LTC2636
OPERATION
up as its voltage output is updated. When a DAC which
is in a powered-down state is powered up and updated,
normal settling is delayed. If less than eight DACs are in
a powered-down state prior to the update command, the
power-up delay time is 10μs. However, if all eight DACs
and the integrated reference are powered down, then the
main bias generation circuit block has been automatically
shut down in addition to the DAC amplifiers and reference
buffers. In this case, the power up delay time is 12μs.
The power-up of the integrated reference depends on
the command that powered it down. If the reference is
powered down using the Select External Reference Com-
mand (0111b), then it can only be powered back up using
Select Internal Reference Command (0110b). However, if
the reference was powered down using Power Down Chip
Command (0101b), then in addition to Select Internal
Reference Command (0110b), any command (in software
or using the LDAC pin) that powers up the DACs will also
power up the integrated reference.
at the REF pin. Adding bypass capacitance to the REF pin
will improve noise performance; and up to 10μF can be
drivenwithoutoscillation.TheREFoutputmustbebuffered
when driving an external DC load current.
Alternatively, the DAC can operate in External Reference
modeusingcommand0111b.Inthismode,aninputvoltage
supplied externally to the REF pin provides the reference
(1V ≤ V ≤ V ) and the supply current is reduced. The
REF
CC
external reference voltage supplied sets the full-scale DAC
output voltage. External Reference mode is the power-on
default for LTC2636-HMX/-LMX.
The reference mode of LTC2636-HZ/-LZ/-HMI/-LMI
(Internal Reference power-on default), can be changed
by software command after power-up. The same is true
for LTC2636-HMX/-LMX (External Reference power-on
default).
Power-Down Mode
Forpower-constrainedapplications,power-downmodecan
be used to reduce the supply current whenever less than
eight DAC outputs are needed. When in power-down, the
buffer amplifiers, bias circuits, and integrated reference
circuitsaredisabled,anddrawessentiallyzerocurrent.The
DAC outputs are put into a high-impedance state, and the
output pins are passively pulled to ground through indi-
vidual 200kΩ resistors. Input- and DAC-register contents
are not disturbed during power-down.
Voltage Outputs
The LTC2636’s integrated rail-to-rail amplifiers have guar-
anteed load regulation when sourcing or sinking up to
10mA at 5V, and 5mA at 3V.
Load regulation is a measure of the amplifier’s ability to
maintain the rated voltage accuracy over a wide range of
load current. The measured change in output voltage per
change in forced load current is expressed in LSB/mA.
Any DAC channel or combination of channels can be put
into power-down mode by using command 0100b in
combination with the appropriate DAC address, (n). The
supplycurrentisreducedapproximately10%foreachDAC
powered down. The integrated reference is automatically
powered down when external reference is selected using
command 0111b. In addition, all the DAC channels and
the integrated reference together can be put into power-
down mode using Power Down Chip command 0101b.
When the integrated reference and all DAC channels are
in power-down mode, the REF pin becomes high imped-
ance (typically > 1GΩ). For all power-down commands
the 16-bit data word is ignored.
DC output impedance is equivalent to load regulation, and
may be derived from it by simply calculating a change in
units from LSB/mA to ohms. The amplifier’s DC output
impedance is 0.1Ω when driving a load well away from
the rails.
When drawing a load current from either rail, the output
voltage headroom with respect to that rail is limited by
the 50Ω typical channel resistance of the output devices
(e.g., when sinking 1mA, the minimum output voltage is
50Ω • 1mA, or 50mV). See the graph “Headroom at Rails
vs. Output Current” in the Typical Performance Charac-
teristics section.
The amplifier is stable driving capacitive loads of up to
500pF.
Normal operation resumes after executing any command
that includes a DAC update, (as shown in Table 1) or using
theasynchronousLDACpin. TheselectedDACispowered
2636f
20
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