Datasheet

Data Sheet AD5755-1
Rev. E | Page 47 of 52
DRIVING INDUCTIVE LOADS
When driving inductive or poorly defined loads, a capacitor
may be required between I
OUT_x
and AGND to ensure stability.
A 0.01 µF capacitor between I
OUT_x
and AGND ensures stability
of a load of 50 mH. The capacitive component of the load may
cause slower settling, although this may be masked by the set-
tling time of the AD5755-1. There is no maximum capacitance
limit for the current output of the AD5755-1.
TRANSIENT VOLTAGE PROTECTION
The AD5755-1 contains ESD protection diodes that prevent
damage from normal handling. The industrial control
environment can, however, subject I/O circuits to much higher
transients. To protect the AD5755-1 from excessively high
voltage transients, external power diodes and a surge current
limiting resistor (R
P
) are required, as shown in Figure 86. A
typical value for R
P
is 10 Ω. The two protection diodes and the
resistor (R
P
) must have appropriate power ratings.
R
LOAD
R
P
D2
D1
AV
SS
AD5755-1
V
BOOST_x
I
OUT_x
AGND
09226-079
R
FILTER
C
FILTER
0.1µF
10Ω
(FROM
DC-TO-DC
CONVERTER)
C
DCDC
4.7µF
Figure 86. Output Transient Voltage Protection
Further protection can be provided using transient voltage
suppressors (TVSs), also referred to as transorbs. These compo-
nents are available as unidirectional suppressors, which protect
against positive high voltage transients, and as bidirectional
suppressors, which protect against both positive and negative
high voltage transients. Transient voltage suppressors are avail-
able in a wide range of standoff and breakdown voltage ratings.
The TVS should be sized with the lowest breakdown voltage
possible while not conducting in the functional range of the
current output.
It is recommended that all field connected nodes be protected.
The voltage output node can be protected with a similar circuit,
where D2 and the transorb are connected to AV
SS
. For the volt-
age output node, the +V
SENSE_x
pin should also be protected with
a large value series resistance to the transorb, such as 5 kΩ. In
this way, the I
OUT_x
and V
OUT_x
pins can also be tied together and
share the same protection circuitry.
MICROPROCESSOR INTERFACING
Microprocessor interfacing to the AD5755-1 is via a serial bus
that uses a protocol compatible with microcontrollers and DSP
processors. The communications channel is a 3-wire minimum
interface consisting of a clock signal, a data signal, and a latch
signal. The AD5755-1 requires a 24-bit data-word with data
valid on the falling edge of SCLK.
The DAC output update is initiated on either the rising edge of
LDAC
or, if
LDAC
is held low, on the rising edge of
SYNC
. The
contents of the registers can be read using the readback function.
AD5755-1-TO-ADSP-BF527 INTERFACE
The AD5755-1 can be connected directly to the SPORT
interface of the ADSP-BF527, an Analog Devices, Inc.,
Blackfin® DSP. Figure 87 shows how the SPORT interface
can be connected to control the AD5755-1.
09226-080
AD5755-1
SYNC
SCLK
SDIN
LDAC
SPORT_TFS
SPORT_TSCK
SPORT_DTO
GPIO0
ADSP-BF527
Figure 87. AD5755-1-to-ADSP-BF527 SPORT Interface
LAYOUT GUIDELINES
Grounding
In any circuit where accuracy is important, careful consideration
of the power supply and ground return layout helps to ensure
the rated performance. The printed circuit board on which the
AD5755-1 is mounted should be designed so that the analog
and digital sections are separated and confined to certain areas of
the board. If the AD5755-1 is in a system where multiple devices
require an AGND-to-DGND connection, the connection should
be made at one point only. The star ground point should be
established as close as possible to the device.
The GNDSW
x
and ground connection for the AV
CC
supply are
referred to as PGND. PGND should be confined to certain areas
of the board, and the PGND-to-AGND connection should be
made at one point only.
Supply Decoupling
The AD5755-1 should have ample supply bypassing of 10 µF
in parallel with 0.1 µF on each supply located as close to the
package as possible, ideally right up against the device. The
10 µF capacitors are the tantalum bead type. The 0.1 µF
capacitor should have low effective series resistance (ESR) and
low effective series inductance (ESL), such as the common
ceramic types, which provide a low impedance path to ground
at high frequencies to handle transient currents due to internal
logic switching.