SLAAE46 November 2021 DAC43701

1 Introduction

LCD panels are widely used in notebooks, TV, automotive display and so on. A typical LCD panel subsystem diagram block is shown in Figure 1-1. VCOM is the reference voltage for each pixel in the LCD panel. VCOM is typically used to maximize contrast and image quality during operation. A VCOM with different amplitudes and frequencies are needed to achieve different LCD electrical characteristics, optical characteristics and functional requirements.

Figure 1-1 Typical LCD Panel Subsystem Diagram Block

Figure 1-2 shows the LCD digital VCOM circuit using the DAC43701. It consists primarily of a DAC43701 and inverting summer circuit. The DAC output can be dynamically adjusted by I2C, and the inverting summer circuit outputs the required VCOM with specific gain and offset.

Figure 1-2 LCD Digital VCOM Using the DAC43701

The DAC43701 uses the power supply (VDD) as a reference by default. It also contains an internal reference that can be enabled by writing a 1 to the REF_EN bit in the GENERAL_CONFIG register (address D1h). The DAC transfer function when VDD is used as reference is given in Equation 1.

Equation 1.

V_{D A C_O U T} = \frac{D A C_D A T A}{2^{N}} \times V_{D D}

Where, N is the DAC resolution, either 8-bits (DAC43701) or 10-bits (DAC53701). DAC_DATA is the decimal equivalent of the data in the DAC_DATA register (address 21h), ranges from 0 to 2^N-1.

The transfer function of the inverting summer circuit is given in Equation 2.

Equation 2.

V_{C O M} = - \frac{R_{3}}{R_{1}} \times V_{D A C_O U T} - \frac{R_{3}}{R_{2}} \times V_{B I A S} = - \frac{R_{3}}{R_{1}} \times V_{D A C_O U T} - \frac{R_{3}}{R_{2}} \times (\frac{R_{5}}{R_{4} + R_{5}} \times V_{E E})

Assuming R₁ and R₂ are equal, Equation 1 and Equation 2 became Equation 3

Equation 3.

V_{C O M} = - \frac{R_{3}}{R_{1}} \times (\frac{D A C_D A T A}{2^{N}} \times V_{D D} + V_{B I A S})

Thus, different amplitudes and frequencies of VCOM can be controlled by a dedicated DAC_DATA value, which is configured in real time by I2C. Figure 1-3 shows the VCOM voltage characteristic when VDD is 5 V.

Figure 1-3 LCD Digital VCOM Voltage Characteristic