isl24201 Intersil Corporation, isl24201 Datasheet - Page 6

• Manufacturer Part Number: isl24201
• Description: Programmable Vcom Calibrator With Eeprom
• Manufacturer: Intersil Corporation

DCP (Digitally Controlled Potentiometer)
Figure 4 shows the relationship between the register value and
the resistor string of the DCP. Note that the register value of zero
actually selects the first step of the resistor string. The output
voltage of the DCP is given by Equation 1:
Output Current Sink
Figure 5 shows the schematic of the OUT pin current sink. The
circuit made up of amplifier A1, transistor Q1, and resistor R
forms a voltage controlled current source.
The external R
determines the lowest output voltage of the external voltage divider
R
V
I
OUT
DCP
1
and R
FIGURE 4. SIMPLIFIED SCHEMATIC OF DIGITAL CONTROL
A
=
=
VDD
V
-------------
R
⎛
⎝
DCP
2
SET
RegisterValue
-------------------------------------------------- -
. I
V
OUT
DCP
A
=
SET
20
POTENTIOMETER (DCP)
VDD
A1
⎛
⎝
FIGURE 5. CURRENT SINK CIRCUIT
is calculated as shown by Equation 2:
256
RegisterValue
-------------------------------------------------- -
resistor sets the full-scale sink current that
R
19R
256
+
Q1
A
1
VDD
⎞
⎠
⎛
⎝
V
A
------------- -
SAT
VDD
20
6
+
I
OUT
1
255
254
253
252
251
⎞
⎠
SET
2
⎞
⎠
OUT
1
0
⎛
⎝
A
------------- -
20
VDD
REGISTER
R
V
SET
VALUE
SET
⎞
⎠
A
= (I
⎛
⎝
VDD
------------ -
R
R
R
SET
1
2
1
OUT
V
V
DCP
)*(R
⎞
⎠
OUT
SET
) = V
ISL24201
DCP
(EQ. 2)
(EQ. 1)
SET
The maximum value of I
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 by removing the Register Value term from it as shown in
Equation 4.
The voltage difference between the OUT pin and SET pin, which are
also the drain and source of the output transistor, should be greater
than the minimum saturation voltage for the I
This will keep the output transistor in its saturation region to
maintain linear operation over the full range of register values.
Figure 6 shows I
"Minimum Saturation Voltage" is the minimum voltage that should
be maintained across the drain and source of Q1. To find the
minimum saturation voltage for a specific condition, locate the
voltage at the intersection of the I
and the line labeled "Minimum Saturation Voltage".
I
I
STEP
OUT
FIGURE 6. I
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
(
MAX
0
=
0
--------------------------------------------- -
(
MINIMUM SATURATION
VOLTAGE
256
)
=
1
) 20
DS
------------------- -
20R
A
(
A
VDD
DS
VDD
vs V
) R
SET
2
(
vs V
SET
DS
DS
3
FOR THE ISL24201 OUTPUT TRANSISTOR
OUT
)
for transistor Q1. The line labeled
can be calculated by substituting the
4
VDS (V)
OUT(MAX)
SATURATION REGION
5
6
value from Equation 3
OUT(MAX)
7
December 9, 2010
8
being used.
9
FN7586.1
(EQ. 3)
(EQ. 4)
10