LTC2657
OPERATION
Write Word Protocol
combination with the appropriate DAC address, (n). The
integrated reference is automatically powered down when
external reference mode is selected using command
0111b. In addition, all the DAC channels and the integrated
reference together can be put into power-down mode
using “Power-Down Chip” command 0101b. For all power-
down commands the 16-bit data word is ignored, but still
required in order to complete a full communication cycle.
The master initiates communication with the LTC2657
with a START condition and a 7-bit slave address followed
by the Write bit (W) = 0. The LTC2657 acknowledges by
pulling the SDA pin low at the 9th clock if the 7-bit slave
address matches the address of the part (set by CA0, CA1
and CA2) or the global address. The master then transmits
three bytes of write data. The LTC2657 acknowledges each
byte of data by pulling the SDA line low at the 9th clock of
eachdatabytetransmission.Afterreceivingthreecomplete
bytesofdata,theLTC2657executesthecommandspecified
in the 24-bit input word. If more than three data bytes are
transmitted after a valid 7-bit slave address, the LTC2657
does not acknowledge the extra bytes of data (SDA is
high during the 9th clock). The first byte of the input word
consists of the 4-bit command followed by 4-bit address.
The next two bytes consist of the 16-bit data word. The
16-bit data word consists of the 16- or 12-bit input code,
MSB to LSB, followed by 0 or 4 don’t care bits (LTC2657-
16 and LTC2657-12, respectively). A typical LTC2657 write
transaction is shown in Figure 2. The command (C3-C0)
and address (A3-A0) assignments are shown in Table 1.
The first four commands in the table consist of write and
update operations. A write operation loads a 16-bit data
word from the 32-bit shift register into the input register.
In an update operation, the data word is copied from the
inputregistertotheDACregisterandconvertedtoananalog
voltageattheDACoutput.Theupdateoperationalsopowers
up the DAC if it had been in power-down mode. The data
path and registers are shown in the Block Diagram.
Normal operation resumes by executing any command
whichincludesaDACupdate,insoftwareasshowninTable
1 or using the asynchronous LDAC pin. The selected DAC
is powered up as its voltage output is updated. When a
DAC which is in a powered-down state is powered up and
updated,normalsettlingisdelayed.IflessthaneightDACs
areinapowered-downstatepriortotheupdatecommand,
the power-up delay time is 12ꢁs. If on the other hand,
all eight DACs and the integrated reference are powered
down,thenthemainbiasgenerationcircuitblockhasbeen
automatically shut down in addition to the individual DAC
amplifiers and reference inputs. In this case, the power
up delay time is 14ꢁs. The power up of the integrated
referencedependsonthecommandthatpowereditdown.
Ifthereferenceispowereddownusingthe“SelectExternal
Reference”command(0111b),thenitcanonlybepowered
back up by sending “Select Internal Reference” command
(0110b). However if the reference was powered down by
sending “Power Down Chip” command (0101b), then in
additionto“SelectInternalReference”command(0110b),
any command that powers up the DACs will also power
up the integrated reference.
Power-Down Mode
Reference Modes
For power-constrained applications, power-down mode
can be used to reduce the supply current whenever less
than eight outputs are needed. When in power-down, the
buffer amplifiers, bias circuits and integrated reference
circuits are disabled, and draw essentially zero current.
The DAC outputs are put into a high-impedance state, and
the output pins are passively pulled to ground through
individual 80k resistors. Input- and DAC-register contents
are not disturbed during power-down.
For applications where an accurate external reference is
notavailable,theLTC2657hasauser-selectable,integrated
reference. The LTC2657-L has a 1.25V reference that
provides a full-scale output of 2.5V. The LTC2657-H has
a 2.048V reference that provides a full-scale output of
4.096V. Both references exhibit a typical temperature drift
of 2ppm/°C. Internal Reference mode can be selected
by using command 0110b, and is the power-on default.
A buffer is needed if the internal reference is required to
drive external circuitry. For reference stability and low
noise, it is recommended that a 0.1μF capacitor be tied
Any channel or combination of channels can be put
into power-down mode by using command 0100b in
between REFCOMP and GND. In this configuration, the
2657f
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