®

XTR101

6

+V

CC

A

1

+V

CC

D

1

V

PS

I

O

7

e

L

10

11

Q

1

R1

Voltage Controlled

Current Source

2.5k

Ω

100µA

1.25k

Ω

8

56

2mA

R2

52.6

Ω

(e

I

B1

I

B2

I

2

I

REF1

1k

Ω

e

IN

I

S

(e

(e

I

3

I

4

R

3

–+

I

1

+

–

A

3

I

+ 40/R

(e

R

S

–In

3

+In

4

e

IN

I

REF2

I

O

+V

CC

1.25k

Ω

R

4

+V

CC

+V

CC

R

L

A

2

FIGURE 1. Simplified Schematic of the XTR101.

and minimizes thermal feedback to the input stage. Also in

R

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’s 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 transis-

tor characteristics and wiring capacitance. The actual “cur-

rent 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.

However, it will heat faster and take a greater share after a

few seconds.

Although any NPN of suitable power rating will operate

with the XTR101, two readily available transistors are

recommended.

1. 2N2222 in the TO-18 package. For power supply volt-

ages above 24V, a 750

Ω, 1/2W resistor should be con-

nected in series with the collector. This will limit the

power dissipation to 377mW under the worst-case condi-

tions shown in Figure 2. Thus the 2N2222 will safely

operate below its 400mW rating at the upper temperature

of +85

°C. Heat sinking the 2N2222 will result in greatly

reduced accuracy improvement and is not recommended.

2. TIP29B in the TO-220 package. This transistor will

operate over the specified temperature and output voltage

range without a series collector resistor. Heat sinking the

TIP29B will result in slightly less accuracy improvement.

It can be done, however, when mechanical constraints

require it.

ACCURACY WITH AND

WITHOUT EXTERNAL TRANSISTOR

The XTR101 has been tested in a circuit using an external

transistor. The relative difference in accuracy with and

without an external transistor is shown in Figure 3. Notice

that a dramatic improvement in offset voltage change with

supply voltage is evident for any value of load resistor.

MAJOR POINTS TO

CONSIDER WHEN USING THE XTR101

reduce noise pick-up and parasitic resistance.

to the unit as possible (pin 8 to 7).

3. Always keep the input voltages within their range of

linear operation, +4V to +6V (e

respect to pin 7).