
6
FN6072.9
April 13, 2011
Expected Output Voltage
The ISL45042 provides an output sink current, which lowers
the voltage on the external voltage divider (VCOM output
voltage). Equations
1 and
2 can be used to calculate the
output current (IOUT) and output voltage (VOUT) values.
(Where “Setting” is an integer between 1 and 128.)
Table 1 gives the calculated value of VOUT for resistors values of: RSET = 24.9kΩ, R1 = 200kΩ, R2 = 243kΩ, and
AVDD = 10V.
RSET Resistor
The external RSET resistor sets the full-scale sink current that
determines the lowest voltage of the external voltage divider R1
and R2 (see Figure 1). The voltage difference between the VOUT pin and ISET pin (see Figure 5) has to be greater than 1.75V. This will keep the output MOS transistor in the saturation
region. Expected current settings and 7-bit accuracy occurs
when the output MOS transistor is operating in the saturation
region. Figure
5 shows the internal connection for the output
MOS transistor. The value of the AVDD supply sets the voltage
at the source of the output transistor. This voltage is equal to
(Setting/128) x (AVDD/20). The ISET current is therefore equal
to (Setting/128) x (AVDD/20 x RSET). The value of the Drain
voltage is found using Equation 2. The values of R1 and R2 in Equation
2, should be determined (setting equal to 128) so the
minimum value of VOUT is greater than 1.75V + AVDD/20.
TABLE 1. CALCULATED VCOM OUTPUT VOLTAGES
SETTING VALUE
VOUT
1
5.468
10
5.313
20
5.141
30
4.969
40
4.797
50
4.625
60
4.453
70
4.281
80
4.109
IOUT
Setting
128
---------------------
x
AVDD
20 RSET
()
---------------------------
=
VOUT
R2
R1 R2
+
---------------------
AVDD 1
Setting
128
---------------------
x
R1
20 RSET
()
---------------------------
–
=
(EQ. 1)
(EQ. 2)
FIGURE 3. EXTERNAL ESD PROTECTION ON CTL PIN
ISL45042
CTL
0.01F
1k
Ω
-
+
Ra
Rb
Rc
RSET
-
+
ISL45042
SET
OUT
AVDD
R1
R2
AVDD
VCOM
AVDD
R1 = Ra
R2 = Rb + Rc
RSET = (Ra(Rb + Rc)) / 20Rb
FIGURE 4. EXAMPLE OF THE REPLACEMENT FOR THE MECHANICAL POTENTIOMETER CIRCUIT USING THE ISL45042
90
3.936
100
3.764
110
3.592
128
3.282
TABLE 1. CALCULATED VCOM OUTPUT VOLTAGES
SETTING VALUE
VOUT
FIGURE 5. OUTPUT CONNECTION CIRCUIT EXAMPLE
AVDD = 15V
RSET
VOUT PIN
R1
R2
AVDD
VSAT
0.5V
ISET PIN
SETTING
128
----------------------------x
AVDD
20
------------------
ISL45042