LTC2633 datasheet(20/24 Pages) AD

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LTC2633

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Reference Modes

For applications where an accurate external reference is

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

the LTC2633 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 LTC2633-LI/LTC2633-LX/LTC2633-LO/LTC2633-LZ

provides a full-scale output of 2.5V. The LTC2633-HI/

LTC2633-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 LTC2633-HZ/LTC2633-LZ, as well

as for LTC2633-HI/LTC2633-LI/LTC2633-LO.

The10ppm/°C,1.25V(LTC2633-LI/LTC2633-LX/LTC2633-

LO/LTC2633-LZ) or 2.048V (LTC2633-HI/LTC2633-HZ)

internal reference is available at the REF pin. Adding

bypass capacitance to the REF pin will improve noise

performance; 0.1µF 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

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 LTC2633-LX.

The reference mode of LTC2633-HZ/LTC2633-LZ/

LTC2633-HI/LTC2633-LI/LTC2633-LO (internal reference

power-on default), can be changed by software command

after power up. The same is true for LTC2633-LX (external

reference power-on default).

Power-Down Mode

Forpower-constrainedapplications,power-downmodecan

be used to reduce the supply current whenever less than

two 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

individual 200kΩ resistors (LTC2633-LI/LTC2633-LX/

LTC2633-LO/LTC2633-LZ/LTC2633-HI/LTC2633-HZ). For

the LTC2633-LO options, the output 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 LTC2633-LO options power-

up with all DAC outputs in this high impedance state. They

remain that way until given a software update command.

For all LTC2633 options, Input- and DAC-register contents

are not disturbed during power-down.

Any channel or combination of channels can be put into

power-down mode by using command 0100b in combi-

nation with the appropriate DAC address, (n). The supply

current is reduced approximately 30% for each DAC

powered down. The integrated reference is automatically

powered down when external reference is selected using

command 0111b. In addition, all the DAC channels and the

integrated reference together can be put into power-down

mode using Power Down Chip command 0101b. When

the integrated reference is in power-down mode, the REF

pin becomes high impedance (typically > 1GΩ). For all

power-down commands the 16-bit data word is ignored.

Normal operation resumes after executing any command

that includes a DAC update, (as shown in Table 1). The

selected DAC is powered up as its voltage output is up-

dated. When a DAC which is in a powered-down state is

powered up and updated, normal settling is delayed. If

less than two DACs are in a powered-down state prior to

the update command, the power-up delay time is 10µs.

However, if both DACs and the integrated reference are

powered down, then the main bias generation circuit block

has been automatically shut down in addition to the DAC

amplifiersandreferencebuffers.Inthiscase,thepowerup

delay time is 12µs. The power-up of the integrated refer-

ence depends on the command that powered it down. If

the reference is powered down using the select external

reference command (0111b), then it can only be powered

backupusingselectinternalreferencecommand(0110b).

However, if the reference was powered down using power

down chip command (0101b), then in addition to select

OPERATION

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