CAT5271ZI-00-GT3 ONSEMI [ON Semiconductor], CAT5271ZI-00-GT3 Datasheet - Page 7

• Manufacturer Part Number: CAT5271ZI-00-GT3
• Description: Dual 256-Position I2C Compatible Digital Potentiometer
• Manufacturer: ONSEMI [ON Semiconductor]

Basic Operation
potentiometer. When power is first applied, the wipers
assume a mid−scale position. Once the power supply is
stable, the wipers may be repositioned via the I
compatible interface.
Programming: Variable Resistor
Rheostat Mode
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.
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
wiper output with its respective contact point. In CAT5271
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.
point, corresponds to 441 W (R
+ 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.
The CAT5271 is a dual 256−position digitally controlled
The resistance between terminals A and B, R
The wiper’s first connection is at the B terminal,
For a 100 kW device, the second connection, or the first tap
Figure 11. CAT5271 Equivalent DPP Circuit
Register
Decoder
Wiper
and
ON
of the FET switch connecting the
R
R
R
R
S
S
S
S
WB
= R
AB
/256 + R
A
W
B
W
AB
= 390.6
http://onsemi.com
, has a
2
C
7
output resistance between W and B is
where D is the decimal equivalent of the binary code loaded
in the 8−bit Wiper register, R
resistance, and R
on resistance of the internal switch.
circuited, the following output resistance R
the indicated Wiper register codes:
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.
the DPP (Digitally Programmed Potentiometer) between the
wiper W and terminal A also produces a digitally controlled
complementary resistance R
used, the B terminal can be opened. Setting the resistance
value for R
decreases as the data loaded in the latch increases in value.
The general equation for this operation is
following output resistance R
Wiper register codes.
dependent and may vary by up to ±20%.
Table 9. CODES AND CORRESPONDING R
RESISTANCE FOR R
Table 10. CODES AND CORRESPONDING R
RESISTANCE FOR R
D (Dec.)
The equation for determining the digitally programmed
In summary, if R
Be aware that in the zero−scale position, the wiper
Similar to the mechanical potentiometer, the resistance of
For R
Typical device to device resistance matching is lot
D (Dec.)
255
128
255
128
1
0
1
0
AB
WA
= 100 kW and the B terminal open circuited, the
R
R
100,050
50,050
99,659
starts at a maximum value of resistance and
WA
WA
R
441
W
99,559
50,050
WB
R
441
(D) + 256 * D
50
(W)
is the wiper resistance contributed by the
AB
WB
(W)
= 100 kW and the A terminal is open
+ D
AB
AB
256
Full Scale
Midscale
1 LSB
Zero Scale
= 100 kW, V
= 100 kW, V
256
Full Scale (R
Midscale
1 LSB
Zero Scale
(Wiper Contact Resistance)
WA
WA
R
AB
. When these terminals are
will be set for the indicated
R
) R
AB
Output State
AB
Output State
DD
DD
) R
is the end−to−end
W
AB
= 5 V
= 5 V
WB
– 1 LSB + R
W
will be set for
WB
WA
(eq. 1)
(eq. 2)
W
)