LM334 datasheet(6/26 Pages) TI

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LM134, LM234, LM334

SNVS746E – MARCH 2000 – REVISED MAY 2013

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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 contact resistance.

Calculating RSET

The total current through the LM134 (ISET) is the sum of the current going through the SET resistor (IR) and the

LM134's bias current (IBIAS), as shown in Figure 13.

Figure 13. Basic Current Source

A graph showing the ratio of these two currents is supplied under Ratio of ISET to IBIAS in Typical Performance

Characteristics. The current flowing through RSET is determined by VR, which is approximately 214μV/°K (64

mV/298°K

∼ 214μV/°K).

(1)

Since (for a given set current) IBIAS is simply a percentage of ISET, the equation can be rewritten

where

•

n is the ratio of ISET to IBIAS as specified in Electrical Characteristics and shown in the graph

(2)

Since n is typically 18 for 2

μA ≤ ISET ≤ 1mA, the equation can be further simplified to

(3)

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 ISET. At ISET = 10μA, maximum dV/dt is 0.01V/μs; at ISET =

1mA, the 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 ISET greater than 100μA. For example, each

1V increase across the LM134 at ISET = 1 mA will increase junction temperature by ≈0.4°C in still air. Output

current (ISET) has a temperature coefficient of ≈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, therefore, must be taken into account when DC regulation is critical and ISET exceeds 100μA. Heat

sinking of the TO package or the TO-92 leads can reduce this effect by more than 3:1.

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LM334 Datasheet(PDF) 6 Page - Texas Instruments