DAC7678SPW datasheet(24/52 Pages) TI

24 / 52 page

REF

Reference

Disable

DAC7678

SBAS493A – FEBRUARY 2010 – REVISED AUGUST 2010

www.ti.com

INTERNAL REFERENCE

using an external reference. The internal reference

can be powered up and powered down using a serial

The DAC7678 includes a 2.5V internal reference that

command that requires a 32-bit write sequence,

is disabled by default. The internal reference is

which consists of 8 bit Address Byte plus 24 bit serial

externally available at the VREFIN/VREFOUT pin. A

command as shown in Table 1. During the time that

minimum 100nF capacitor is recommended between

the internal reference is disabled, the DAC functions

the reference output and GND for noise filtering. The

normally using an external reference. However, when

internal

reference

the

DAC7678

switching to the external reference the internal gain is

bipolar-transistor based precision bandgap voltage

dynamically switched to one. Therefore appropriate

reference. Figure 88 shows the basic bandgap

value of external reference should be used per the

topology. Transistors Q1 and Q2 are biased such that

desired output voltage. At this point, the internal

the current density of Q1 is greater than that of Q2.

reference is disconnected from the VREFIN/VREFOUT pin

The difference of the two base-emitter voltages (VBE1

(3-state

output).

not

attempt

drive

the

– VBE2) has a positive temperature coefficient and is

VREFIN/VREFOUT pin externally and internally at the

forced across resistor R1. This voltage is gained up

same time indefinitely. There are two modes that

and added to the base-emitter voltage of Q2, which

allow communication with the internal reference:

has a negative temperature coefficient. The resulting

Regular/Static and Flexible. In Flexible mode DB14

output voltage is virtually independent of temperature.

needs to be set to '1' as shown in Table 17.

The short-circuit current is limited by design to

approximately 100mA.

Regular/Static Mode (see Table 1 and Table 2)

Enabling Internal Reference:

To enable the internal reference, write the 24-bit

serial command shown in Table 1 after properly

addressing the device. When performing a power

cycle to reset the device, the internal reference is

switched off (default mode). In the default mode, the

internal reference is powered down until a valid write

sequence is applied to power up the internal

reference. Setting DB4 to '1' turns on the internal

reference.If the internal reference is powered up, it

automatically powers down when all DACs power

down in any of the power-down modes (see Table 17

and Power Down Modes section). The internal

reference automatically powers up when any DAC is

powered up.

Disabling Internal Reference:

To disable the internal reference, address the device

Figure 88. Simplified Schematic of the Bandgap

by writing the 8-bit address byte and then writing the

Reference

24-bit serial command shown in Table 1. When

performing a power cycle to reset the device, the

Enable/Disable Internal Reference

internal reference is put back into the default mode

(switched off).

The internal reference in the DAC7678 is disabled by

default for debugging, evaluation purposes, or when

SPACER

Table 1. Write Sequence for Enabling Internal Reference (Static Mode) (Internal Reference Powered On)

COMMAND AND ACCESS BYTE

MOST SIGNIFICANT DATA BYTE

LEAST SIGNIFICANT BYTE

DB15

DB14

DB13

DB12

DB11

DB10

DB9

DB8

DB7

DB6

DB5

DB4

DB3

DB2

DB1

DB0

Table 2. Write Sequence for Disabling Internal Reference (Static Mode) (Internal Reference Powered Off)

COMMAND AND ACCESS BYTE

MOST SIGNIFICANT DATA BYTE

LEAST SIGNIFICANT BYTE

DB15

DB14

DB13

DB12

DB11

DB10

DB9

DB8

DB7

DB6

DB5

DB4

DB3

DB2

DB1

DB0

Submit Documentation Feedback

Product Folder Link(s): DAC7678

DAC7678SPW Datasheet(PDF) 24 Page - Texas Instruments

DAC7678SPW Datasheet(HTML) 24 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

Datasheet Download

Link URL