Datasheet

AD5246 Data Sheet
Rev. C | Page 14 of 16
A repeated write function gives the user flexibility to update the
RDAC output a number of times after addressing the part only
once. For example, after the RDAC has acknowledged its slave
address in write mode, the RDAC output updates on each succes-
sive byte. If different instructions are needed, the write/read mode
has to start again with a new slave address and data byte.
Similarly, a repeated read function of the RDAC is also allowed.
LEVEL SHIFTING FOR BIDIRECTIONAL INTERFACE
While most legacy systems may be operated at one voltage,
a new component may be optimized at another. When two
systems operate the same signal at two different voltages, proper
level shifting is needed. For instance, one can use a 1.8 V
E
2
PROM to interface with a 5 V digital potentiometer. A level
shifting scheme is needed to enable a bidirectional communi-
cation so that the setting of the digital potentiometer can be
stored to and retrieved from the E
2
PROM. Figure 30 shows
one of the implementations. M1 and M2 can be any N channel
signal FETs, or if V
DD
falls below 2.5 V, M1 and M2 can be low
threshold FETs such as the FDV301N.
E
2
PROM
AD5246
SDA1
SCL1
D
G
R
P
R
P
1.8V
5V
S
M1
SCL2
SDA2
R
P
R
P
G
S
M2
V
DD1
= 1.8V V
DD2
=
5V
D
03875-011
Figure 30. Level Shifting for Operation at Different Potentials
ESD PROTECTION
All digital inputs are protected with a series input resistor
and parallel Zener ESD structures, as shown in Figure 31.
This applies to the digital input pins SDA and SCL.
LOGIC
340
Ω
GND
03875-002
Figure 31. ESD Protection of Digital Pins
TERMINAL VOLTAGE OPERATING RANGE
The AD5246 V
DD
and GND power supply defines the boundary
conditions for proper 3-terminal digital potentiometer
operation. Supply signals present on Terminal B and
Terminal W that exceed V
DD
or GND are clamped by
the internal forward biased diodes (see Figure 32).
B
V
DD
W
GND
03875-016
Figure 32. Maximum Terminal Voltages Set by V
DD
and GND
MAXIMUM OPERATING CURRENT
At low code values, the user should be aware that due to low
resistance values, the current through the RDAC may exceed
the 5 mA limit. In Figure 33, a 5 V supply is placed on the
wiper, and the current through Terminal W and Terminal B is
plotted with respect to code. A line is also drawn denoting the
5 mA current limit. Note that at low code values (particularly
for the 5 kΩ and 10 kΩ options), the current level increases
significantly. Care should be taken to limit the current flow
between W and B in this state to a maximum continuous
current of 5 mA and a maximum pulse current of no more than
20 mA. Otherwise, degradation or possible destruction of the
internal switch contacts can occur.
CODE (Decimal)
IWB CURRENT (mA)
0
0.01
0.1
1
10
16 32 48
64 80 96 112 128
100
5mA CURRENT LIMIT
R
AB
= 5kΩ
R
AB
= 10kΩ
R
AB
= 100kΩ
R
AB
= 50kΩ
03875-034
Figure 33. Maximum Operating Current
POWER-UP SEQUENCE
Since the ESD protection diodes limit the voltage compliance
at Terminal B and Terminal W (see Figure 32), it is important
to power V
DD
/GND before applying any voltage to Terminal B
and Terminal W; otherwise, the diode is forward biased such
that V
DD
is powered unintentionally and may affect the rest of
the user’s circuit. The ideal power-up sequence is in the follow-
ing order: GND, V
DD
, digital inputs, and then V
B
/V
W
. The
relative order of powering V
B
and V
W
and the digital inputs
is not important, providing they are powered after V
DD
/GND.