LTC2657 datasheet(18/24 Pages) LINER

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LTC2657

2657f

OPERATION

The LTC2657 is a family of octal voltage output DACs

in 20-lead 4mm

× 5mm QFN and in 20-lead thermally

enhanced TSSOP packages. Each DAC can operate rail-

to-rail in external reference mode, or with its full-scale

voltage set by an integrated reference. Four combinations

of accuracy (16- and 12-bit), and full-scale voltage (2.5V

or 4.096V) are available. The LTC2657 is controlled using

a 2-wire I2C compatible interface.

Power-On Reset

The LTC2657-L/-H clear the output to zero-scale if the

PORSEL pin is tied to GND when power is ﬁrst applied,

making system initialization consistent and repeatable. For

some applications, downstream circuits are active during

DAC power-up, and may be sensitive to nonzero outputs

from the DAC during this time. The LTC2657 contains

circuitry to reduce the power-on glitch. The analog outputs

typically rise less than 10mV above zero-scale during power

on if the power supply is ramped to 5V in 1ms or more. In

general, the glitch amplitude decreases as the power supply

ramp time is increased. See “Power-On Reset Glitch” in

the Typical Performance Characteristics section.

Alternatively, if PORSEL is tied to VCC, The LTC2657-L/-H

set the output to mid-scale when power is ﬁrst applied.

Power Supply Sequencing and Start-Up

For the LTC2657 family of parts, the internal reference is

powered-up at start-up by default. If an external reference is

to be used, the REFCOMP pin (Pin 4 –TSSOP, Pin 2 -QFN )

must be hardwired to GND. This conﬁguration allows the

use of an external reference at start-up and converts the

REFIN/OUT pin to an input. However, the internal reference

will still be ON and draw supply current. In order to use

an external reference, command 0111b should be used

to turn the Internal Reference OFF.(See Table1.)

The voltage at REFIN/OUT (Pin 7 –TSSOP, Pin 5 -QFN)

should be kept within the range –0.3V ≤ REFIN/OUT

≤ VCC + 0.3V (see Absolute Maximum Ratings). Particular

care should be taken to observe these limits during power

supply turn-on and turn-off sequences, when the voltage

at VCC (Pin 19 –TSSOP, Pin 17 -QFN ) is in transition.

Transfer Function

The digital-to-analog transfer function is:

REFLO

REFL

OUT IDEAL

REF

()

•

⎛

⎝

⎜

⎞

⎠

⎟

•

−

⎡⎣

⎤⎦+

where k is the decimal equivalent of the binary DAC input

code, N is the resolution, and VREF is the voltage at the

REFIN/OUT Pin. The resulting DAC output span is 0V to

2 • VREF, as it is necessary to tie REFLO to GND. VREF is

nominally 1.25V for LTC2657-L and 2.048V for LTC2657-H,

in Internal Reference Mode.

Table 1. Command and Address Codes

COMMAND*

Write to Input Register n

Update (Power Up) DAC Register n

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

Write to and Update (Power Up) n

Power Down n

Power Down Chip (All DACs and Reference)

Select Internal Reference (Power-Up Reference)

Select External Reference (Power-Down

Reference)

No Operation

ADDRESS (n)*

DAC A

DAC B

DAC C

DAC D

DAC E

DAC F

DAC G

DAC H

All DACs

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

Serial Interface

The LTC2657 communicates with a host using the stan-

dard 2-wire I2C interface. The Timing Diagrams (Figures

1 and 2) show the timing relationship of the signals on

the bus. The two bus lines, SDA and SCL, must be high

when the bus is not in use. External pull-up resistors or

current sources are required on these lines. The value of

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