Guide to Using the MAX25221 VCOM and Temperature Compensation in TFT-LCD Panels

Sep 14 2021

Abstract

This application note explains how to use the MAX25221 programmable VCOM buffer in TFT LCD applications.

Introduction

The MAX25221 is a 4-channel TFT-LCD power IC that provides symmetrical positive AVDD and negative NAVDD supplies, VGON and VGOFF gate supplies, and an internal digitally programmable VCOM buffer (Figure 1).

The VCOM voltage provides a tuned reference DC voltage to the TFT-LCD panel backplane and functions as the path of current. The MAX25221 is designed to hold a constant VCOM voltage while mitigating the current spikes from the backplane. Spikes and fluctuations of the VCOM voltage cause crosstalk and deteriorate the display image of TFT-LCD. The larger TFT-LCD panels become, the more important it is to consider these fluctuations. Stability of the VCOM voltage and a low drift over the operation temperature range improve picture quality and reduce brightness variation. The VCOM buffer outputs peak currents up to ±120mA and is powered from the IN supply pin and a regulated voltage at VCOMN.

Figure 1. Partial block diagram of MAX25221.

VCOM Buffer

The VCOM output voltage is programmed using the built-in I²C interface to a value between - 2.49V and +1V in 6.83mV steps. The 9-bit value can also be stored in a nonvolatile register. Table 1 describes the configuration of the VCOM register bits. The most-significant bits of the VCOM voltage setting are in the VCOM25 register while the least-significant bit is the vcom25_0 bit in the DELAY VCOM_LSB register.

Table 1. VCOM Register Bits.
Address	Name	Bit
		7	6	5	4	3	2	1	0
0x08	DELAY-VCOM_LSB	delayt1[1:0]		delayt2[1:0]		Delayt3[1:0]		T_comp_en	vcom25_0
0x09	VCOM25	[7:0]

Drive Capability

The maximum capacitive load on the VCOM output is 10nF. When driving larger capacitive loads, a series resistor should be employed to maintain stability. Figure 2 displays the simulation results of a capacitive load sweep from 10nF to 14.1µF, series resistor value to 1?, and different VCOM values.

Figure 2. Simulated phase margins at different voltage and capacitance values.

VCOM Fault Detection

If the VCOM output voltage deviates from the set value by more than 0.25V or a current limitation is verified for a time longer than tfault, a VCOM fault is detected and flagged with the vcom_flt bit in the FAULT2 register. When this fault is detected, the VCOM buffer continues to function — it is not automatically disabled. However, a fault condition can lead to high power dissipation in the VCOM buffer and could lead to thermal shutdown of the entire device. If the VCOM buffer is in continuous current-limit mode for more than the time set by tfault[1:0], the vcom_flt bit is flagged, and outputs including VCOM, AVDD, NAVDD, VGH and VGL are disabled to avoid damage to the IC.

VCOM Settings

To calculate the VCOM value (9 bits), write to the VCOM25 register using the following equation:

Equation 1.

The correspondence between the VCOM set value and the VCOM voltage is shown in Table 2.

Table 2. VCOM Settings.
VCOM Register Value	REG 0X09 [7:0]	REG 0X08 [0]	VCOM Voltage (V)
0X1FF	FF	1	+1
0X1FE	FF	0	+0.99317
0X1B6	DB	0	+0.501
—	—	—	—
0X18A	C5	0	0.2008
0X16C	B6	0	-0.0040
0X002	01	0	-2.4763
0X001	00	1	-2.4832
0X000	00	0	-2.49

Different Functional Modes

The VCOM output can operate in different modes:

VCOM without temperature compensation.
VCOM with external temperature compensation.
VCOM with internal temperature compensation.

VCOM Without Temperature Compensation

If this option is selected, the IC operates in a simple mode where the VCOM voltage is not compensated for temperature variations and is always set to the value according to VCOM25. It is possible to limit the VCOM output to a range between the values set in the VCOM_MIN and VCOM_MAX registers.

If this limitation is desired, the pins 29 (R_REF) and 30 (TEMP) should be floating. Table 3 lays out the register settings for this mode of operation.

Table 3. VCOM Without Temperature VCOM Register Bits.
Register		Bits	Description
0x08	DELAY-VCOM_LSB [7:0]	1	T_comp_en bit enables/disables the temperature compensation	[0] disables this functionality.
		0	vcom25_0	LSB of VCOM setting at +25°C
0x09	VCOM25[7:0]	[7:0]	Sets the VCOM value between the range defined from the registers VCOM_MIN and VCOM_MAX.
0x11	VCOM_MIN[7:0]	[7:0]	Defines minimum value of VCOM.
0x12	VCOM_MAX[7:0]	[7:0]	Defines maximum value of VCOM.

VCOM Temperature Compensation

The VCOM output voltage can compensate for temperature changes using a temperature-sensitive component connected to the TEMP input (Figure 3). A temperature-compensated VCOM is sometimes needed to maintain display quality in environments with temperature variations.

The temperature compensation is enabled by setting the T_comp_en bit in the DELAY-VCOM_LSB register and the sensor to be used is selected according to the int_sensor bit in the CONFIG register.

These registers are further detailed in Table 4.

The external compensation guarantees a more accurate measurement related to the display panel. The temperature-sensitive components are placed on the display panel to guarantee a correct measurement. The internal compensation is less accurate as the temperature is measured inside to the device using the internal temperature sensor. However, this solution has the benefit of not requiring external components.

Figure 3. Temperature compensation circuit.

Table 4. VCOM Temperature Compensation Registers.
Register		Bits	Description
0x07	CONFIG [7:0]	7	int_sensor	[0]* External T Sensor [1] Internal Sensor
0x08	DELAY-VCOM_LSB [7:0]	1	T_comp_en bit enable/disable the temperature compensation	[1] Enable
*Default position

VCOM External Temperature Compensation

The VCOM output voltage compensates for temperature changes externally using an NTC thermistor connected to the TEMP pin. The VCOM value functions according to the value of ADC reading which varies with temperature (Figure 4).

Figure 4. Top-level NTC+VCOM system.

The TEMP pin is forced to 625mV and the current drawn from it is mirrored on the R_REF pin. The voltage generated due to the resistor on R_REF is fed to the internal 8-bit ADC, which has a reference voltage of 1.25V.

The input to the ADC is therefore as follows:

Equation 2.

The highly nonlinear NTC characteristic can be modified depending on which temperature (cold, room, or hot) necessitates the highest resolution.

Two different NTC connections can be implemented with different transfer functions of the internal ADC as show in Figure 5 and Figure 6.

Figure 5. NTC Connection Mode 1.

Figure 6. Push-Pull Configuration.

Figure 7. VADC_in vs. Temp Mode 1 and Mode 2. Shows a comparison between the voltage at the ADC in Mode 1 and Mode 2 vs. temperature.

Figure 7. V_{ADC_in} vs. Temp Mode 1 and Mode 2. Shows a comparison between the voltage at the ADC in Mode 1 and Mode 2 vs. temperature.

In Mode 2, the values of R_REF, NTC, R1, and R2 are chosen to optimize the ADC input range (0 to 1.25V) in the operating temperature range of the display (-30°C to +90°C).

As you can see, Mode 2 implementation is more linear than Mode 1 in the range of -30°C to +90°C, enhancing the resolution of the ADC. Overall, it is possible to optimize the resistor values to obtain the best resolution reference to customer requirements.

In Mode 1 implementation, the best results are only in the range of +40°C to +90°C (the red curve is more slope in this range), but the worst resolution is in the range of -40°C to +20°C.

As shown in Table 1, the VCOM value at +25°C is the value written in the VCOM25 register together with the LSB from the DELAY-VCOM_LSB register. During the startup phase, the temperature compensation function is not instantly enabled. For this reason, the device always starts up with the VCOM25 voltage value on VCOM. To obviate the highly nonlinear NTC characteristic but keep a high resolution, four breakpoints are introduced as shown in Figure 8. For each individual breakpoint, the VTEMP values and the desired VCOM values are defined related to temperature (Table 5).

Table 5. VCOM Break Points.
Breakpoint	Temp	vtemp	vcom
1	LOW	VTEMP_L	VCOM_L *
2	25	VTEMP25	VCOM_25
3	H1	VTEMP_H1	VCOM_H1*
4	H2	VTEMP_H2	VCOM_H2 **
The 5-bit values in the VCOM_L, and VCOM_H1 registers represent the change in VCOM from the VCOM25 value at the temperature represented by an ADC reading of VTEMP_L and VTEMP_H1. *The value in the VCOM_H2 register represents the positive shift in VCOM from VCOM_H1. The VCOM_L value represents a negative shift in VCOM while VCOM_H1 and VCOM_H2 represents positive shifts.

Figure 8. Temperature compensation curve.

Table 6 describes the registers to configure for this operation mode.

Table 6. VCOM with Temperature Compensation Registers.
Register		Bits	Description
0x07	CONFIG [7:0]	7	int_sensor	[0]* External T Sensor
0x08	DELAY-VCOM_LSB [7:0]	1	T_comp_en bit enable/disable the temperature compensation	[1] Enable
0x08	DELAY-VCOM_LSB [7:0]	0	vcom25_0	LSB of VCOM setting at 25°C
0x09	VCOM25[7:0]	[7:0]	Set the VCOM value at 25°C
0x0A	VCOM_L[7:0]	[4:0]	Delta VCOM at the temperature corresponding to VTEMP_L. This value sets the difference between the VCOM value at 25°C and that at VTEMP_L.
0x0B	VCOM_H1[7:0]	[4:0]	Delta VCOM at VTEMP_H1. This value sets the difference between the VCOM value at 25°C and that at VTEMP_H1.
0x0C	VCOM_H2[7:0]	[4:0]	Delta VCOM at VTEMP_H2. This value sets the difference between the VCOM value at VTEMP_H1 and that at VTEMP_H2.
0x0D	VTEMP25	[7:0]	Voltage at the TEMP* pin at 25°C.
0x0E	VTEMPL	[7:0]	Voltage at the TEMP* pin corresponding to the low-temperature breakpoint in the VCOM compensation curve.
0xF	VTEMP_H1	[7:0]	Voltage at the TEMP pin corresponding to the first high-temperature breakpoint in the VCOM compensation curve.
0x10	VTEMP_H2	[7:0]	Voltage at the TEMP pin corresponding to the second high-temperature breakpoint in the VCOM compensation curve

VCOM with Internal Temperature Compensation

In this mode, the VCOM output voltage compensates for temperature changes using the internal temperature sensor. The internal temperature sensor senses the junction temperature of the IC, which may be significantly different to the ambient temperature (Figure 9).

Figure 9. Internal temperature compensation system.

To use the internal sensor, set the int_sensor bit in the CONFIG register to 1. The internal temperature sensor has a temperature coefficient of 2mV/°C and a nominal output voltage of 620mV at 25°C as demonstrated in Figure 10.

Figure 10. V<sub>ADC in</sub> vs. temperature.

Figure 10. V_{ADC in} vs. temperature.

Sample ADC Results vs. Temperature

This is an example of how to configure the VCOM output voltage registers.

An NTC with 10kO resistance at 25°C and R1 with 330O resistance are connected from TEMP to GND. The R_REF resistor is 2400O with the Mode 2 connection.

At various temperatures, the following voltages will be observed on R_REF as well as the ADC measurement result will be as follows in Table 7. For examples of various VCOM register settings, see Table 8

The VTEMP HEX column indicates the VADC voltage for each temperature. The VCOM column indicates the delta between the desired VCOM voltage and VCOM25 for each temperature. The DES VCOM VOLTAGE indicates the desired VCOM voltage values at each temperature.

* lsb 4.89mV
**lsb 6.83mV

Table 7. ADC Voltage vs. Temperature.
Temp (°C)	NTC (k?)	VADC (mV)	vtemp		VCOM			DES VCOM Voltage (V)
Temp (°C)	NTC (k?)	VADC (mV)	HEX*	__	__	(mV)	HEX**	DES VCOM Voltage (V)
-30	113.3	13.2	0x02	VTEMPL	VCOML	90	0x0D	-1.09
25	10.0	145	0x1D	VTEMP25	VCOM25	0	__	- 1.0
60	3.0	448	0x5B	VTEMP_H1	VCOM_H1	20	0x03	-0.98
85	1.5	881	0xAC	VTEMP_H2	VCOM_H2	70	0x0A	-0.91

Table 8. VCOM Registers Setting Example.
Register	Field	Setting	Notes
DELAYVCOM_LSB[7:0]	vcom25_0	0	9-bit value is 011011010 or 0xDA which corresponds to -1V
VCOM25	vcom25[7:0]	0x6D	9-bit value is 011011010 or 0xDA which corresponds to -1V
VCOM_L	vcom_l[4:0]	0x0D	Represents shift of -89mV from VCOM25
VCOM_H1	vcom_h1[4:0]	0x03	Represents shift of +20mV from VCOM25
VCOM_H2	vcom_h2[4:0]	0x0A	Represents shift of +68mV from VCOM_H1
VTEMP25	vtemp25[7:0]	0x1D	ADC result at +25°C
VTEMP_L	vtemp_l[7:0]	0x02	ADC result at -30°C
VTEMP_H1	vtemp_h1[7:0]	0x5B	ADC result at +60°C
VTEMP_H2	vtemp_h2[7:0]	0xAC	ADC result at +85°C

With these settings, the VCOM output voltage is -1V at +25°C. However, if the temperature changes and the value at the TEMP voltage adjusts to 13.2mV, the VCOM voltage would decrease to -1.09V according to the setting in the VCOM_L register. In an analogous fashion, the VCOM_H1 and VCOM_H2 values are output on VCOM when the TEMP voltage is 448mV and 881mV, respectively. Between these values, the device interpolates the correct VCOM voltage value with a resolution of 6.83mV. The complete curve is shown in Figure 11.

Figure 11. Temperature compensation curve example value.

Conclusion

The VCOM buffer provides a high-accuracy voltage in a range between -2.55V and +1V, in 6.83mV steps (9 bits). It is programmed using a 9-bit word to set a value, which can be stored in nonvolatile memory. This functionality guarantees a reference backplane voltage for TFT LCDs that can be tuned for different panels, helping to reduce display aberrations. Further advantages include the use of minimal external components with good configurability to guarantee a low-cost solution that has both high quality and high reliability. In addition, the thermal compensation functionality guarantees low drift over temperature to meet customers' requirements.