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LTC2637

2637fa

operation

Figure 3. Command and Data Input Format

1ST DATA BYTE

Input Word (LTC2637-12)

Write Word Protocol for LTC2637

C2 C1 C0 A3 A2 A1 A1

D10

D11

W ACK

SLAVE ADDRESS

1ST DATA BYTE

D8 D7 D6 D5 D4 D3 D2 D1 D0

ACK 2ND DATA BYTE ACK 3RD DATA BYTE ACK P

2637 F03

2ND DATA BYTE

INPUT WORD

3RD DATA BYTE

1ST DATA BYTE

Input Word (LTC2637-10)

C2 C1 C0 A3 A2 A1 A0

D6 D5 D4 D3 D2 D1 D0 X

2ND DATA BYTE

3RD DATA BYTE

1ST DATA BYTE

Input Word (LTC2637-8)

C2 C1 C0 A3 A2 A1 A0

D4 D3 D2 D1 D0 X

2ND DATA BYTE

3RD DATA BYTE

The LTC2637-LMI/ LTC2637-LMX/ LTC2637-LZ provides

a full-scale output of 2.5V. The LTC2637-HMI/ LTC2637-

HMX/ LTC2637-HZ provides 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 0110b, and is

the power-on default for LTC2637-HZ/ LTC2637-LZ, as

well as for LTC2637-HMI/ LTC2637-LMI.

The 10ppm/°C, 1.25V (LTC2637-LMI/ LTC2637-LMX/

LTC2637-LZ) or 2.048V (LTC2637-HMI/ LTC2637-HMX/

LTC2637-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. The REF output must be buffered when driving

an external DC load current.

Alternatively, the DAC can operate in External Reference

modeusingcommand0111b.Inthismode,aninputvoltage

supplied externally to the REF pin provides the reference

(1V ≤ VREF ≤ VCC) 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 LTC2637-HMX/ LTC2637-LMX.

ThereferencemodeofLTC2637-HZ/LTC2637-LZ/LTC2637-

HMI/ LTC2637-LMI (internal reference power-on default),

can be changed by software command after power up. The

same is true for LTC2637-HMX/ LTC2637-LMX (external

reference power-on default).

Power-Down Mode

Forpower-constrainedapplications,power-downmodecan

be used to reduce the supply current whenever less than

eight 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 outputs are put into a high-impedance state, and

the output pins are passively pulled to ground through in-

dividual 200kΩ resistors. Input and DAC register contents

are not disturbed during power down.

Any DAC channel or combination of channels can be put

into power-down mode by using command 0100b in

combination with the appropriate DAC address, (n). The

supplycurrentisreducedapproximately10%foreachDAC

powered down. The integrated reference is automatically

powered down when external reference is selected using

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