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ISL24202 Folha de dados(PDF) 7 Page - Intersil Corporation |
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ISL24202 Folha de dados(HTML) 7 Page - Intersil Corporation |
7 / 12 page ISL24202 7 FN7587.0 March 15, 2011 Determination of RSET The ultimate goal for the ISL24202 is to generate an adjustable voltage between two endpoints, VCOM_MIN and VCOM_MAX, with a fixed power supply voltage, AVDD. This is accomplished by choosing the correct values for RSET, R1 and R2. The exact value of RSET is not critical. Values from 1k to more than 100k will work under most conditions. The following expression calculates the minimum RSET value: Note that this is the absolute minimum value for RSET. Larger RSET values reduce quiescent power, since R1 and R2 are proportional to RSET. The ISL24202 is tested with a 5kΩ RSET. Determination of R1 and R2 With AVDD, VCOM(MIN) and VCOM(MAX) known and RSET chosen per the above requirements, R1 and R2 can be determined using Equations 6 and 7: Final Transfer Function The voltage at the OUT pin can be calculated from Equation 8: With external amplifier A2 in the unity-gain configuration, VOUT =VCOM. Example As an example, suppose the AVDD supply is 15V, the desired VCOM_MIN = 6.5V and the desired VCOM_MAX = 8.5V. RSET is arbitrarily chosen to be 7.5kΩ. First, verify that our chosen RSET meets the minimum requirement described in Equation 5: Using Equations 6 and 7, calculate the values of R1 and R2: Table 1 shows the resulting VCOM voltage as a function of register value for these conditions. Output Voltage Span Calculation It is also possible to calculate VCOM(MIN) and VCOM(MAX) from the existing resistor values. VCOM_MIN occurs when the greatest current, IOUT(MAX), is drawn from the middle node of the R1/R2 divider. Substituting RegisterValue = 255 into Equation 8 gives the following: Similarly, RegisterValue = 0 for VCOM(MAX): By finding the difference of Equation 13 and Equation 12, the total span of VCOM can be found: RSET MIN () AVDD 16 -------------- VOUT MIN () AVDD 20 -------------- – ⎝⎠ ⎛⎞ ------------------------------------------------------ ⎝⎠ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎛⎞ k Ω () = (EQ. 5) R1 5120 RSET VCOM MAX () VCOM MIN () – 256 VCOM MAX () ⋅ VCOM MIN () – --------------------------------------------------------------------------------- ⎝⎠ ⎜⎟ ⎛⎞ ⋅ = (EQ. 6) R2 5120 RSET VCOM MAX () VCOM MIN () – 255 AVDD ⋅ VCOM MIN () 256 VCOM MAX () ⋅ – + --------------------------------------------------------------------------------------------------------------------- ⎝⎠ ⎜⎟ ⎛⎞ ⋅ = (EQ. 7) VOUT AVDD R2 R1 R2 + -------------------- ⎝⎠ ⎜⎟ ⎛⎞ 1 RegisterValue 1 + 256 --------------------------------------------------- R1 20RSET -------------------- ⎝⎠ ⎜⎟ ⎛⎞ – ⎝⎠ ⎜⎟ ⎛⎞ = (EQ. 8) 7.5k Ω () RSET MIN () 15 16 ------- 6.5V 15 20 ------- – ⎝⎠ ⎛⎞ ------------------------------ ⎝⎠ ⎜⎟ ⎜⎟ ⎜⎟ ⎛⎞ 0.163k Ω == ⎝⎠ ⎜⎟ ⎜⎟ ⎜⎟ ⎛⎞ > (EQ. 9) TABLE 1. EXAMPLE VOUT vs REGISTER VALUE REGISTER VALUE VOUT (V) 08.49 20 8.34 40 8.18 60 8.02 80 7.87 100 7.71 120 7.55 127 7.50 140 7.40 160 7.24 180 7.09 200 6.93 220 6.77 240 6.62 255 6.50 R1 5120 7500 8.5 6.5 – 256 8.5 ⋅ 6.5 – -------------------------------------- ⎝⎠ ⎛⎞ ⋅⋅ 35.4k Ω == (EQ. 10) R2 5120 7500 8.5 6.5 – 255 15 ⋅ 6.5 256 8.5 ⋅ – + ------------------------------------------------------------------ ⎝⎠ ⎛⎞ ⋅⋅ 46.4k Ω == (EQ. 11) VCOM MIN () AVDD R2 R1 R2 + -------------------- ⎝⎠ ⎜⎟ ⎛⎞ 1 R1 20RSET -------------------- ⎝⎠ ⎜⎟ ⎛⎞ – ⎝⎠ ⎜⎟ ⎛⎞ = (EQ. 12) VCOM MAX () AVDD R2 R1 R2 + -------------------- ⎝⎠ ⎜⎟ ⎛⎞ 1 1 256 ---------- R1 20RSET -------------------- ⎝⎠ ⎜⎟ ⎛⎞ – ⎝⎠ ⎜⎟ ⎛⎞ = (EQ. 13) VCOMSPAN AVDD R2 R1 R2 + -------------------- ⎝⎠ ⎜⎟ ⎛⎞ 1 1 256 ---------- – ⎝⎠ ⎛⎞ R1 20RSET -------------------- ⎝⎠ ⎜⎟ ⎛⎞ = (EQ. 14) |
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