Datasheet

LM134, LM234, LM334
SNVS746C –MAY 2004–REVISED MARCH 2005 www.ti.com
Figure 5. Basic Current Source
A graph showing the ratio of these two currents is supplied under Ratio of I
SET
to I
BIAS
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).
(1)
Since (for a given set current) I
BIAS
is simply a percentage of I
SET
, the equation can be rewritten
(2)
where n is the ratio of I
SET
to I
BIAS
as specified in the 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
(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 I
SET
. At I
SET
= 10μA, maximum dV/dt is 0.01V/μs; at I
SET
=
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 I
SET
greater than 100μA. For example, each
1V increase across the LM134 at I
SET
= 1 mA will increase junction 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 regulation by at least an order of magnitude. DC characteristics (with the exception
of minimum input voltage), are not affected.
NOISE
Current noise generated by the LM134 is approximately 4 times the shot noise of a transistor. If the LM134 is
used as an active load for a transistor amplifier, input referred noise will be increased by about 12dB. In many
cases, this is acceptable and a single stage amplifier can be built with a voltage gain exceeding 2000.
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