DAC8564ICPWG4 datasheet(28/51 Pages) TI1

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REF

Reference

Disable

DAC8564

SBAS403D

– JUNE 2007 – REVISED MAY 2011

www.ti.com

INTERNAL REFERENCE

The DAC8564 includes a 2.5V internal reference that

is enabled by default. The internal reference is

externally available at the VREFH/VREFOUT pin. A

minimum 100nF capacitor is recommended between

the reference output and GND for noise filtering.

The internal reference of the DAC8564 is a bipolar

transistor-based,

precision

bandgap

voltage

reference. Figure 94 shows the basic bandgap

topology. Transistors Q1 and Q2 are biased such that

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

The difference of the two base-emitter voltages

(VBE1 – VBE2) has a positive temperature coefficient

and is forced across resistor R1. This voltage is

gained up and added to the base-emitter voltage of

Q2, which has a negative temperature coefficient. The

resulting output voltage is virtually independent of

Figure 94. Simplified Schematic of the Bandgap

temperature. The short-circuit current is limited by

Reference

design to approximately 100mA.

Enable/Disable Internal Reference

To then enable the internal reference, either perform

a power-cycle to reset the device, or write the 24-bit

The internal reference in the DAC8564 is enabled by

serial command shown in Table 2. These actions put

default and operates in automatic mode; however, the

the internal reference back into the default mode. In

reference can be disabled for debugging, evaluation

the default mode, the internal reference powers down

purposes, or when using an external reference. A

automatically when all DACs power down in any of

serial command that requires a 24-bit write sequence

the power-down modes (see the Power-Down Modes

(see the Serial Interface section) must be used to

section);

the

internal

reference

powers

disable the internal reference, as shown in Table 1.

automatically when any DAC is powered up.

During the time that the internal reference is disabled,

the

DAC

functions

normally

using

external

The DAC8564 also provides the option of keeping the

reference. At this point, the internal reference is

internal reference powered on all the time, regardless

disconnected from the VREFH/VREFOUT pin (3-state

of the DAC(s) state (powered up or down). To keep

output). Do not attempt to drive the VREFH/VREFOUT

the internal reference powered on, regardless of the

pin externally and internally at the same time

DAC(s) state, write the 24-bit serial command shown

indefinitely.

in Table 3.

Table 1. Write Sequence for Disabling Internal Reference

(internal reference always powered down

—012000h)

DB23

DB16

DB13

DB0

—————————————————– Data Bits ––—————————————————|

Table 2. Write Sequence for Enabling Internal Reference

(internal reference powered up to default mode

—010000h)

DB23

DB16

DB0

—————————————————– Data Bits ––—————————————————|

Table 3. Write Sequence for Enabling Internal Reference

(internal reference always powered up

—011000h)

DB23

DB16

DB12

DB0

—————————————————– Data Bits ––—————————————————|

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