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)
(3)
Since n is typically 18 for 2μA ≤ ISET ≤ 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 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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