Datasheet

LTC2605/LTC2615/LTC2625
13
2605fa
OPERATION
The format of the three data bytes is shown in Figure 2.
The fi rst byte of the input word consists of the 4-bit com-
mand and 4-bit DAC address. The next two bytes consist
of the 16-bit data word. The 16-bit data word consists of
the 16-, 14- or 12-bit input code, MSB to LSB, followed by
0, 2 or 4 don’t care bits (LTC2605, LTC2615 and LTC2625
respectively). A typical I
2
C write transaction is shown in
Figure 3.
The command (C3-C0) and address (A3-A0) assignments
are shown in Table 1. The fi rst four commands in the table
consist of write and update operations. A write operation
loads the 16-bit data word from the 32-bit shift register
into the input register of the selected DAC, n. An update
operation copies the data word from the input register to
the DAC register. Once copied into the DAC register, the
data word becomes the active 16-, 14- or 12-bit input
code, and is converted to an analog voltage at the DAC
output. The update operation also powers up the selected
DAC if it had been in power-down mode. The data path
and registers are shown in the Block Diagram.
Power-Down Mode
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 amplifi ers and reference inputs 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 90k resistors. When
all eight DACs are powered down, the bias generation
circuit is also disabled. Input and DAC registers are not
disturbed during power down.
Any channel or combination of channels can be put into
power-down mode by using command 0100
b
in combi-
nation with the appropriate DAC address, (n). The 16-bit
data word is ignored. The supply and reference currents
are reduced by approximately 1/8 for each DAC powered
down; the effective resistance at REF (Pin 6) rises accord-
ingly, becoming a high impedance input (typically >1GΩ)
when all eight DACs are powered down.
Normal operation can be resumed by executing any com-
mand which includes a DAC update, as shown in Table 1.
The selected DAC is powered up as its voltage output is
updated.
There is an initial delay as the DAC powers up before it
begins its usual settling behavior. If less than eight DACs
are in a powered-down state prior to the updated com-
mand, the power-up delay is 5μs. If, on the other hand,
all eight DACs are powered down, then the bias genera-
tion circuit is also disabled and must be restarted. In this
case, the power-up delay is greater: 12μs for V
CC
= 5V,
30μs for V
CC
= 3V.
S
INPUT WORD
WRITE WORD PROTOCOL FOR LTC2605/LTC2615/LTC2625
INPUT WORD (LTC2605)
SLAVE ADDRESS
W
A
A1ST DATA BYTE
2ND DATA BYTE A
3RD DATA BYTE
AP
2605 F02
1ST DATA BYTE 2ND DATA BYTE 3RD DATA BYTE
1ST DATA BYTE 2ND DATA BYTE 3RD DATA BYTE
1ST DATA BYTE 2ND DATA BYTE 3RD DATA BYTE
C3
C2
C1
C0
A3
A2
A1
A0
D13D14D15
D12
D11 D10 D9 D8
D7
D6
D5 D4 D3 D2 D1
D0
INPUT WORD (LTC2615)
C3
C2
C1
C0
A3
A2
A1
A0
D11D12D13
D10
D9 D8 D7 D6
D5
D4
D3 D2 D1 D0 X
X
INPUT WORD (LTC2625)
C3
C2
C1
C0
A3
A2
A1
A0
D9D10D11
D8
D7 D6 D5 D4
D3
D2
D1 D0 X X X
X
Figure 2