ISL24201
7
FN7586.1
December 9, 2010
The maximum voltage on the SET pin is A
VDD
/20 and is added to
the minimum voltage difference between the V
OUT
and SET pins to
calculate the minimum V
OUT
voltage, as shown in Equation 5.
Output Voltage
The output voltage, V
OUT
, of the OUT pin can be calculated from
Equation 6:
While Equation 6 can be used to calculate the output voltage, it
does not help select the values of R
1
, R
2
and R
SET
to obtain a
specific range of V
COM
voltages.
Output Voltage Span Calculation
The span of the output voltage is typically centered around the
nominal V
COM
voltage value, which is typically near half of the
A
VDD
voltage. The high V
COM
voltage occurs with the register
value of zero, while the low V
COM
voltage occurs with the register
value of 255. Figure 7 shows the definition of several terms used
later in the text.
There are three variables that control the V
COM
calibrator’s
operating point; the span of the V
COM
voltage, the maximum
current sink and the source impedance of the resistive divider.
Figure 8 shows a range of operating points for these three variables
and a quick way to estimate a specific operating point. The X-axis is
the span of the V
COM
voltage (High V
COM
Voltage - Low V
COM
Voltage), and the Y-axis is the maximum sink current set by R
SET
.
The individual plots of each R
TH
show the V
COM
span plotted against
the maximum OUT sink current given that value of source
impedance of the voltage divider. R
TH
is the Thevenin equivalent
resistance of the voltage divider R
1
and R
2
, which is the resistance
of the parallel combination of R
1
and R
2
, as shown in Equation 7.
The span of the V
COM
voltage is shown by Equation 8.
To make a final selection of the resistor values for R
1
and R
2
, The
supply voltage A
VDD
and the value of R
SET
are specified. The
calculations for R
1
and R
2
are shown in Equations 9 and 10:
The R
1
and R
2
calculations are based on the span of the V
COM
voltage being centered at half the A
VDD
voltage.
As an example, A
VDD
= 15V, the maximum value for I
SET
is
selected to be 100µA and the required span is 2V. Using Figure 8
as a guide, the V
COM
maximum is equal to 8.5V and the V
COM
minimum is equal to 6.5V. Rearranging equation and calculation
the value of R
SET
:
Calculating the value of R
1
is shown in Equation 12.
Calculating the value of R
2
is shown in Equation 13.
V
OUT
MIN()
A
VDD
20
--------------
MinimumSaturationVoltage+≥
(EQ. 5
V
OUT
A
VDD
R
2
R
1
R
2
+
--------------------
⎝⎠
⎜⎟
⎛⎞
1
RegisterValue 1+
256
---------------------------------------------------
R
1
20R
SET
--------------------
⎝⎠
⎜⎟
⎛⎞
–
⎝⎠
⎜⎟
⎛⎞
=
(EQ. 6)
GND
A
VDD
NOMINAL V
COM
VOLTAGE
HIGH V
COM
VOLTAGE
LOW V
COM
VOLTAGE
SPAN
FIGURE 7. VOLTAGE LEVELS FOR V
COM
R
TH
R
1
R
2
R
1
R
2
+
--------------------
=
(EQ. 7
V
COM
SPAN I
SET
R
TH
()=
(EQ. 8
FIGURE 8. GRAPH of V
COM
SPAN vs MAXIMUM OUTCURRENT
AND R
TH
0.6
0.5
0.4
0.3
0.2
0.1
0
0123456
V
COM
SPAN (V)
OUT PIN MAXIMUM CURRENT (mA)
R
T
H
=
1
0
k
Ω
R
T
H
=
2
5
k
Ω
R
T
H
=
5
0
k
Ω
R
T
H
=
1
0
0
k
Ω
R
1
40R
SET
SPAN()
A
VDD
SPAN+
----------------------------------------- -
=
(EQ. 9
R
2
40R
SET
SPAN()
A
VDD
SPAN–
----------------------------------------- -
=
(EQ. 10)
R
SET
A
VDD
20I
OUT
MAX()
-------------------------------------
15
20 0.000100()
--------------------------------------
7500Ω== =
(EQ. 11)
R
1
40 7500()2()
15 2+
-----------------------------------
39.29kΩ==
(EQ. 12)
R
2
40 7500()2()
15 2–
-----------------------------------
46.15kΩ==
(EQ. 13)
