LTC2635 datasheet(23/32 Pages) LINER

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LTC2635

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operation

Write Word Protocol

The master initiates communication with the LTC2635

with a START condition and a 7-bit slave address followed

by the Write bit (W) = 0. The LTC2635 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

or CA2) or the global address. The master then transmits

three bytes of data. The LTC2635 acknowledges each byte

of data by pulling the SDA line low at the 9th clock of each

data byte transmission. After receiving three complete

bytes of data, the LTC2635 executes the command speci-

fied in the 24-bit input word.

If more than three data bytes are transmitted after a valid

7-bit slave address, the LTC2635 does not acknowledge

(NAK) the extra bytes of data (SDA is high during the 9th

clock).

TheformatofthethreedatabytesisshowninFigure3.The

first byte of the input word consists of the 4-bit command,

followed by the 4-bit DAC address. The next two bytes

contain the 16-bit data word, which consists of the 12-,

10- or 8-bit input code, MSB to LSB, followed by 4, 6 or 8

don’t-care bits (LTC2635-12, -10 and -8, respectively). A

typical LTC2635 write transaction is shown in Figure 4.

The command bit assignments (C3-C0) and address (A3-

A0) assignments are shown in Tables 3 and 4. 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 input

8-bit input code, and is converted to an analog voltage at

the DAC output. Write to and Update combines the first

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 3. Command Codes

COMMAND*

C2 C1 C0

0 Write to Input Register n

1 Update (Power Up) DAC Register n

0 Write to Input Register n, Update (Power Up) All

1 Write to and Update (Power Up) DAC Register n

0 Power Down n

1 Power Down Chip (All DAC’s and Reference)

0 Select Internal Reference (Power Up Reference)

1 Select External Reference (Power Down Internal

Reference)

1 No Operation

*Command codes not shown are reserved and should not be used.

Table 4. Address Codes

ADDRESS (n)*

A2 A1 A0

0 DAC A

1 DAC B

0 DAC C

1 DAC D

1 ALL DACs

* Address codes not shown are reserved and should not be used.

Reference Modes

For applications where an accurate external reference is

either not available, or not desirable due to limited space,

the LTC2635 has a user-selectable, integrated reference.

The integrated reference voltage is internally amplified by

2x to provide the full-scale DAC output voltage range. The

LTC2635-LMI/-LMX/-LMO/-LZprovidesafull-scaleoutput

of2.5V.TheLTC2635-HMI/-HZprovidesafull-scaleoutput

of 4.096V. The internal reference can be useful in applica-

tionswherethesupplyvoltageispoorlyregulated.Internal

Referencemodecanbeselectedbyusingcommand0110b,

and is the power-on default for LTC2635-HZ/-LZ, as well

as for LTC2635-HMI/-LMI/-LMO.

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