Datasheet

Data Sheet AD5751
Rev. B | Page 29 of 32
APPLICATIONS INFORMATION
TRANSIENT VOLTAGE PROTECTION
The AD5751 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 AD5751 from excessively high voltage transients, external
power diodes and a surge current limiting resistor may be
required, as shown in Figure 55. The constraint on the resistor
value is that during normal operation the output level at IOUT
must remain within its voltage compliance limit of AV
DD
−
2.75 V and the two protection diodes and resistor must have
appropriate power ratings. Further protection can be added
with transient voltage suppressors if needed.
I
OUT
AV
DD
AV
DD
AD5751
R
P
R
LOAD
07269-011
Figure 55. Output Transient Voltage Protection
THERMAL CONSIDERATIONS
It is important to understand the effects of power dissipation
on the package and how it affects junction temperature. The
internal junction temperature should not exceed 125°C. The
AD5751 is packaged in a 32-lead, 5 mm × 5 mm LFCSP pack-
age. The thermal impedance, θ
JA
, is 42 ° C / W. It is important that
the devices not be operated under conditions that cause the
junction temperature to exceed its limit. Worst-case conditions
occur when the AD5751 are operated from the maximum AV
DD
(55 V) and driving the maximum current (24 mA) directly to
ground. The quiescent current of the AD5751 should also be
taken into account, nominally ~4 mA. The following calculations
estimate maximum power dissipation under these worst-case
conditions, and determine maximum ambient temperature based
on this. These figures assume that proper layout and grounding
techniques are followed to minimize power dissipation, as
outlined in the Layout Guidelines section.
Table 12. Thermal and Supply Considerations
Considerations 32-Lead LFCSP Package
Maximum allowed power dissipation when operating at an ambient
temperature of 85°C
W95.0
42
85125
=
−
=
−
JA
AJMAX
θ
TT
Maximum allowed ambient temperature when operating from a supply of
55 V and driving 24 mA directly to ground (include 4 mA for internal AD5751
current)
T
JMAX
− (P
D
× θ
JA
) = 125 − ((55 × 0.028) × 42) = 60.3°C
Maximum allowed supply voltage when operating at an ambient
temperature of 85°C and driving 24 mA directly to ground
(
)
V34
42028.0
85
125
=
×
−
=
×
−
JA
DD
AJMAX
θ
AI
TT