ltc2640-lz8 Linear Technology Corporation, ltc2640-lz8 Datasheet - Page 18

• Manufacturer Part Number: ltc2640-lz8
• Description: Single 12-/10-/8-bit Spi Vout Dacs With Bidirectional Reference
• Manufacturer: Linear Technology Corporation

LTC2640
OPERATION
Serial Interface
The CS/LD input is level triggered. When this input is
taken low, it acts as a chip-select signal, enabling the SDI
and SCK buffers and the input shift register. Data (SDI
input) is transferred at the next 24 rising SCK edges.
The 4-bit command, C3-C0, is loaded fi rst, followed by
4 don’t-care bits, and fi nally the 16-bit data word. The
data word comprises the 12-, 10- or 8-bit input code,
ordered MSB-to-LSB, followed by 4, 6 or 8 don’t-care bits
(LTC2640-12, LTC2640-10 and LTC2640-8 respectively;
see Figure 2). Data can only be transferred to the device
when the CS/LD signal is low, beginning on the fi rst rising
edge of SCK. SCK may be high or low at the falling edge
of CS/LD. The rising edge of CS/LD ends the data transfer
and causes the device to execute the command specifi ed
in the 24-bit input sequence. The complete sequence is
shown in Figure 3a.
The command (C3-C0) assignments are shown in Table 1.
The fi rst three commands in the table consist of write and
update operations. A Write operation loads a 16-bit data
word from the 24-bit shift register into the input register.
In an Update operation, the input register is copied to the
DAC register and converted to an analog voltage at the
DAC output. Write to and Update combines the fi rst two
commands. The Update operation also powers up the
DAC if it had been in power-down mode. The data path
and registers are shown in the Block Diagram.
Table 1. Command Codes
18
COMMAND*
*Command codes not shown are reserved and should not be used
C3
0
0
0
0
0
0
C2
0
0
0
1
1
1
C1
0
0
1
0
1
1
C0
0
1
1
0
0
1
Write to Input Register
Update (Power Up) DAC Register
Write to and Update (Power Up) DAC Register
Power Down
Select Internal Reference
Select External Reference
While the minimum input sequence is 24 bits, it may
optionally be extended to 32 bits to accommodate micro-
processors that have a minimum word width of 16 bits (2
bytes). To use the 32-bit width, 8 don’t-care bits are trans-
ferred to the device fi rst, followed by the 24-bit sequence
described. Figure 3b shows the 32-bit sequence.
The 16-bit data word is ignored for all commands that do
not include a Write operation.
Reference Modes
For applications where an accurate external reference is
not available, the LTC2640 has a user-selectable, inte-
grated reference. The LTC2640-LM/LTC2640-LZ provide
a full-scale output of 2.5V. The LTC2640-HM/LTC2640-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
LTC2640-HZ/LTC2640-LZ, as well as for LTC2640-HM/
LTC2640-LM when REF_SEL is tied high.
The 10ppm/°C, 1.25V (LTC2640-LM/LTC2640-LZ) or
2.048V (LTC2640-HM/LTC2640-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 an
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
External Reference mode is the power-on default for
LTC2640-HM/LTC2640-LM when REF_SEL is tied low.
The reference mode of LTC2640-HZ/LTC2640-LZ can be
changed only by software command. The same is true for
LTC2640-HM/LTC2640-LM after power-on, after which the
logic state on REF_SEL is ignored.
REF
≤ V
CC
) and the supply current is reduced.
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