Datasheet
LTC2635
24
2635fb
operation
The 10ppm/°C, 1.25V (LTC2635-LMI/-LMX/-LMO/-LZ) or
2.048V (LTC2635-HMI/-HZ) internal reference is available
at the REF pin. Adding bypass capacitance to the REF pin
will improve noise performance; and up to 10µF can be
driven without oscillation. This output must be buffered
when driving an external DC load current.
Alternatively, the DAC can operate in External Reference
mode using command 0111b. In this mode, an input voltage
supplied externally to the REF pin provides the reference
(1V ≤ V
REF
≤ V
CC
) and the supply current is reduced. The
external reference voltage supplied sets the full-scale DAC
output voltage. External Reference mode is the power-on
default for LTC2635-LMX.
The reference mode of LTC2635-HZ/-LZ/-HMI/-LMI/-LMO
(Internal Reference power-on default), can be changed by
software command after power up. The same is true for
LTC2635-LMX (External Reference power-on default).
Power-Down Mode
For power-constrained applications, power-down mode can
be used to reduce the supply current whenever less than
four DAC 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 amplifier outputs are put into a high-impedance
state, and the output pins are passively pulled to ground
through individual 200k resistors (LTC2635-LMI/-LMX/
-LZ/-HMI/-HZ). For the LTC2635-LMO options, the out-
put pins are not passively pulled to ground, but are also
placed in a high-impedance state (open-circuited state)
during power-down, typically drawing less than 0.1µA.
The LTC2635-LMO options power-up with all DAC outputs
in this high-impedance state. They remain that way until
given a software or hardware update command. For all
LTC2635 options, input- and DAC-register contents are
not disturbed during power-down.
C3
Input Word (LTC2635-12)
1ST DATA BYTE 2ND DATA BYTE 3RD DATA BYTE
C2
C1
C0
A3
A2
A1
A0
D9D10D11
D8
D7 D6 D5 D4
D3 D2
D1 D0 X X X
X
1ST DATA BYTE 2ND DATA BYTE 3RD DATA BYTE
1ST DATA BYTE 2ND DATA BYTE 3RD DATA BYTE
S
Write Word Protocol for LTC2635
INPUT WORD
SLAVE ADDRESS
ACK ACK ACKW 1ST DATA BYTE 2ND DATA BYTE 3RD DATA BYTE
ACK P
C3
Input Word (LTC2635-10)
C2
C1
C0
A3
A2
A1
A0
D7D8D9
D6
D5 D4 D3 D2
D1
D0
X X X X X
X
C3
Input Word (LTC2635-8)
C2
C1
C0
A3
A2
A1
A0
D5D6D7
D4
D3 D2 D1 D0
X
X
X X X X X
X
2635 F03
Figure 3. Command and Data Input Format