CAT5171
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7
BASIC OPERATION
The CAT5171 is a 256-position digitally controlled
potentiometer. When power is first applied, the wiper
assumes a mid-scale position. Once the power supply is
stable, the wiper may be repositioned via the I
2
C compatible
interface.
PROGRAMMING: VARIABLE RESISTOR
Rheostat Mode
The resistance between terminals A and B, R
AB
, has a
nominal value of 50 kW or 100 kW and has 256 contact
points accessed by the wiper terminal, plus the B terminal
contact. Data in the 8-bit Wiper register is decoded to select
one of these 256 possible settings.
The wiper’s first connection is at the B terminal,
corresponding to control position 0x00. Ideally this would
present a 0 W between the Wiper and B, but just as with a
mechanical rheostat there is a small amount of contact
resistance to be considered, there is a wiper resistance
comprised of the R
ON
of the FET switch connecting the
wiper output with its respective contact point. In CAT5171
this ‘contact’ resistance is typically 50 W. Thus a connection
setting of 0x00 yields a minimum resistance of 50 W
between terminals W and B.
For a 100 kW device, the second connection, or the first tap
point, corresponds to 441 W (R
WB
= R
AB
/256 + R
W
= 390.6
+ 50 W) for data 0x01. The third connection is the next tap
point, is 831 W (2 x 390.6 + 50 W) for data 0x02, and so on.
Figure 11 shows a simplified equivalent circuit where the
last resistor string will not be accessed; therefore, there is
1 LSB less of the nominal resistance at full scale in addition
to the wiper resistance.
Figure 11. CAT5171 Equivalent Digital POT Circuit
R
S
Wiper
Register
and
Decoder
W
B
R
S
R
S
R
S
The equation for determining the digitally programmed
output resistance between W and B is
R
WB
+
D
256
R
AB
) R
W
(eq. 1)
where D is the decimal equivalent of the binary code loaded
in the 8-bit Wiper register, R
AB
is the end-to-end resistance,
and R
W
is the wiper resistance contributed by the on
resistance of the internal switch.
In summary, if R
AB
= 100 kW and the A terminal is open
circuited, the following output resistance R
WB
will be set for
the indicated Wiper register codes:
Table 9. CODES AND CORRESPONDING R
WB
RESISTANCE FOR R
AB
= 100 kW, V
DD
= 5 V
D (Dec.)
R
WB
(W)
Output State
255 99,559 Full Scale (R
AB
– 1 LSB + R
W
)
128 50,050 Midscale
1 441 1 LSB
0 50 Zero Scale
(Wiper Contact Resistance)
Be aware that in the zero-scale position, the wiper
resistance of 50 W is still present. Current flow between W
and B in this condition should be limited to a maximum
pulsed current of no more than 20 mA. Failure to heed this
restriction can cause degradation or possible destruction of
the internal switch contact.
Similar to the mechanical potentiometer, the resistance of
the digital POT between the wiper W and terminal A also
produces a digitally controlled complementary resistance
R
WA
. When these terminals are used, the B terminal can be
opened. Setting the resistance value for R
WA
starts at a
maximum value of resistance and decreases as the data
loaded in the latch increases in value. The general equation
for this operation is
R
WA
(D) +
256 * D
256
R
AB
) R
W
(eq. 2)
For R
AB
= 100 kW and the B terminal open circuited, the
following output resistance R
WA
will be set for the indicated
Wiper register codes.
Table 10. CODES AND CORRESPONDING R
WA
RESISTANCE FOR R
AB
= 100 kW, V
DD
= 5 V
D (Dec.)
R
WA
(W)
Output State
255 441 Full Scale
128 50,050 Midscale
1 99,659 1 LSB
0 100,050 Zero Scale
Typical device to device resistance matching is lot
dependent and may vary by up to ±20%.
