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LM334SMX Datasheet(PDF) 5 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. LM334SMX
Description  3-Terminal Adjustable Current Sources
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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Typical Performance Characteristics (Continued)
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
SET) is the sum of the
current going through the SET resistor (I
R) and the LM134’s
bias current (I
BIAS), as shown in Figure 1.
A graph showing the ratio of these two currents is supplied
under Ratio of I
SET to IBIAS in the Typical Performance
Characteristics section. The current flowing through R
SET is
determined by V
R, which is approximately 214µV/˚K (64 mV/
298˚K
∼ 214µV/˚K).
Since (for a given set current) I
BIAS is simply a percentage of
I
SET, the equation can be rewritten
where n is the ratio of I
SET to IBIAS as specified in the Elec-
trical 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
SET.At
I
SET = 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 regu-
lation for I
SET greater than 100µA. For example, each 1V in-
crease across the LM134 at I
SET = 1 mA will increase junc-
tion temperature by
≈0.4˚C in still air. Output current (I
SET)
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 I
SET exceeds
100µA. Heat sinking of the TO-46 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 regu-
lation by at least an order of magnitude. DC characteristics
(with the exception of minimum input voltage), are not af-
fected.
Turn-On Voltage
DS005697-29
Ratio of I
SET to IBIAS
DS005697-3
DS005697-27
FIGURE 1. Basic Current Source
www.national.com
5




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