ISL24211
7
FN7585.0
February 23, 2011
The maximum value of I
DVR_OUT
can be calculated by
substituting the maximum register value of 255 into Equation 2,
resulting in Equation 3:
Equation 2 can also be used to calculate the unit sink current
step size per Register Code, resulting in Equation 4:
Determination of R
SET
The ultimate goal for the ISL24211 is to generate an adjustable
voltage between two endpoints, V
COM_MIN
and V
COM_MAX
, with
a fixed power supply voltage, AV
DD
. This is accomplished by
choosing the correct values for R
SET
, R
1
and R
2
. The exact value
of R
SET
is not critical. Values from 1k to more than 100k will
work under most conditions. The following expression calculates
the minimum R
SET
value:
Note that this is the absolute minimum value for R
SET
. Larger
R
SET
values reduce quiescent power, since R
1
and R
2
are
proportional to R
SET
. The ISL24211 is tested with a 5kΩ R
SET
.
Determination of R
1
and R
2
With AV
DD
, V
COM(MIN)
and V
COM(MAX)
known and R
SET
chosen
per the above requirements, R
1
and R
2
can be determined using
Equations 6 and 7:
Final Transfer Function
The voltage at DVR_OUT can be calculated from Equation 8:
With amplifier A2 in the unity-gain configuration (V
COM_OUT
tied
to IN
N
as shown in Figure 5), V
DVROUT
=V
COM_OUT
=V
COM
.
Example
As an example, suppose the A
VDD
supply is 15V, the desired
V
COM_MIN
= 6.5V and the desired V
COM_MAX
= 8.5V. R
SET
is
arbitrarily chosen to be 7.5kΩ.
First, verify that our chosen R
SET
meets the minimum
requirement described in Equation 5:
Using Equations 6 and 7, calculate the values of R
1
and R
2
:
Table 1 shows the resulting V
COM
voltage as a function of register
value for these conditions.
Output Voltage Span Calculation
It is also possible to calculate V
COM(MIN)
and V
COM(MAX)
from the
existing resistor values.
V
COM_MIN
occurs when the greatest current, I
DVR(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 V
COM(MAX)
:
I
DVROUT
MAX()
A
VDD
20R
SET
--------------------
=
(EQ. 3)
I
STEP
AV
DD
256()20()R
SET
()
----------------------------------------------
=
(EQ. 4)
R
SET
MIN()
AV
DD
16
--------------
V
OUT MIN()
AV
DD
20
--------------
–
⎝⎠
⎛⎞
------------------------------------------------------
⎝⎠
⎜⎟
⎜⎟
⎜⎟
⎜⎟
⎛⎞
kΩ()=
(EQ. 5)
R
1
5120 R
SET
V
COM MAX()
V
COM MIN()
–
256 V
COM MAX()
⋅ V
COM MIN()
–
-------------------------------------------------------------------------------- -
⎝⎠
⎜⎟
⎛⎞
⋅=
(EQ. 6)
R
2
5120 R
SET
V
COM MAX()
V
COM MIN()
–
255 AV
DD
⋅ V
COM MIN()
256 V
COM MAX()
⋅–+
---------------------------------------------------------------------------------------------------------------------
⎝⎠
⎜⎟
⎛⎞
⋅=
(EQ. 7)
V
DVROUT
AV
DD
R
2
R
1
R
2
+
--------------------
⎝⎠
⎜⎟
⎛⎞
1
RegisterValue 1+
256
---------------------------------------------------
R
1
20R
SET
--------------------
⎝⎠
⎜⎟
⎛⎞
–
⎝⎠
⎜⎟
⎛⎞
=
(EQ. 8)
TABLE 1. EXAMPLE V
DVR_OUT
vs REGISTER VALUE
REGISTER VALUE V
DVR_OUT
(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
7.5kΩ()R
SET
MIN()
15
16
------ -
6.5V
15
20
------ -
–
⎝⎠
⎛⎞
------------------------------
⎝⎠
⎜⎟
⎜⎟
⎜⎟
⎛⎞
0.163kΩ==
⎝⎠
⎜⎟
⎜⎟
⎜⎟
⎛⎞
>
(EQ. 9)
R
1
5120 7500
8.5 6.5–
256 8.5⋅ 6.5–
------------------------------------- -
⎝⎠
⎛⎞
⋅⋅ 35.4kΩ==
(EQ. 10)
R
2
5120 7500
8.5 6.5–
255 15⋅ 6.5 256 8.5⋅–+
------------------------------------------------------------------
⎝⎠
⎛⎞
⋅⋅ 46.4kΩ==
(EQ. 11)
V
COM MIN()
AV
DD
R
2
R
1
R
2
+
--------------------
⎝⎠
⎜⎟
⎛⎞
1
R
1
20R
SET
--------------------
⎝⎠
⎜⎟
⎛⎞
–
⎝⎠
⎜⎟
⎛⎞
=
(EQ. 12)
V
COM MAX()
AV
DD
R
2
R
1
R
2
+
--------------------
⎝⎠
⎜⎟
⎛⎞
1
1
256
----------
R
1
20R
SET
--------------------
⎝⎠
⎜⎟
⎛⎞
–
⎝⎠
⎜⎟
⎛⎞
=
(EQ. 13)
