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ISL24211IRTZ-EVALZ Datasheet(PDF) 6 Page - Intersil Corporation |
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ISL24211IRTZ-EVALZ Datasheet(HTML) 6 Page - Intersil Corporation |
6 / 12 page ISL24211 6 FN7585.0 February 23, 2011 Application Information LCD panels have a VCOM (common voltage) that must be precisely set to minimize flicker. Figure 3 shows a typical VCOM adjustment circuit using a mechanical potentiometer, and the equivalent circuit replacement using the ISL24211. Having a digital I2C interface enables automatic, digital flicker minimization during production test and alignment. After programming, the I2C interface has no further use therefore, the ISL24211 automatically powers up with the correct VCOM voltage programmed previously. The ISL24211 uses a digitally controllable potentiometer (DCP), with 256 steps of resolution (see Figure 4) to change the current drawn at the DVR_OUT pin, which then changes the voltage created by the R1 to R2 resistor divider (see Figure 5). The DVR_OUT voltage is then buffered by A2 to generate a buffered output voltage at the VCOM_OUT pin, capable of directly driving the VCOM input of an LCD panel. The amount of current sunk is controlled by the setting of the DCP, which is recalled at power-up from the ISL24211’s internal EEPROM. The EEPROM is typically programmed during panel manufacture. As noted in the Electrical Specifications on page 4, the ISL24211 requires a minimum AVDD voltage of 10.8V for EEPROM programming, but will work in normal operation (with no EEPROM programming) down to 4.5V. DCP (Digitally Controllable Potentiometer) The DCP controls the voltage that ultimately controls the SET current. Figure 4 shows the relationship between the register value and the DCP’s tap position. Note that a register value of 0 selects the first step of the resistor string. The output voltage of the DCP is given in Equation 1: Output Current Sink Figure 5 shows the schematic of the DVR_OUT current sink. The combination of amplifier A1, transistor Q1, and resistor RSET forms a voltage-controlled current source, with the voltage determined by the DCP setting. The external RSET resistor sets the full-scale (maximum) sink current that can be pulled from the DVR_OUT node. The relationship between IDVR_OUT and Register Value is shown in Equation 2. FIGURE 3. MECHANICAL ADJUSTMENT REPLACEMENT DVR_OUT SET ISL24211 R1 R2 AVDD AVDD VCOM IOUT VDD RSET RA RC AVDD VCOM RB R1 = RA R2 = RB+RC RSET = RARB + RARC 20RB INN VCOM_OUT VDCP RegisterValue 1 + 256 --------------------------------------------------- ⎝⎠ ⎛⎞ AVDD 20 -------------- ⎝⎠ ⎛⎞ = (EQ. 1) FIGURE 4. SIMPLIFIED SCHEMATIC OF DCP AVDD 19R R 0 1 2 255 254 253 252 251 REGISTER VALUE AVDD 20 VDCP FIGURE 5. CURRENT SINK CIRCUIT AVDD RSET VDCP SET DVR_OUT AVDD IOUT R1 R2 VSAT VSET = VDCP = IOUT * RSET Q1 A1 VOUT VCOM_OUT INN GND A2 IDVR_OUT IDVROUT VDCP RSET ------------- RegisterValue 1 + 256 --------------------------------------------------- ⎝⎠ ⎛⎞ AVDD 20 -------------- ⎝⎠ ⎛⎞ 1 RSET ------------- ⎝⎠ ⎛⎞ == (EQ. 2) |
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