Datasheet

Data Sheet AD5412/AD5422
Rev. I | Page 39 of 44
Table 25. Thermal and Supply Considerations for Each Package
Considerations TSSOP LFCSP
Maximum Allowed Power
Dissipation When Operating at an
Ambient Temperature of 85°C
mW14
.
1
35
85
125
=
−
=
−
JA
A
J
T
max
T
θ
W21.1
33
85125
=
−
=
−
JA
A
J
TmaxT
θ
Maximum Allowed Ambient
Temperature When Operating
from a Supply of 40 V and Driving
24 mA Directly to Ground
C8635)028.040(125 °=××−=×−
JA
DJ
PmaxT θ
C8833)028.040(125 °=××−=×−
JA
DJ
PmaxT θ
Maximum Allowed Supply
Voltage When Operating at an
Ambient Temperature of 85°C and
Driving 24 mA Directly to Ground
V40
35028.0
85125
=
×
−
=
×
−
JA
DD
A
J
AI
TmaxT
θ
V43
33028.0
85125
=
×
−
=
×
−
JA
DD
A
J
AI
TmaxT
θ
INDUSTRIAL ANALOG OUTPUT MODULE
Many industrial control applications have requirements for
accurately controlled current and voltage output signals. The
AD5412/AD5422 are ideal for such applications. Figure 83 shows
the AD5412/AD5422 in a circuit design for an output module,
specifically for use in an industrial control application. The design
provides for a current or voltage output. The module is powered
from a field supply of 24 V. This supplies AV
DD
directly. An inverting
buck regulator generates the negative supply for AV
SS
. For transient
overvoltage protection, transient voltage suppressors (TVS) are
placed on all field accessible connections. A 24 V volt TVS is placed
on each I
OUT
, V
OUT
, +V
SENSE
, and −V
SENSE
connection, and a 36 V TVS
is placed on the field supply input. For added protection, clamping
diodes are connected from the I
OUT
, V
OUT
, +V
SENSE
, and −V
SENSE
pins to the AV
DD
and AV
SS
power supply pins. If remote voltage load
sensing is not required, the +V
SENSE
pin can be directly connected to
the V
OUT
pin and the –V
SENSE
pin can be connected to GND.
Isolation between the AD5412/AD5422 and the backplane
circuitry is provided with ADuM1400 and ADuM1200 iCoupler
digital isolators; further information on iCoupler products is
available at www.analog.com/isolators. The internally generated
digital power supply of the AD5412/AD5422 powers the field
side of the digital isolaters, removing the need to generate a digital
power supply on the field side of the isolation barrier. The AD5412/
AD5422 digital supply output supplies up to 5 mA, which is more
than enough to supply the 2.8 mA requirements of the ADuM1400
and ADuM1200 operating at a logic signal frequency of up to
1 MHz. To reduce the number of isolators required, nonessen-
tial signals such as CLEAR can be connected to GND.
FAULT
and SDO can be left unconnected, reducing the isolation
requirements to just three signals. See Circuit Note CN0321 for
an example of a built and tested circuit of a fully isolated, single
channel voltage and 4 mA to 20 mA output with HART.
INDUSTRIAL HART CAPABLE ANALOG OUTPUT
APPLICATION
Many industrial control applications have requirements for
accurately controlled current output signals, and the AD5412/
AD5422 are ideal for such applications. Figure 82 shows the
AD5412/AD5422 in a circuit design for a HART-enabled output
module, specifically for use in an industrial control application in
which both the voltage output and current output are available—
one at a time—on one pin, thus reducing the number of screw
connections required. There is no conflict with tying the two output
pins together because only the voltage output or the current output
can be enabled at any one time. For further information on this
circuit, see Circuit Note CN0278, Complete 4 mA to 20 mA
HART Solution with Additional Voltage Output Capability.
The design provides for a HART-enabled current output, with
the HART capability provided by the AD5700/AD5700-1 HART
modem, the industry’s lowest power and smallest footprint HART-
compliant IC modem. For additional space-savings, the AD5700-1
offers a 0.5% precision internal oscillator. The HART_OUT signal
from the AD5700 is attenuated and ac-coupled into the RSET pin
of the AD5412/AD5422. Because the RSET pin is used to couple
the HART signal into the AD5412/AD5422, either the TSSOP or
LFCSP package option can be used for this configuration. It should
be noted however, that since the TSSOP package does not have
a CAP1 pin, C1 (see Figure 82) cannot be inserted in this case.
While the TSSOP equivalent circuit (as in Figure 82 but without
C1 in place) still passes the HART Communication Foundation
physical layer specs, the results with C1 in place are superior to
those without C1 in place. Further information on an alternative
configuration, whereby the HART signal is coupled into the CAP2
pin can be found in Application Note AN-1065. This is based
on the AD5410/AD5420 but can also be applied to the AD5412/
AD5422. Use of either configuration results in the AD5700 HART
modem output modulating the 4 mA to 20 mA analog current
without affecting the dc level of the current. This circuit adheres
to the HART physical layer specifications as defined by the
HART Communication Foundation.
The module is powered from a field supply of ±10.8 V to ±26.4 V.
This supplies AV
DD
/AVSS directly. For transient overvoltage
protection, transient voltage suppressors (TVS) are placed on
both the I
OUT
and field supply connections. A 24 V TVS is
placed on the I
OUT
connection, and a 36 V TVS is placed on the
field supply input(s). For added protection, clamping diodes are
connected from the I
OUT
pin to t he AV
DD
and GND power
supply pins. A 10 kΩ current limiting resistor is also placed in
series with the positive terminal of the +V
SENSE
buffer input.
This is to limit the current to an acceptable level during a
transient event.