METHOD FOR VCOM LEVEL ADJUSTMENT WITH INTEGRATED PROGRAMMABLE RESISTIVE ARRAYS

Abstract

A calibrator circuit and method for VCOM voltage adjustment for an LCD includes using integrated programmable resistive arrays. The method uses two DACs and three integrated circuit arrays to provide all of the advantages of VCOM calibrator circuits using external resistive voltage-dividers. The integrated circuit resistor arrays reduce the number of external components and PCB space. The method used is suitable for higher resolution adjustment of the VCOM voltage and no calculation is required in the whole adjustment procedure, which saves labor cost, time and enables automation of the calibrator fabrication.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the invention will be more readily understood from the following detailed description of the invention which is provided in connection with the accompanying drawings, in which:

FIG. 1 is a diagram that shows the relationship between VCOM level distribution and LCD flicker intensity according the prior art;

FIG. 2 is a schematic diagram of two prior art VCOM calibrator circuits each using a mechanical potentiometer;

FIG. 3 is a schematic diagram of a prior art VCOM calibrator circuit using an external resistive voltage-divider;

FIG. 4 is a schematic diagram of a VCOM calibrator circuit having internal programmable resistive arrays in an integrated circuit with the other calibrator circuitry according to the present invention;

FIG. 5 is a flow chart of a VCOM adjustment procedure using the integrated programmable resistors of the present invention;

FIG. 6 is a diagram that illustrates the coarse adjustment of the VCOM level according to the present invention;

FIG. 7 is a diagram that illustrates the fine adjustment of the VCOM level according to the present invention;

FIG. 8 is a schematic diagram of two integrated circuit arrays used to set the R1/R2 ratio according to the present invention; and

FIG. 9 is a schematic diagram of a resistor array and simplified circuit diagram used to set the value of the RSET resistor according to the present invention.

DESCRIPTION OF THE INVENTION

Referring now to FIG. 4, a schematic diagram of a VCOM calibrator 400 is shown having integrated programmable resistors according to the present invention. Calibrator 400 includes an interface and control block 402, a bus 416 for communicating with a first DAC (“Digital to Analog Converter”) 418, a bus 404 for communicating with a second DAC 408, and a bus 406 for communicating with non-volatile memory block 410. The output of DAC 408 is coupled to the positive input of buffer 412, which drives the gate of transistor M1. The source of transistor M1 is coupled to the R_SET resistor at the SET node. The drain of transistor M1 is coupled to the output of an internal integrated resistor divider at the OUT node. The internal integrated resistor divider includes resistors R1 and R2, which is coupled between the AVDD power supply and ground. The OUT node is buffered through buffer 414 to provide the VCOM voltage. The output of the second DAC 408 is coupled to the control inputs of adjustable integrated resistors R1, R2, and RSET as is shown in FIG. 4. Note that the reference inputs of DAC 408 are coupled between AVDD and ground and include “Y” resistors having a value of “R” each. Thus, a total resistance of YR is coupled between AVDD and ground.

In FIG. 4, the functional blocks of the VCOM calibrator 400 with internal programmable resistors are different from the prior art calibrators with an external resistive voltage-divider as follows:

• • 1) R1, R2 and R_SET are external resistors in PRIOR ART FIG. 3 whereas they are integrated on chip in FIG. 4, according to the present invention.
  • 2) The values of R1, R2 and R_SET are fixed in PRIOR ART FIG. 3 whereas they are programmable in FIG. 4.
  • 3) There is only one DAC to change the VCOM level in PRIOR ART FIG. 3 whereas there are two DACs for adjusting the VCOM level in FIG. 4. The first DAC 418 is used to program resistors R1, R2 and R_SET for selection of the adjustment range. The second DAC 408 has a similar function to the sole DAC shown in FIG. 3, whose function is to change the VCOM level.

The basic operating procedure of VCOM calibrator 400 with integrated programmable resistors can be summarized in two main steps:

• • 1) Change the value of the first DAC 418 (k bits) to select a sub-range that contains the desired VCOM level within the probable range of the VCOM level distribution (generally between 1/3*AVDD and 2/3*AVDD, where AVDD is the analog voltage supply). This step is called coarse adjustment.
  • 2) After the desired sub-range is found, change the value of the second DAC 408 (n bits) to achieve the desired VCOM level within this sub-range. This step is called fine adjustment.

The flow chart of the adjustment procedure 500 is shown in FIG. 5. In FIG. 5, “VCOM at boundary point” means that the optimum flicker intensity of LCD panel is already achieved before adjustment and so there is no need to further adjust the VCOM level at all. Step 502 loads the default value of VCOM from the NVMEM 410. Steps 504 starts the coarse adjustment. Decision diamond 506 asks whether or not the VCOM level has reached the boundary point as described above. If the answer is yes, the adjustment procedure proceeds to step 514, described below. If the answer is no, the adjustment procedure proceeds to step 508. Step 508 starts the fine adjustment. Decision diamond 510 asks whether or not the VCOM level has now reached the boundary point as described above. If the answer is yes, the adjustment procedure proceeds to step 514, described below. If the answer is no, the adjustment procedure proceeds to decision diamond 512. Decision diamond asks whether or not a precise value of the VCOM voltage is obtained. If the answer is no, the procedure returns to initial step 502. If the answer is yes, the procedure proceeds to step 516 and the precise VCOM value is stored in NVMEM 410. For a yes answer at 506 or 510, the procedure proceeds to step 514 wherein the user sets the VCOM level, and then proceeds to step 516 for storing the final VCOM value.

The desired value of VCOM is generally in the range between (1/3)*AVDD and (2/3)*AVDD, depending upon the manufacturer of the LCD display. And so the range for the coarse adjustment should cover this entire range. In the coarse adjustment, the overall range from (1/3)*AVDD to (2/3)*AVDD is separated into 2^k−1 identical sub-ranges whose serial numbers are from 0 to 2^k−2 as shown in diagram 600 of FIG. 6.

During coarse adjustment, the value of the first DAC, “i”, is changed between 0 and 2^k−1 with a default value 2^k−−1 and the value of the second DAC is kept constant at zero. Thus, the ratio of R1/R2 is changed and the output of VCOM is:

$\begin{array}{cc}V\mathrm{COM}\left(i\right)=\frac{\mathrm{AVDD}}{\left(1+\frac{R1}{R2}\right)}=\left(\frac{2}{3}-\frac{i}{3*\left(2^k-1\right)}\right)*\mathrm{AVDD}i=0,1\dots 2^k-1& \left(7\right)\end{array}$

According to Equation (7), if “i” is increased by one, the VCOM output decreases by AVDD/(3*(2^k−1)). Thus the resolution for the coarse adjustment of VCOM is AVDD/(3*(2^k−1)).

The ratio of R1 to R2 can be obtained from Equation (7):

$\begin{array}{cc}\frac{R1}{R2}=\left(\frac{2^k-1+i}{2^{k+1}-2-i}\right)i=0,1\dots 2^k-1& \left(8\right)\end{array}$

With this procedure, the sub-range that contains the desired VCOM level can be found by observing the flicker intensity degree of the LCD panel. For example, if “i” is changed from 1 to 3 and the flicker intensity is: flicker intensity (i=1)>flicker intensity (i=3)>flicker intensity (i=2), then the desired VCOM level is in sub-range 2.

After the sub-range is found, the fine adjustment of VCOM level is carried out within this sub-range. FIG. 7 is the illustration of the fine adjustment of VCOM level. In FIG. 7, it is assumed that the desired VCOM level is in sub-range “i”, so we obtain V_MAX=VCOM(i) and V_MIN=VMAX(i+1)=VCOM(i+1) for fine adjustment. The fine adjustment step is similar to that of the VCOM calibrator with an external resistive voltage-divider. The value of the second DAC, “j”, is changed between zero and 2ⁿ−1 to vary the VCOM output and find the optimum VCOM level. VCOM(i, j) equals:

$\begin{array}{cc}V\mathrm{COM}\left(i,j\right)=\left(\frac{2}{3}-\frac{i}{3*\left(2^k-1\right)}\right)*\mathrm{AVDD}*\left(1-\frac{j}{2^n-1}*\frac{R1}{Y*R_{\mathrm{SET}}}\right)i=0,1\dots 2^k-2;j=0,1\dots 2^k-1& \left(9\right)\end{array}$

where Y=number of resistor R in the second DAC.

• • V_MAX(i) is:

$\begin{array}{cc}{V}_{\mathrm{MAX}}\left(i\right)=V\mathrm{COM}\left(i\right)\approx \left(\frac{2}{3}-\frac{i}{3*\left(2^k-1\right)}\right)*\mathrm{AVDD}i=0,1\dots 2^k-2& \left(10\right)\end{array}$

• • V_MIN(i) is:

$\begin{array}{cc}{V}_{\mathrm{MIN}}\left(i\right)=\left(\frac{2}{3}-\frac{i}{3*\left(2^k-1\right)}\right)*\mathrm{AVDD}*\left(1-\frac{R1}{Y*R_{\mathrm{SET}}}\right)i=0,1\dots 2^k-2& \left(11\right)\end{array}$

where Y=number of resistors R in the second DAC.

Due to V_MIN(i)=V_MAX(i+1), it can be shown that:

$\begin{array}{cc}\frac{R1}{R_{\mathrm{SET}}}=\left(\frac{Y}{2^{k+1}-2-i}\right)i=0,1\dots 2^k-2;& \left(12\right)\end{array}$

where Y=the number of resistors R in the second DAC.Within these 2ⁿ−1 adjustment steps, the optimum “fine” VCOM level can be located. The sub-range for fine adjustment is:

$\begin{array}{cc}\text{Sub-range}=\frac{\mathrm{AVDD}}{3*\left(2^k-1\right)}& \left(13\right)\end{array}$

The resolution for fine adjustment is:

$\begin{array}{cc}\mathrm{Resolution}=\frac{\mathrm{AVDD}}{3*\left(2^k-1\right)*\left(2^n-1\right)}& \left(14\right)\end{array}$

From Equation (8), it can be shown that the sum of the numerator and the denominator of R1/R2 (i.e. R1+R2) is a constant and it is equal to 3*(2^k−1). We can use R_UNIT to denote them where R_UNIT is a unit resistor whose value should be determined by its electrical characteristics.

The parameter implementation of R1/R2 and R1/R_SET obtained from Equation (8) and Equation (12) are listed in Table 1.

TABLE 1
PARAMETER IMPLEMENTATION of R1, R2 AND RSET
		R1	R2	RSET
	i	(RUNIT)	(RUNIT)	(RUNIT)
	0	2k − 1	2k+1 − 2	(2k − 1) * (2k+1 − 2)/Y
	1	2k	2k+1 − 3	2k * (2k+1 − 3)/Y
	2	2k + 1	2k+1 − 4	(2k + 1) * (2k+1 − 4)/Y
	3	2k + 2	2k+1 − 5	(2k + 2) * (2k+1 − 5)/Y
	—	—	—	—
	2k − 4	2k+1 − 5	2k + 2	(2k + 2) * (2k+1 − 5)/Y
	2k − 3	2k+1 − 4	2k + 1	(2k + 1) * (2k+1 − 4)/Y
	2k − 2	2k+1 − 3	2k	2k * (2k+1 − 3)/Y
	2k − 1	2k+1 − 3	2k − 1	—

In this way, the values of R1, R2 and R_SET can be determined very simply and conveniently.

Two examples of using integrated programmable resistive arrays to adjust VCOM level are given below. In these examples, a 3-bit first DAC and a 7-bit second DAC are utilized.

EXAMPLE 1

AVDD=20V, VCOM=13V

Coarse Adjustment

Step 1:

Load default settings: k:=3 (011₂)

• • n:=0 (000 0000₂)

Thus R1=10*R_UNIT, R2=11*R_UNIT

• • VMAX=40/3V, VMIN=20/3V.

Step 2:

Change k until the desired VCOM level is found in the sub-range between k=0 (000₂) and k=1 (001₂).

The coarse adjustment range is from 20/3V to 40/3V.

The coarse adjustment resolution is 20/21 V.

Fine Adjustment

Step 1:

Setting: k:=5 (101₂)

• • n:=64 (100 0000₂)

Thus R1=7*R_UNIT, R2=14*R_UNIT, RSET=4.9*R_UNIT;

• • VMAX=40/3V, VMIN=260/21 V.

Step 2:

Change n until the desired VCOM level is found between n=44 (010 1100₂) and n=45 (010 1011₂).

If n is chosen at 44, the output Value is 13.003V (very close to desired VCOM level of 13V) and only has a 3 mv deviation.

The fine adjustment range is 20/21 V.

The fine adjustment resolution is 20/2667≈7.5 mV.

EXAMPLE 2

AVDD=10V, VCOM=4.131V

Coarse Adjustment

Step 1:

Load default settings: k:=3 (011₂)

• • n:=0 (000 0000₂)

Thus R1=10*R_UNIT, R2=11*R_UNIT

• • VMAX=40/3V, VMIN=20/3V.

Step 2:

Change k until the desired VCOM level is found in the sub-range between k=5 (101₂) and k=6 (110₂);

The coarse adjustment range is from 20/3V to 40/3V.

The coarse adjustment resolution is 20/21V.

Fine Adjustment

Step 1:

Setting: k:=5 (000₂)

• • n:=64 (100 0000₂)

Thus R1=12*R_UNIT, R2=9*R_UNIT, RSET=4.9*R_UNIT;

• • VMAX=30/7V, VMIN=80/21V.

Step 2:

Change n until the desired VCOM level is found between n=64 (100 0000₂) and n=63 (011 1111₂);

If n is chosen at 64, the output Value is 4.131V which just equals the desired VCOM level.

• • In this case, the desired VCOM level is just at the “boundary point” before fine adjustment, so there is no need to carry on fine adjustment.

FIGS. 8 and 9 depict the implementation of realizing the R1/R2 and R_SET values according to the present invention in greater detail. In particular, note that resistors R1 and R2 are switched arrays of resistors.

In FIG. 8, altogether 4*(2^k−1) resistors, which have the same unit value of resistance (R_UNIT), are put in series between AVDD and ground. Among them, 2*(2^k−1) resistors on the left of “OUT” constitute R1 and the remaining ones on the right constitute R2. Overall, 2*(2^k−1) switches which include N1˜N2^k−1 and P1˜P2^k−1 are used to determine the valid R2 and R1 values which in turn determine the output result. The control signals for N1˜N2^k−1 and P1˜P2^k−1 are listed in Table 2 below. For each “i”, overall 3*(2^k−1) resistors are valid between AVDD and GND, others are shorted by the switches which are turned on. For example, if i=2, R1=(2^k+1)*R_UNIT, R2=(2^k+1−4)*R_UNIT and N2=1, P3=1; Other switches are off.

Among all of these switches, N2^k−1 or P2^k−1 may need to withstand a maximum Vgd of 6.5V, which is substantially lower than that of MOS transistors used in prior art methods. Hence, the simplicity and reliability are greatly increased by using this structure and procedure according to the present invention. The VCOM output for this structure is more precise and its total used resistance value is much less than other prior art methods.

Generally, 2^k−1 R_UNIT and 2^k−1 switches are required for the implementation of R_SET. In fact, the numbers of resistors and switches can be reduced further as long as the resolution can be guaranteed. The case below shows how it can be done.

TABLE 2
CONTROL SIGNALS FOR SWITCHES IN FIG. 8
I	P2k − 1	P2k − 2	P2k − 3	. . .	P3	P2	P1	N2k − 1	N2k − 2	N2k − 3	. . .	N3	N2	N1
0	0	0	0	. . .	0	0	1	0	0	0	. . .	0	0	0
1	0	0	0	. . .	0	1	0	0	0	0	. . .	0	0	1
2	0	0	0	. . .	1	0	0	0	0	0	. . .	0	1	0
—	—	—
2k − 3	0	1	0	. . .	0	0	0	0	0	1	. . .	0	0	0
2k − 2	1	0	0	. . .	0	0	0	0	1	0	. . .	0	0	0
2k − 1	0	0	0	. . .	0	0	0	1	0	0	. . .	0	0	0

Example

Y=20, k=3 and n=7, the parameters for R2/R1 and R_SET/R1 is provided in Table 3.

TABLE 3
PARAMETERS OF R2/R1 AND RSET/R1 FOR Y = 20, k = 3 and n = 7
		R1	R2	RSET
	I	(RUNIT)	(RUNIT)	(RUNIT)
	0	7	14	4.9
	1	8	13	5.2
	2	9	12	5.4
	3	10	11	5.5
	4	11	10	5.5
	5	12	9	5.4
	6	13	8	5.2
	7	14	7	—

In fact, R_SET can use just one type of resistor (4.9*R_UNIT) so as to decrease the resistor numbers from 4 to 1 and decrease the switch number from 7 to 1. This decreases V_MIN a bit and increases the sub-range slightly. However, it is reasonable as long as the resolution is not compromised too much. The ratios of new sub-ranges when R_SET=4.9*R_UNIT compared to original sub-ranges are provided in Table 4.

TABLE 4
RESULT COMPARISON FOR THE SIMPLIFIED CIRCUIT
I	Ratio of new sub-range to original sub-range	% Change
0	1.00	+0%
1, 6	1.06	+6%
2, 5	1.10	+10%
3, 4	1.12	+12%

In the worst case when AVDD=20V, the sub-range is changed from 0.950V to 1.065V and the resolution is changed from 7.5 mV to 8.9 mV (with an increase for +12%). The difference is quite insignificant compared to the overall range. If one resistor is not enough to guarantee the resolutions for all sub-ranges, we can add some more resistors. However, the total number of resistors used is still less than 2^k−1. As a result, the number of resistors and switches can be greatly decreased.

While there have been described above the principles of the present invention, it is to be clearly understood that the foregoing description is made only by way of example and not as a limitation to the scope of the invention. Particularly, it is recognized that the teachings of the foregoing disclosure will suggest other modifications to those persons skilled in the relevant art. Such modifications may involve other features which are already known per se and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure herein also includes any novel feature or any novel combination of features disclosed either explicitly or implicitly or any generalization or modification thereof which would be apparent to persons skilled in the relevant art, whether or not such relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as confronted by the present invention. The applicants hereby reserve the right to formulate new claims to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

1. A calibrator comprising: a first DAC;a second DAC;a first resistor coupled between a power supply voltage and a calibrator output having an adjustable value controlled by the first DAC;a second resistor coupled between the calibrator output and ground having an adjustable value controlled by the first DAC; anda set resistor coupled between an output of the second DAC and ground, wherein the current flowing through the set resistor is sourced from the calibrator output.

2. The calibrator of claim 1 wherein the first DAC receives an input word having a first number of bits.

3. The calibrator of claim 2 wherein the input word comprises three bits.

4. The calibrator of claim 2 wherein the second DAC receives an input word having a second number of bits.

5. The calibrator of claim 4 wherein the input word comprises seven bits.

6. The calibrator of claim 1 wherein the first resistor comprises a programmable integrated circuit resistor.

7. The calibrator of claim 1 wherein the second resistor comprises a programmable integrated circuit resistor.

8. The calibrator of claim 1 wherein the set resistor comprises a programmable integrated circuit resistor.

9. The calibrator of claim 1 further comprising an interface and control logic block for receiving a clock signal and a data input signal, for providing a first digital input signal for the first DAC, and for providing a second digital input signal for the second DAC.

10. The calibrator of claim 9 further comprising a non-volatile memory coupled to the interface and control logic block.

11. The calibrator of claim 1 further comprising a buffer having a first input coupled to the second DAC, a second input coupled to the set resistor, and a current output coupled to the calibrator output.

12. The calibrator of claim 1 further comprising a buffer coupled to the calibrator output.

13. The calibrator of claim 1 further comprising changing an output value of the first DAC to select a sub-range that contains a desired calibrator output value.

14. The calibrator of claim 1 wherein the desired calibrator output value is between one-third and two-thirds of the power supply voltage.

15. The calibrator of claim 13 further comprising changing an output value of the second DAC to achieve the desired calibrator output value.

16. A calibration method comprising: providing a first DAC;providing a second DAC;coupling a first resistor between a power supply voltage and a calibrator output having an adjustable value controlled by the first DAC;coupling a second resistor coupled between the calibrator output and ground having an adjustable value controlled by the first DAC;coupling a set resistor between an output of the second DAC and ground; andsourcing the current flowing through the set resistor is from the calibrator output.

17. The method of claim 16 further comprising changing an output value of the first DAC to select a sub-range that contains a desired calibrator output value.

18. The method of claim 17 wherein the desired calibrator output value is between one-third and two-thirds of the power supply voltage.

19. The method of claim 17 further comprising changing an output value of the second DAC to achieve the desired calibrator output value.

20. A calibrator comprising: a first resistor, a second resistor, and set resistor in an integrated circuit having an adjustable value controlled by a first DAC;a resistor divider formed by the first and second resistors having a calibrator output; anda second DAC for controlling the voltage across the set resistor, wherein the current through the set resistor is sourced from the calibrator output.