LTC2634 datasheet(21/28 Pages) LINER

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LTC2634

2634f

OPERATION

Power-Down Mode

For power-constrained applications, power-down mode can

be used to reduce the supply current whenever less than

four 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 200k 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

supply current is reduced approximately 20% 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 and all DAC channels are

in power-down mode, the REF pin becomes high imped-

ance (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) or pull-

ing the asynchronous LDAC pin low. The selected DAC

is powered up as its voltage output is updated. When

a DAC which is in a powered-down state is powered up

and updated, normal settling is delayed. If less than four

DACs are in a powered-down state prior to the update

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

all four 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 ampliﬁers

and reference buffers. In this case, the power-up delay time

is 12μs. The power-up of the integrated reference depends

on the command that powered it down. If the reference is

powered down using the select external reference com-

mand (0111b), then it can only be powered back up using

select internal reference command (0110b). However, if

the reference was powered down using power-down chip

command (0101b), then in addition to select internal

reference command (0110b), any command (in software

or using the LDAC pin) that powers up the DACs will also

power up the integrated reference.

Voltage Output

The LTC2634’s integrated rail-to-rail ampliﬁer has guaran-

teed load regulation when sourcing or sinking up to 10mA

at 5V, and 5mA at 3V.

Load regulation is a measure of the ampliﬁer’s ability to

maintain the rated voltage accuracy over a wide range of

load current. The measured change in output voltage per

change in forced load current is expressed in LSB/mA.

DC output impedance is equivalent to load regulation, and

may be derived from it by simply calculating a change in

units from LSB/mA to ohms. The ampliﬁer’s DC output

impedance is 0.1Ω when driving a load well away from

the rails.

When drawing a load current from either rail, the output

voltage headroom with respect to that rail is limited by

the 50Ω typical channel resistance of the output devices

(e.g., when sinking 1mA, the minimum output voltage is

50Ω • 1mA, or 50mV). See the graph “Headroom at Rails

vs Output Current” in the Typical Performance Character-

istics section.

The ampliﬁer is stable driving capacitive loads of up to

500pF.

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