AD694 Analog Devices, AD694 Datasheet - Page 7

• Manufacturer Part Number: AD694
• Description: 4.20 mA Transmitter
• Manufacturer: Analog Devices

REV. A
POWER DISSIPATION CONSIDERATIONS
The AD694 is rated for operation over its specified temperature
without the use of an external pass transistor. However, it is
possible to exceed the absolute maximum power dissipation,
with some combinations of power supply voltage and voltage
reference load. The internal dissipation of the part can be calcu-
lated to determine if there is a chance that the absolute maxi-
mum dissipation may be exceeded. The die temperature must
never exceed 150 C.
Total power dissipation (P
by the internal amplifiers, P (Standing), the voltage reference,
P(V
where:
Definitions:
An appropriate safety factor should be added to P
The junction temperature may be calculated with the following
formula:
(case),
surroundings and is determined by the characteristics of the
thermal connection of the case to ambient.
For example, assume that the part is operating with a V
in the cerdip package at 50 C, with a 1 mA load on the 10 V
reference. Assume that I
would be 20 mA. The internal dissipation would be:
JC
is the thermal resistance between the chip and the package
REF
P (Standing) = 2 mA (max) V
P (V
P(I
V
I
V
V
VREF
REF
S
OUT
) and the current output stage, P(I
OUT
= supply voltage
CA
P
I
rent, or the overdriven current of the device.
P(I
is used.
T
Figure 4. Using Optional Pass Transistor to Minimize Self-Heating Errors; Dual Supply Operation Shown
REF
= output voltage of reference
OUT
= output current of reference
= voltage at I
TOT
J
) (V
is the thermal resistance between the case and its
= P
) = (V
OUT
(max) may be the max expected operating cur-
= P (Standing) + P (V
S
TOT
) drops to (2 Volts
– V
S
(
– V
OUT
JC
OUT
REF
+
OUT
) I
TOT
) I
CA
pin.
is grounded and that the max I
OUT
) + T
), is the sum of power dissipated
VREF
(max):
AMBIENT
S
REF
I
OUT
) + P (I
OUT
) if a pass transistor
) as follows:
OUT
TOT
)
.
S
of 24 V
OUT
–7–
P(
Using
the junction temperature is:
The junction temperature is in the safe region.
Internal power dissipation can be reduced either by reducing the
value of
ducing P(
transistor. Figure 5 shows the maximum case and still air tem-
peratures for a given level of power dissipation.
ADJUSTMENT PROCEDURES
The following sections describe methods for trimming the out-
put current offset, the span and the voltage reference.
ADJUSTING 4 mA ZERO
The 4 mA zero current may be adjusted over the range of 2 mA
to 4.8 mA to accommodate large input signal offsets, or to allow
small adjustment in the zero current. The zero may be adjusted
by pulling up or down on Pin 6 (4 mA Adj) to increase or de-
crease the nominal offset current. The 4 mA Adj. (Pin 6) should
not be driven to a voltage greater than 1 V. The arrangement of
TOT
) = 2 mA
= 48 mW + 14 mW + 480 mW = 542 mW
JC
Figure 5. Internal Power Dissipation in mW
T
CA
TOT
of 30 C/Watt and
J
= 542 mW (30 C/W + 70 C/W) + 50 C = 104.2 C
through the use of air flow or heat sinks, or by re-
) of the AD694 through the use of an external pass
24 V + (24 V – 10 V)
CA
of 70 C/Watt, (from spec page)
1 mA + (24 V – 0 V)
AD694
20 mA