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LM334M Datasheet(PDF) 6 Page - Texas Instruments

Part No. LM334M
Description  LM134/LM234/LM334 3-Terminal Adjustable Current Sources
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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LM134, LM234, LM334
SNVS746C – MAY 2004 – REVISED MARCH 2005
www.ti.com
(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 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.
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.
Lead Resistance
The sense voltage which determines operating current of the LM134 is less than 100mV. At this level,
thermocouple or lead resistance effects should be minimized by locating the current setting resistor physically
close to the device. Sockets should be avoided if possible. It takes only 0.7
Ω contact resistance to reduce output
current by 1% at the 1 mA level.
Sensing Temperature
The LM134 makes an ideal remote temperature sensor because its current mode operation does not lose
accuracy over long wire runs. Output current is directly proportional to absolute temperature in degrees Kelvin,
according to the following formula:
(4)
Calibration of the LM134 is greatly simplified because of the fact that most of the initial inaccuracy is due to a
gain term (slope error) and not an offset. This means that a calibration consisting of a gain adjustment only will
trim both slope and zero at the same time. In addition, gain adjustment is a one point trim because the output of
the LM134 extrapolates to zero at 0°K, independent of RSET or any initial inaccuracy.
6
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Copyright © 2004–2005, Texas Instruments Incorporated
Product Folder Links: LM134 LM234 LM334




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