Application information LM134-LM234-LM334
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4 Application information
4.1 Slew rate
At slew rates above a threshold (see Figure 4 and Figure 5), the LM134, LM234, LM334 can
have a non-linear current characteristic. The slew rate at which this takes place is directly
proportional to I
set
. At I
set
= 10µA, dv/dt max. = 0.01V/µs ; at I
set
= 1mA, dv/dt max. = 1V/µs.
Slew rates of more than 1V/µs do not damage the circuit nor do they produce high currents.
4.2 Thermal effects
Internal heating can have a significant effect on current regulation for an I
set
above 100µA.
For example, each increase of 1V in the voltage across the LM134 at I
set
= 1mA will
increase the junction temperature by ≈ 0.4°C (in still air). The output current (I
set
) 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 a 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 I
set
is higher than 100µA.
4.3 Shunt capacitance
In certain applications, the 15pF value for the shunt capacitance should be reduced:
● because of loading problems,
● because of limitation of output impedance of the current source in a.c. applications.
You can easily reduce the capacitance by adding a FET as shown in Typical applications on
page 8.
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 input voltage).
4.4 Noise
The current noise produced by LM134, LM234, and 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 acceptable, and a
single amplifier can be built with a voltage gain higher than 2000.
4.5 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 locating the current setting resistor close to the device. Do not use sockets for
the ICs. A contact resistance of 0.7Ω is sufficient to decrease the output current by 1% at the
1mA level.
