Datasheet

V
REF
R
1
1 Q
2
NQ
1
R
2
Reference
Disable
DAC7565
SBAS412B –FEBRUARY 2008–REVISED MARCH 2011 www.ti.com
INTERNAL REFERENCE
The DAC7565 includes a 2.5V internal reference that
is enabled by default. The internal reference is
externally available at the V
REF
H/V
REF
OUT pin. A
minimum 100nF capacitor is recommended between
the reference output and GND for noise filtering.
The internal reference of the DAC7565 is a bipolar
transistor-based, precision bandgap voltage
reference. Figure 94 shows the basic bandgap
topology. Transistors Q
1
and Q
2
are biased such that
the current density of Q
1
is greater than that of Q
2
.
The difference of the two base-emitter voltages
(V
BE1
– V
BE2
) has a positive temperature coefficient
and is forced across resistor R
1
. This voltage is
gained up and added to the base-emitter voltage of
Q
2
, 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 DAC7565 is enabled by
serial command shown in Table 3. 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 up
disable the internal reference, as shown in Table 2.
automatically when any DAC is powered up.
During the time that the internal reference is disabled,
the DAC functions normally using an external
The DAC7565 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 V
REF
H/V
REF
OUT pin (3-state
of the DAC(s) state (powered up or down). To keep
output). Do not attempt to drive the V
REF
H/V
REF
OUT
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 4.
Table 2. Write Sequence for Disabling Internal Reference
(internal reference always powered down—012000h)
DB23 DB16 DB13 DB0
0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 0 X X X X
|———————————— Data Bits —————————————|
Table 3. Write Sequence for Enabling Internal Reference
(internal reference powered up to default mode—010000h)
DB23 DB16 DB0
0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 X X X X
|———————————— Data Bits —————————————|
Table 4. Write Sequence for Enabling Internal Reference
(internal reference always powered up—011000h)
DB23 DB16 DB12 DB0
0 0 0 0 0 0 0 1 0 0 0 1 0 0 0 0 0 0 0 0 X X X X
|———————————— Data Bits —————————————|
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