LM334SMX National Semiconductor, LM334SMX Datasheet - Page 5

• Manufacturer Part Number: LM334SMX
• Description: 3-Terminal Adjustable Current Sources
• Manufacturer: National Semiconductor

Typical Performance Characteristics
Turn-On Voltage
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
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
298˚K
Since (for a given set current) I
I
SET
, the equation can be rewritten
214µV/˚K).
FIGURE 1. Basic Current Source
BIAS
R
SET
, which is approximately 214µV/˚K (64 mV/
), as shown in Figure 1 .
SET
to I
BIAS
BIAS
in the Typical Performance
is simply a percentage of
DS005697-27
SET
R
) is the sum of the
) and the LM134’s
DS005697-29
SET
is
Ratio of I
(Continued)
5
where n is the ratio of I
trical Characteristics Section and shown in the graph. Since
n is typically 18 for 2µA
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 regu-
lation for I
crease across the LM134 at I
tion temperature by 0.4˚C in still air. Output current (I
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
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.
SET
SET
= 10µA, maximum dV/dt is 0.01V/µs; at I
to I
SET
BIAS
greater than 100µA. For example, each 1V in-
SET
I
SET
to I
SET
BIAS
1mA, the equation can be
= 1 mA will increase junc-
as specified in the Elec-
DS005697-3
SET
SET
www.national.com
= 1mA, the
exceeds
SET
SET
. At
)