Turn-On Voltage

Ratio of I

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Application Hints

The LM134 has been designed for ease of application, but a

general discussion of design features is presented here to

familiarize the designer with device characteristics which

may not be immediately obvious. These include the effects

of slewing, power dissipation, capacitance, noise, and con-

tact resistance.

CALCULATING R

SET

The total current through the LM134 (I

current going through the SET resistor (I

bias current (I

A graph showing the ratio of these two currents is supplied

under Ratio of I

Characteristics section. The current flowing through R

determined by V

mV/298˚K

∼ 214µV/˚K).

Since (for a given set current) I

I

where n is the ratio of I

Electrical Characteristics Section and shown in the graph.

Since n is typically 18 for 2µA

≤ I

SET

≤ 1mA, the equation can

be further simplified to

for most set currents.

SLEW RATE

At slew rates above a given threshold (see curve), the

LM134 may exhibit non-linear current shifts. The slewing

rate at which this occurs is directly proportional to I

I

limit is 1V/µs. Slew rates above the limit do not harm the

LM134, or cause large currents to flow.

THERMAL EFFECTS

Internal heating can have a significant effect on current

regulation for I

1V increase across the LM134 at I

junction temperature by

≈0.4˚C in still air. Output current

(I

≈0.33%/˚C, so the

change in current due to temperature rise will be (0.4)

(0.33) = 0.132%. This is a 10:1 degradation in regulation

compared to true electrical effects. Thermal effects, there-

fore, must be taken into account when DC regulation is

critical and I

package or the TO-92 leads can reduce this effect by more

than 3:1.

SHUNT CAPACITANCE

In certain applications, the 15 pF shunt capacitance of the

LM134 may have to be reduced, either because of loading

problems or because it limits the AC output impedance of the

current source. This can be easily accomplished by buffering

the LM134 with an FET as shown in the applications. This

can reduce capacitance to less than 3 pF and improve

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FIGURE 1. Basic Current Source

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