Datasheet

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SBOS146A − OCTOBER 1986 − REVISED AUGUST 2004
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7
INSTALLATION AND
OPERATING INSTRUCTIONS
BASIC CONNECTION
See Figure 1 for the basic connection of the XTR101. A
difference voltage applied between input pins 3 and 4 will
cause a current of 4-20mA to circulate in the two-wire
output loop (through R
L
, V
PS
, and D
1
). For applications
requiring moderate accuracy, the XTR101 operates very
cost-effectively with just its internal drive transistor. For
more demanding applications (high accuracy in high gain),
an external NPN transistor can be added in parallel with
the internal one. This keeps the heat out of the XTR101
package and minimizes thermal feedback to the input
stage. Also, in such applications where the e
IN
full-scale
is small (< 50mV) and R
SPAN
is small (< 150Ω), caution
should be taken to consider errors from the external span
circuit plus high amplification of offset drift and noise.
OPTIONAL EXTERNAL TRANSISTOR
The optional external transistor, when used, is connected
in parallel with the XTR101 internal transistor. The purpose
is to increase accuracy by reducing heat change inside the
XTR101 package as the output current spans from
4-20mA. Under normal operating conditions, the internal
transistor is never completely turned off, as shown in
Figure 2. This maintains frequency stability with varying
external transistor characteristics and wiring capacitance.
The actual current sharing between internal and external
transistors is dependent on two factors:
1. relative geometry of emitter areas, and
2. relative package dissipation (case size and thermal
conductivity).
For best results, the external device should have a larger
base-emitter area and smaller package. It will, upon
turn-on, take about [0.95(I
O
− 3.3mA)]mA. However, it will
heat faster and take a greater share after a few seconds.
1mA
10
7
1mA
11
Q
EXT
23.6V, 377mW
12
B
210
Ω
52.6
Ω
1.5mA
Quiescent
18mA
20mA
R
L
250
Ω
Short−Circuit
Worst−Case
V
PS
40V
20mA4mA
16mA
750
Ω
12V, 200mW
Q
INT
18mW
+V
CC
8
I
OUT
NOTES: (1) An external transistor is used in the manufacturing test circuit for testing electrical specifications.
(2) This resistor is required for the 2N2222 with V
PS
> 24V to limit power dissipation.
3.47V, 60mW
XTR101
3.5mA
0.95V, 17mW
E
(2)
2N2222
(1)
2mA
9
0.5mA
Type
2N4922
TIP29B
TIP31B
Package
TO−225
TO−220
TO−220
Other Suitable Types
Figure 2. Power Calculation of the XTR101 with an External Transistor