Datasheet
LTC2631
22
2631fc
For more information www.linear.com/LTC2631
operation
Table 3. Command Codes
COMMAND*
C3 C2 C1 C0
0 0 0 0 Write to Input Register
0 0 0 1 Update (Power Up) DAC Register
0 0 1 1 Write to and Update (Power Up) DAC Register
0 1 0 0 Power Down
0 1 1 0 Select Internal Reference
0 1 1 1 Select External Reference
*Command codes not shown are reserved and should not be used.
Reference Modes
For applications where an accurate external reference is not
available, the LTC2631 has a user-selectable, integrated
reference. The LTC2631-LM/LTC2631-LZ provide a full-
scale output of 2.5V. The LTC2631-HM/LTC2631-HZ
provide a full-scale output of 4.096V. The internal reference
can be useful in applications where the supply voltage is
poorly regulated. Internal Reference mode can be selected
by using command 0110, and is the power-on default for
LTC2631-HZ/LTC2631-LZ, as well as for LTC2631-HM/
LTC2631-LM when REF_SEL is tied high.
The 10ppm/°C, 1.25V (LTC2631-LM/LTC2631-LZ) or
2.048V (LTC2631-HM/LTC2631-HZ) internal reference
is available at the REF pin. Adding bypass capacitance
to the REF pin will improve noise performance; 0.33µF
is recommended, and up to 10µF can be driven without
oscillation. This output must
be buffered when driving
external DC load current.
Alternatively, the
DAC can operate in External Reference
mode using command 0111. In this mode, an input voltage
supplied externally to the REF pin provides the reference
(0V ≤
V
REF
≤ V
CC
) and the supply current is reduced. External
Reference mode is the power-on default for LTC2631-HM/
LTC2631-LM when REF_SEL is tied low.
The reference mode of LTC2631-HZ/LTC2631-LZ can be
changed only by software command. The same is true for
LTC2631-HM/LTC2631-LM after power-on, after which
the logic state on REF_SEL is ignored.
Power-Down Mode
For power-constrained applications, the LTC2631’s power-
down mode can be used to reduce the supply current
whenever the DAC output is not needed. When in power
down, the buffer amplifier, bias circuit, and reference
circuit are disabled and draw essentially zero current. The
DAC output is put into a high-impedance state, and the
output pin is passively pulled to ground through a 200kΩ
resistor. Input and DAC register contents are not disturbed
during power down.
Figure 3. Command and Data Input Format
C3
1ST DATA BYTE
Input Word (LTC2631-12)
Write Word Protocol for LTC2631
C2
C1
C0
X
X
X
X
D9D10D11
S
W A
SLAVE ADDRESS
1ST DATA BYTE
D8
D7 D6 D5 D4
D3
D2
D1 D0 X X X
X
A 2ND DATA BYTE A 3RD DATA BYTE A P
2631 F03
2ND DATA BYTE
INPUT WORD
3RD DATA BYTE
C3
1ST DATA BYTE
Input Word (LTC2631-10)
C2
C1
C0
X
X
X
X
D7D8D9
D6
D5 D4 D3 D2
D1
D0
X X X X X
X
2ND DATA BYTE 3RD DATA BYTE
C3
1ST DATA BYTE
Input Word (LTC2631-8)
C2
C1
C0
X
X
X
X
D5D6D7
D4
D3 D2 D1 D0
X
X
X X X X X
X
2ND DATA BYTE 3RD DATA BYTE