LTC2607 LINER [Linear Technology], LTC2607 Datasheet - Page 20

• Manufacturer Part Number: LTC2607
• Manufacturer: LINER [Linear Technology]

LTC2633
operation
Reference Modes
For applications where an accurate external reference is
either not available, or not desirable due to limited space,
the LTC2633 has a user-selectable, integrated reference.
The integrated reference voltage is internally amplified
by 2x to provide the full-scale DAC output voltage range.
The LTC2633-LI/LTC2633-LX/LTC2633-LO/LTC2633-LZ
provides a full-scale output of 2.5V. The LTC2633-HI/
LTC2633-HZ provides a full-scale output of 4.096V. The
internal reference can be useful in applications where
the supply voltage is poorly regulated. Internal reference
mode can be selected by using command 0110b, and is
the power-on default for LTC2633-HZ/LTC2633-LZ, as well
as for LTC2633-HI/LTC2633-LI/LTC2633-LO.
The 10ppm/°C, 1.25V (LTC2633-LI/LTC2633-LX/LTC2633-
LO/LTC2633-LZ) or 2.048V (LTC2633-HI/LTC2633-HZ)
internal reference is available at the REF pin. Adding
bypass capacitance to the REF pin will improve noise
performance; 0.1µF is recommended and up to 10µF can
be driven without oscillation. This output must be buffered
when driving an external DC load current.
Alternatively, the DAC can operate in external reference
mode using command 0111b. In this mode, an input voltage
supplied externally to the REF pin provides the reference
(1V ≤ V
external reference voltage supplied sets the full-scale DAC
output voltage. External reference mode is the power-on
default for LTC2633-LX.
The reference mode of LTC2633-HZ/LTC2633-LZ/
LTC2633-HI/LTC2633-LI/LTC2633-LO (internal reference
power-on default), can be changed by software command
after power up. The same is true for LTC2633-LX (external
reference power-on default).
Power-Down Mode
For power-constrained applications, power-down mode can
be used to reduce the supply current whenever less than
two DAC outputs are needed. When in power-down, the
buffer amplifiers, bias circuits, and integrated reference
circuits are disabled, and draw essentially zero current.
The DAC outputs are put into a high impedance state, and
20
REF
≤ V
CC
) and the supply current is reduced. The
the output pins are passively pulled to ground through
individual 200kΩ resistors (LTC2633-LI/LTC2633-LX/
LTC2633-LO/LTC2633-LZ/LTC2633-HI/LTC2633-HZ). For
the LTC2633-LO options, the output pins are not passively
pulled to ground, but are also placed in a high impedance
state (open-circuited state) during power-down, typically
drawing less than 0.1µA. The LTC2633-LO options power-
up with all DAC outputs in this high impedance state. They
remain that way until given a software update command.
For all LTC2633 options, Input- and DAC-register contents
are not disturbed during power-down.
Any channel or combination of channels can be put into
power-down mode by using command 0100b in combi-
nation with the appropriate DAC address, (n). The supply
current is reduced approximately 30% for each DAC
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 is in power-down mode, the REF
pin becomes high impedance (typically > 1GΩ). For all
power-down commands the 16-bit data word is ignored.
Normal operation resumes after executing any command
that includes a DAC update, (as shown in Table 1). The
selected DAC is powered up as its voltage output is up-
dated. When a DAC which is in a powered-down state is
powered up and updated, normal settling is delayed. If
less than two DACs are in a powered-down state prior to
the update command, the power-up delay time is 10µs.
However, if both 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 refer-
ence depends on the command that powered it down. If
the reference is powered down using the select external
reference command (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
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