LTC2621 LINER [Linear Technology], LTC2621 Datasheet - Page 11

• Manufacturer Part Number: LTC2621
• Description: 16-/14-/12-Bit Rail-to-Rail DACs in 10-Lead DFN
• Manufacturer: LINER [Linear Technology]

OPERATIO
Power-On Reset
The LTC2601/LTC2611/LTC2621 clear the outputs to zero
scale when power is first applied, making system initializa-
tion consistent and repeatable.
For some applications, downstream circuits are active
during DAC power-up, and may be sensitive to nonzero
outputs from the DAC during this time. The LTC2601/
LTC2611/LTC2621 contain circuitry to reduce the power-
on glitch; furthermore, the glitch amplitude can be made
arbitrarily small by reducing the ramp rate of the power
supply. For example, if the power supply is ramped to 5V
in 1ms, the analog outputs rise less than 10mV above
ground (typ) during power-on. See Power-On Reset Glitch
in the Typical Performance Characteristics section.
Power Supply Sequencing
The voltage at REF (Pin 6) should be kept within the range
– 0.3V
Ratings). Particular care should be taken to observe these
limits during power supply turn-on and turn-off sequences,
when the voltage at V
Transfer Function
The digital-to-analog transfer function is:
where k is the decimal equivalent of the binary DAC input
code, N is the resolution and V
(Pin 6).
Serial Interface
The CS/LD input is level triggered. When this input is taken
low, it acts as a chip-select signal, powering-on the SDI
and SCK buffers and enabling the input shift register. Data
(SDI input) is transferred at the next 24 rising SCK edges.
The 4-bit command, C3-C0, is loaded first; then 4 don’t
care bits; and finally the 16-bit data word. The data word
comprises the 16-, 14- or 12-bit input code, ordered MSB-
to-LSB, followed by 0, 2 or 4 don’t care bits (LTC2601,
LTC2611 and LTC2621 respectively). Data can only be
transferred to the device when the CS/LD signal is low.The
rising edge of CS/LD ends the data transfer and causes the
V
OUT IDEAL
(
V
REF
)
U
V
2
CC
k
N
CC
+ 0.3V (see Absolute Maximum
V
REF
(Pin 16) is in transition.
REF
is the voltage at REF
device to execute the command specified in the 24-bit
input word. The complete sequence is shown in Figure 2a.
The command (C3-C0) assignments are shown in Table 1.
The first four commands in the table consist of write and
update operations. A write operation loads a 16-bit data
word from the 32-bit shift register into the input register
of the DAC. In an update operation, the data word is copied
from the input register to the DAC register and converted
to an analog voltage at the DAC output. The update
operation also powers up the DAC if it had been in power-
down mode. The data path and registers are shown in the
Block Diagram.
While the minimum input word is 24 bits, it may optionally
be extended to 32 bits. To use the 32-bit word width,
8 don’t-care bits are transferred to the device first, fol-
lowed by the 24-bit word as just described. Figure 2b
shows the 32-bit sequence. The 32-bit word is required for
daisy-chain operation, and is also available to accommo-
date microprocessors which have a minimum word width
of 16 bits (2 bytes).
Daisy-Chain Operation
The serial output of the shift register appears at the SDO
pin. Data transferred to the device from the SDI input is
delayed 32 SCK rising edges before being output at the
next SCK falling edge.
The SDO output can be used to facilitate control of multiple
serial devices from a single 3-wire serial port (i.e., SCK,
SDI and CS/LD). Such a “daisy chain” series is configured
by connecting SDO of each upstream device to SDI of the
next device in the chain. The shift registers of the devices
are thus connected in series, effectively forming a single
input shift register which extends through the entire chain.
Table 1.
COMMAND*
*Command codes not shown are reserved and should not be used.
C3 C2 C1 C0
0
0
0
0
1
0
0
0
1
1
LTC2601/LTC2611/LTC2621
0
0
1
0
1
0
1
1
0
1
Write to Input Register
Update (Power Up) DAC Register
Write to and Update (Power Up)
Power Down
No Operation
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