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LM334 Datasheet(PDF) 5 Page - STMicroelectronics

Part No. LM334
Description  THREE TERMINAL ADJUSTABLE CURRENT SOURCES
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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LM134 - LM234 - LM334
5/11
APPLICATION HINTS
SLEW RATE
At slew rates above a threshold (see curve) the
LM134, LM234, LM334 can have a non-linear cur-
rent characteristic. The slew rate at which this
takes place is directly proportional to Iset. At Iset =
10µA, dv/dt max. = 0.01V/µs ; at Iset = 1mA, dv/dt
max. = 1V/µs. Slew rates of more than 1V/µs do
not damage the circuit nor do they produce high
currents.
THERMAL EFFECTS
Internal heating can have a significant effect on
current regulation for an Iset above 100µA. For ex-
ample, each increase of 1V in the voltage across
the LM134 at Iset = 1mA will increase the junction
temperature by
≈ 0.4°C (in still air). The output
current (Iset) has a temperature coefficient of
about 0.33%/°C. Thus the change in current due
to the increase in temperature will be (0.4) (0.33) =
0.132%. This is degradation of 10 : 1 in regulation
versus the true electrical effects. Thermal effects
should be taken into account when d.c. regulation
is critical and Iset is higher than 100µA.
SHUNT CAPACITANCE
In certain applications, the 15pF value for the
shunt capacitance should be reduced :
u because of loading problems,
u because of limitation of output impedance
of the current source in a.c. applications.
This reduction of capacitance can be easily
carried out by adding a FET as indicated in
the typical applications.
The value of this capacitance can be reduced by
at least 3pF and regulation can be improved by an
order of magnitude without any modifications of
the d.c. characteristics (except for the minimum in-
put voltage).
NOISE
The current noise produced by LM134, LM234,
LM334 is about 4 times that of a transistor. If the
LM134, LM234, LM334 is used as an active load
for a transistor amplifier, the noise at the input will
increase by about 12dB. In most cases this is ac-
ceptable, and a single amplifier can be built with a
voltage gain higher than 2000.
LEAD RESISTANCE
The sense voltage which determines the current
of the LM134, LM234, LM334 is less than 100mV.
At this level, the thermocouple effects and the
connection resistance should be reduced by locat-
ing the current setting resistor close to the device.
Do not use sockets for the ICs. A contact resis-
tance of 0.7
Ω is sufficient to decrease the output
current by 1% at the 1mA level.
SENSING TEMPERATURE
The LM134, LM234, LM334 are excellent remote
controlled temperature sensors because their op-
eration as current sources preserves their accura-
cy even in the case of long connecting wires. The
output current is directly proportional to the abso-
lute temperature in Kelvin degrees according to
the following equation.
The calibration of the LM134, LM234, LM334 is
simplified by the fact that most of the initial accura-
cy is due to gain limitation (slope error) and not an
offset. Gain adjustment is a one point trim be-
cause the output of the device extrapolates to zero
at 0°K.
This particularity of the LM134, LM234, LM334 is
illustrated in the above diagram. Line abc repre-
sents the sensor current before adjustment and
line a’b’c’ represents the desired output. A gain
adjustment provided at T2 will move the output
from b to b’ and will correct the slope at the same
time so that the output at T1 and T3 will be correct.
This gain adjustment can be carried out by means
of Rset or the load resistor used in the circuit. After
adjustment, the slope error should be less than
1%. A low temperature coefficient for Rset is nec-
essary to keep this accuracy. A 33ppm/°C temper-
ature drift of Rset will give an error of 1% on the
slope because the resistance follows the same
temperature variations as the LM134, LM234,
LM334. Three wires are required to isolate Rset
from the LM134, LM234, LM334. Since this solu-
tion is not recommended. Metal-film resistors with
a drift less than 20ppm/°C are now available.
Wirewound resistors can be used when very high
stability is required.
Iset =
227µV/°K) (T
()
Rset
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