MCP40D17 Microchip Technology Inc., MCP40D17 Datasheet - Page 49

• Manufacturer Part Number: MCP40D17
• Description: 7-bit Single I 2 C? With Command Code Digital Pot With Volatile Memory In Sc70
• Manufacturer: Microchip Technology Inc.

8.0
Digital potentiometers have a multitude of practical
uses in modern electronic circuits. The most popular
uses include precision calibration of set point
thresholds, sensor trimming, LCD bias trimming, audio
attenuation, adjustable power supplies, motor control
overcurrent trip setting, adjustable gain amplifiers and
offset trimming. The MCP40D17/18/19 devices can be
used to replace the common mechanical trim pot in
applications where the operating and terminal voltages
are within CMOS process limitations (V
5.5V).
8.1
Applications that need accurate detection of an input
threshold event often need several sources of error
eliminated. Use of comparators and operational
amplifiers (op amps) with low offset and gain error can
help achieve the desired accuracy, but in many
applications, the input source variation is beyond the
designer’s control. If the entire system can be
calibrated after assembly in a controlled environment
(like factory test), these sources of error are minimized
if not entirely eliminated.
Figure 8-1
configuration. This configuration is often referred to as
a “windowed voltage divider”. Note that R
necessary to create the voltage divider, but its
presence is useful when the desired threshold has
limited range. It is “windowed” because R
the adjustable range of V
V
magnitude of each output step is reduced. This
effectively increases the trimming resolution for a fixed
digital potentiometer resolution. This technique may
allow a lower-cost digital potentiometer to be utilized
(64 steps instead of 256 steps).
The MCP40D18’s low DNL performance is critical to
meeting calibration accuracy in production without
having to use a higher precision digital potentiometer.
EQUATION 8-1:
© 2009 Microchip Technology Inc.
DD
D = Digital Potentiometer Wiper Setting (0-127)
– V
V
D =
TRIP
R
R
APPLICATIONS EXAMPLES
Set Point Threshold Trimming
SS
WB
AB
illustrates a common digital potentiometer
. If the output range is reduced, the
=
= R
= R
V
V
V
Nominal
AB
TRIP
DD
DD
•
⎛
⎝
------------------ -
R
127
CALCULATING THE
WIPER SETTING FROM
THE DESIRED V
1
R
D
• (R
+
WB
TRIP
R
2
1
⎞
⎠
to a value much less than
+ R
AB
)
•
DD
1
127
TRIP
can narrow
= 2.7V to
1
is not
FIGURE 8-1:
Potentiometer to Set a Precise Output Voltage.
8.1.1
If the application has to calibrate the threshold of a
diode, transistor or resistor, a variation range of 0.1V is
common. Often, the desired resolution of 2 mV or
better is adequate to accurately detect the presence of
a precise signal. A “windowed” voltage divider, utilizing
the MCP40D18, would be a potential solution.
Figure 8-2
FIGURE 8-2:
Calibration.
MCP40D18
SDA
SCL
SDA
SCL
MCP40D17/18/19
R
illustrates this example application.
TRIMMING A THRESHOLD FOR AN
OPTICAL SENSOR
1
V
MCP40D18
DD
B
A
W
0.1 µF
V
DD
Using the Digital
Set Point or Threshold
R
V
DD
sense
V
A
B
R
TRIP
1
W
Comparator
DS22152B-page 49
V
V
CC+
V
CC-
OUT
MCP6021