Datasheet

V
REF
R
1
Q
2
Q
1
R
2
Reference
Disable
DAC7678
SBAS493B –FEBRUARY 2010–REVISED JULY 2012 www.ti.com
INTERNAL REFERENCE which consists of 8 bit Address Byte plus 24 bit serial
command as shown in Table 1. During the time that
The DAC7678 includes a 2.5V internal reference that
the internal reference is disabled, the DAC functions
is disabled by default. The internal reference is
normally using an external reference. However, when
externally available at the V
REFIN
/V
REFOUT
pin. A
switching to the external reference the internal gain is
minimum 100nF capacitor is recommended between
dynamically switched to one. Therefore appropriate
the reference output and GND for noise filtering. The
value of external reference should be used per the
internal reference of the DAC7678 is a bipolar-
desired output voltage. At this point, the internal
transistor based precision bandgap voltage reference.
reference is disconnected from the V
REFIN
/V
REFOUT
pin
Figure 88 shows the basic bandgap topology.
(3-state output). Do not attempt to drive the
Transistors Q
1
and Q
2
are biased such that the
V
REFIN
/V
REFOUT
pin externally and internally at the
current density of Q
1
is greater than that of Q
2
. The
same time indefinitely. There are two modes that
difference of the two base-emitter voltages (V
BE1
–
allow communication with the internal reference:
V
BE2
) has a positive temperature coefficient and is
Regular/Static and Flexible. In Flexible mode DB14
forced across resistor R
1
. This voltage is gained up
needs to be set to '1' as shown in Table 17.
and added to the base-emitter voltage of Q
2
, which
has a negative temperature coefficient. The resulting
Regular/Static Mode (see Table 1 and Table 2)
output voltage is virtually independent of temperature.
The short-circuit current is limited by design to Enabling Internal Reference:
approximately 100mA.
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
by writing the 8-bit address byte and then writing the
24-bit serial command shown in Table 1. When
performing a power cycle to reset the device, the
Figure 88. Simplified Schematic of the Bandgap
internal reference is put back into the default mode
Reference
(switched off).
SPACER
Enable/Disable Internal Reference
The internal reference in the DAC7678 is disabled by
default for debugging, evaluation purposes, or when
using an external reference. The internal reference
can be powered up and powered down using a serial
command that requires a 32-bit write sequence,
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
C3 C2 C1 C0 A3 A2 A1 A0 DB15 DB14 DB13 DB12 DB11 DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0
1 0 0 0 X X X X X X X X X X X X X X X 1 X X X X
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
C3 C2 C1 C0 A3 A2 A1 A0 DB15 DB14 DB13 DB12 DB11 DB10 DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0
1 0 0 0 X X X X X X X X X X X X X X X 0 X X X X
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