. 8,421,659 to von Staudt et al., discloses circuits using a digital-to-analog converter (DAC) controlled by a state machine to produce an analog output that is within a least significant bit (LSB) of a digital input bits. Trim solutions are proposed for major transitions of the digital bits.

As discussed in published U.S. Patent Application US 2011/0057825 to Marraccini et al., discloses systems to achieve logarithmic digital-to-analog converter (DAC). The logarithmic DAC is a linear DAC whose output voltage is converted into a logarithmic current value directly from an I-V characteristics of a diode element. The patent application utilizes a diode in the network to establish the voltage-current transformation.

As illustrated inFIG. 1, an IDAC architecture is shown. In the implementation ofFIG. 1, there is a LSB network that separates the LSB base current Io and the exponential part ILSB_exp. In the implementation ofFIG. 1, there is a MSB network that separates the MSB base current To and the exponential part IMSB_exp.FIG. 1shows a power pad VDD1, a ground rail VSS2, a VIDAC_FB pad3, a first PFET current mirror (20and21), a second PFET current mirror (30and31), a LSB network40, and a MSB network50, a buffer element60, a transistor70, and output transistor80. The LSB network40contains a LSB diode41, a transistor42for the base current Io, a variable transistor43for the exponent term ILSB_exp, and a LSB responsive switch44. The MSB network50contains a MSB diode51, a variable transistor52for the exponent term ILSB_exp, and a MSB responsive switch54. Implementing equation for low differential non-linearity (DNL) values using one LSBdiode is feasible task to undertake, considering that at the first part of the characteristic is at relatively low currents and the exponential characteristic is easily approximated with a linear function plus correction terms. But, the difficulty is in the defining of the MSB. Implementing an equation becomes quite tricky if DNLMSBis an irrational number, because that means synthesizing and implementing an MSB current mirror at high currents with an irrational gain. The only way to approximate an irrational gain is to implement several diodes with different gains within the MSB. Systematic error will occur in the overall IDAC DNL. Furthermore, matching between the different branches to contribute to the DNLMSBwill be not achievable due to the different plurality of elements of different sizes (e.g. diode elements). These design challenges can easily turn a low DNL exponential IDAC into a high complexity, unmanageable design. In the attempt to simplify matters and to contain the expense of silicon area, a technique called “dithering” is being commonly used. This consists in targeting the design for higher differential non-linearity (DNL) for then getting lower DNL by switching the digital input between two consecutive codes, using an appropriate frequency and duty cycle. The dithering usually creates more issues than it was meant to solve, as it requires good dynamic performance by a structure that is inherently slow, being made of several big current mirrors. Furthermore, it doesn't resolve the problem of having an irrational MSB gain, and so a low DNL accuracy, if a proper architecture wasn't chosen in the first place.

As illustrated inFIG. 2, a high level diagram of a system to drive light emitting diode (LED) devices is shown. The block diagram shows a system100, to drive a string of LED elements. The system contains a power output pad101, a ground connection102, an output bondpad103, a error amplifier110, a pulse width modulation (PWM) comparator120, a DC-DC converter130, a series configured LED elements140to141, a current source150. The PWM used in the PWM comparator is not to be confused with the PWM used for dimming LEDs as noted in the prior discussion and description ofFIG. 1. The IDAC that provides current control for the LEDs used in portable displays needs to be low voltage compliance to reduce the system power loss, so vital for the battery life. The lower the system is able to regulate the node VIDAC_FB the better the efficiency of the system. In the architecture shown inFIG. 1, the operating point of the MSB stage is set by the MSB diode branch, so if VIDAC_FB goes below the drain voltage saturation, VDsat, of the MSB diode device (referred to as IDAC voltage compliance) the current accuracy will be heavily affected. The IDAC voltage compliance (VDsat) is inversely proportional to the size of the MSB output devices. This means that to get good efficiency performance in driving the LEDs, area must be spent in designing the MSB. If the MSB is already large and complex to meet the DNL specification, it will be very difficult to meet the low voltage compliance requirement as well, which is inversely proportional to the MSB devices size. Also considering that different branches with different diodes, so different VDsat, are involved in generating the final output current. So it might not be possible to simply increase the MSB size to push the output voltage VOUT further down without squeezing some current branches. The voltage drop from the transistor NOUT also impacts the system efficiency. The system efficiency can be written as

η=POUTPIN
where POUTis the output power, and PINis the input power. The output power can be defined as a function of IDAC, as follows:
POUT=(Vout−VIDAC_FB)*IDAC
To maximize the efficiency is mandatory to reduce the loss in the system. The first visible loss affecting the efficiency of the system is the voltage VIDAC_FB required to obtain the desired current. It's evident that current accuracy and efficiency are in contrast, as to get better efficiency we need to push VIDAC_FB down as much as possible but if we go further down than VDsat we will lose current accuracy.

In these prior art embodiments, the solution to design a low differential non-linearity (DNL) and low voltage compliant exponential digital-to-analog converter (IDAC) that minimizes architectural complexity and mismatch variations is not achieved.

SUMMARY

It is an object of the invention to provide a circuit with an output stage architecture that provides flexibility to work at very low compliant voltages without losing current accuracy.

It is an object of the invention to provide a circuit that obtains a simple low size MSB solution that allows to obtain a low voltage compliant IDAC to reduce the power loss.

A principal object of the present disclosure is to provide a system using an exponential IDAC to drive current-controlled light emitting diodes with improved system efficiency.

A principal object of the present disclosure is to provide a circuit with an output stage architecture that provides flexibility to work at very low compliant voltages without losing current accuracy.

Another further object of the present disclosure is to provide a method to obtain a simple low size MSB solution that allows to obtain a low voltage compliant IDAC to reduce the power loss.

This is achieved using defining an apparatus for driving light emitting diode (LED) elements comprising of an error amplifier, a pulse width modulation comparator electrically connected the output of said error amplifier, a DC-DC converter electrically connected to the output of said pulse width modulation comparator and an output pad, at least one light emitting diode (LED) electrically connected to the input of said error amplifier, and an exponential current digital-to-analog converter (IDAC) using a binary weighted most significant bit (MSB) electrically connected to said cathode of said light emitting diode (LED) and input of said error amplifier wherein said binary weighted MSB is defined according to a specified relationship for IMSB providing an MSB code for the MSB network initiating the MSB switches.

This is further achieved using an apparatus of an exponential current digital-to-analog converter (IDAC) using a binary weighted most significant bit (MSB) comprising of a current source Iin, a buffer element, a first PFET current mirror, a second PFET current mirror, a LSB network, and a binary-weighted MSB network.

This is further achieved by defining a method of exponential current digital-to-analog converter (IDAC) using a binary weighted most significant bit (MSB) wherein a most significant bit (MSB) current mirror comprising the steps of (1) defining a differential non-linearity (DNL), (2) defining number of LSB bits needed for the targeted DNL with a binary weighted MSB, (3) calculating the number of bits to be used for the binary-weighted MSB to get the desired IDAC base, (4) deriving the minimum current for the Imax, (5) defining the LSB as an exponential current mirror according to the specified relationship for the ILSB, and (6) defining a binary weighted MSB according to the specified relationship for IMSB.

A method of design synthesis of an exponential current digital-to-analog converter (IDAC) having improved accuracy using a LSB exponential IDAC and an binary-weighted most significant bit (MSB) exponential IDAC whose method of design synthesis comprising the steps of (1) defining a differential non-linearity (DNL), (2) defining number of LSB bits needed for the targeted DNL with a binary weighted MSB, (3) calculating the number of bits to be used for the binary-weighted MSB to get the desired IDAC base, (4) deriving the minimum current for the Imax, (5) defining the LSB as an exponential current mirror according to the specified relationship for the ILSB, (6) defining a binary weighted MSB according to the specified relationship for IMSB, (7) synthesizing said LSB exponential IDAC, (8) synthesizing said binary-weighted most significant bit (MSB) exponential IDAC, (9) integrating said LSB exponential IDAC and said binary-weighted most significant bit (MSB) exponential IDAC, and (10) optimize said exponential current digital-to-analog converter (IDAC).

These and other objects are achieved by a circuit to achieve an exponential current digital-to-analog converter (IDAC) having improved accuracy using a binary-weighted most significant bit (MSB). As such, an apparatus of an exponential current digital-to-analog converter (IDAC) to efficiently drive current controlled light emitting diode (LED) devices is desired. Other advantages will be recognized by those of ordinary skill in the art.

DETAILED DESCRIPTION

FIG. 1is a circuit schematic diagram illustrating a prior art embodiment of an IDAC architecture in accordance with a prior art embodiment. In the implementation ofFIG. 1, there is a LSB network that separates the LSB base current Io and the exponential part ILSB_exp. In the implementation ofFIG. 1, there is a MSB network that separates the MSB base current Io and the exponential part IMSB_exp.FIG. 1shows a power pad VDD1, a ground rail VSS2, a VIDAC_FB pad3, a first PFET current mirror (20and21), a second PFET current mirror (30and31), a LSB network40, and a MSB network50, a buffer element60, a transistor70, and output transistor80. The LSB network40contains a LSB diode41, a transistor42for the base current Io, a variable transistor43for the exponent term ILSB_exp, and a LSB responsive switch44. The MSB network50contains a MSB diode51, a variable transistor52for the exponent term ILSB_exp, and a MSB responsive switch54.

FIG. 2is a circuit schematic diagram illustrating a prior art embodiment of a system for driving light emitting diode (LED) elements in accordance with prior art. As illustrated inFIG. 2, a high level diagram of a system to drive light emitting diode (LED) devices is shown. The block diagram shows a system100, to drive a string of LED elements. The system contains a power output pad101, a ground connection102, a error amplifier110, a pulse width modulation (PWM) comparator120, a DC-DC converter130, a series configured LED elements140to141, a current source150. The PWM used in the PWM comparator is not to be confused with the PWM used for dimming LEDs as noted in the prior discussion and description ofFIG. 1.

In the preferred embodiment,FIG. 3is a circuit schematic diagram illustrating an exponential current digital-to-analog converter (IDAC) in accordance with one embodiment of the disclosure. In the implementation ofFIG. 3, there is a LSB network that separates the LSB base current Io and the exponential part ILSB_exp. In the implementation ofFIG. 3, there is a MSB network that separates the MSB base current Io and the exponential part IMSB_exp.

As illustrated inFIG. 3, shows a power pad VDD201, a ground rail VSS202, a VIDAC_FB pad203, a current source Iin210, a first PFET current mirror (220and221), a second PFET current mirror (230and231), a LSB network240, a MSB network250, a buffer element260, and a transistor270.

The LSB network240contains a LSB diode241, a transistor242for the base current Io, a variable transistor243for the exponent term ILSB_exp, and a LSB responsive switch244. The LSB network240has the gate of the transistor elements241,242, and243electrically connected to the drain of p-type current mirror transistor221. The LSB diode241drain is connected to the drain of p-type current mirror transistor221. The LSB transistor242drain is electrically connected to the p-type transistor drain230of the p-type current mirror formed from transistor230and231. The drain of LSB transistor243is electrically connected to the LSB switch244. The LSB switch is in series between the drain of LSB transistor243and the drain of p-type current mirror transistor230. The LSB code initiates LSB switch244.

The MSB network250contains a MSB diode251, and a plurality of transistor elements and series switch elements. The plurality of transistor and series switch elements are illustrated as transistor252A, and series switch254A; transistor252B, and series switch254B; transistor252C, and series switch254C. The MSB code initiates the parallel switches for the MSB switch254A,254B, and254C, as illustrated. The MSB diode251element is electrically connected to the transistor270and buffer input260. The gates of the MSB diode251, and transistors252A,252B, and252C is electrically connected to the p-type current mirror element231from current mirror formed from transistors230and231.

The apparatus of the IDAC inFIG. 3, follows the formulation as shown:

(2nLSB-1)+X*(2nMSB-1)(2n-1)-1→X=2nLSB
The differential non-linearity (DNL) is typically defined as the current step percentage,

DNL⁢⁢%=IDAC⁡(n+1)-IDAC⁡(n)IDAC⁡(n)*100
In this embodiment, the DNL will be defined as the ratio of two consecutive steps, or as a percentage, as shown below,

DNL=IDAC⁡(n+1)IDAC⁡(n)orDNL=1+DNL⁢⁢%100
As such, it is possible to represent the DNL as follows:

The exponential IDAC network can be defined as a function of the LSB, the DNL, the nLSB, and MSBcode. The relationship for the implementation can be defined according to the following derivation

The design synthesis of the exponential IDAC circuit is then defined by forming two exponential current mirror networks from the derivations, where one of the current source networks is for the least significant bit (LSB), and a second for the most significant bit (MSB), as illustrated inFIG. 3.

The current through the LSB, the ILSB, can be defined as a function of the linear current Io, the gain parameter G, the DNL, and the LSBcode.

ILSB=I0G*(DNL)LSBcode
The current through the MSB, the IMSB, can be defined as a function of the gain parameter G, the ILSB, the DNL of the MSB, and the MSBcode.
IMSB=G*ILSB*(DNLMSB)MSBcode
Defining the number of bits needed for the LSB to achieve DNLMSB=2.
2=(DNL)2nLSB
The base required for the synthesis of a design of an exponential IDAC with a binary-weighted MSB, will determine the number of MSB bits that are required.

Defining a differential non-linearity (DNL) according to the definition

Defining number of LSB bits needed for the targeted DNL with a binary weighted MSB
DNLMSB=(DNL)2nLSB

Calculating the IDAC base using the number of bits used for the MSB

Deriving the minimum current for the Imax
Io=Imax/base

Defining the LSB as an exponential current mirror according to the specified relationship for the ILSB

Defining a binary weighted MSB according to the specified relationship for IMSB,
IMSB=G*ILSB*(DNLMSB)MSBcode
For MSB code=0, . . . n MSB.

As an example of the preferred embodiment ofFIG. 3, considering that, according to definition, it is usually DNL about 1, the number of nMSB will depend on how much contrast is needed. Typically, an IDAC to drive a light emitting diode (LED) string, needs a maximum current in the order of 25 mA. To guarantee an appropriate range of dimming you might want to be able to go down to tens of uA as minimum current. This means that, based on equation, nMSB=3 is possible, as illustrated inFIG. 3.

base=(2)2nDNL≅256→I⁢⁢min=25⁢⁢mA256≅98⁢⁢uA
Note that the formulas described are also applicable to the architecture ofFIG. 1, as well as the architecture that employs an LSB and MSB current mirror to generate an exponential current.

FIG. 3is an example embodiment associated with the methodology defined in the disclosure to create an exponential IDAC with a 3-bits binary-weighted MSB. It is possible to design an exponential IDAC with a 3-bits binary-weighted MSB, which will be easy to layout, more accurate and simple enough to allow spending area to meet the low voltage compliance specification.

As such, an exponential current digital-to-analog converter (IDAC) circuit, and, more particularly, an apparatus of an exponential current digital-to-analog converter (IDAC) using a binary-weighted MSB to efficiently drive current controlled light emitting diode (LED) devices are herein described. The apparatus comprises of a exponential current digital-to-analog converter (IDAC) current source, a voltage buffer to create an active cascode at the output stage, and an error amplifier that by means of a DC-DC converter voltage loop imposes an appropriate voltage at the output of the IDAC, depending on the current load set to drive the LEDs. The definition of the apparatus involves defining an exponential LSB and exponential MSB current mirrors according to a defined methodology. The improvement is achieved providing a solution for low voltage compliance without losing current accuracy. The improvement allows for an output stage that allows flexibility for the IDAC to be operable in both saturation and linear region, as well as allow for improved efficiency.

FIG. 4is a high level diagram for the method of design synthesis for improving the accuracy of an exponential IDAC using a binary weighted MSB in accordance with one embodiment of the disclosure. A method to achieve an exponential current digital-to-analog converter (IDAC) having improved accuracy using a binary-weighted most significant bit (MSB) comprising:

defining a differential non-linearity (DNL);

defining number of LSB bits needed for the targeted DNL with a binary weighted MSB;

calculating the number of bits to be used for the binary-weighted MSB to get the desired IDAC base;

deriving the minimum current for the Imax;

defining the LSB as an exponential current mirror according to the specified relationship for the ILSB; and

defining a binary weighted MSB according to the specified relationship for IMSB.

As the first step300, a differential non-linearity (DNL) is defined. The second step310defines the number of LSB bits needed for the targeted DNL with a binary weighted MSB. The third step320calculates the number of bits for the binary-weighted MSB to get the desired IDAC base. The fourth step330derives the minimum current for the Imax. The fifth step340defines the LSB as an exponential current mirror according to the specified relationship for the ILSB. The sixth step350defines a binary weighted MSB according to the specified relationship for IMSB.

As such, a method of exponential current digital-to-analog converter (IDAC) using a binary weighted most significant bit (MSB) wherein said most significant bit (MSB) current mirror is defined according the following steps, (1) defining a differential non-linearity (DNL), (2) defining number of LSB bits needed for the targeted DNL with a binary weighted MSB, (3) calculating the number of bits to be used for the binary-weighted MSB to get the desired IDAC base, (4) deriving the minimum current for the Imax, (5) defining the LSB as an exponential current mirror according to the specified relationship for the ILSB, and (6) defining a binary weighted MSB according to the specified relationship for IMSB.

A method of design synthesis to achieve an exponential current digital-to-analog converter (IDAC) having improved accuracy using a LSB exponential IDAC and an binary-weighted most significant bit (MSB) exponential IDAC whose method of design synthesis comprises (1) defining a differential non-linearity (DNL), (2) defining number of LSB bits needed for the targeted DNL with a binary weighted MSB, (3) calculating the number of bits used for the binary-weighted MSB to get the desired IDAC base, (4) deriving the minimum current for the Imax,

(5) defining the LSB as an exponential current mirror according to the specified relationship for the ILSB, (6) defining a binary weighted MSB according to the specified relationship for IMSB, (7) synthesizing the LSB exponential IDAC, (8) synthesizing the binary-weighted most significant bit (MSB) exponential IDAC, (9) integrating the LSB exponential IDAC and the binary-weighted most significant bit (MSB) exponential IDAC, and (10) optimize the exponential current digital-to-analog converter (IDAC).

Other advantages will be recognized by those of ordinary skill in the art. The above detailed description of the disclosure, and the examples described therein, has been presented for the purposes of illustration and description. While the principles of the disclosure have been described above in connection with a specific device, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the disclosure.