XTR101

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

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

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

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

20mA

4mA

16mA

750

Ω

12V, 200mW

Q

+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

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