NSC LM134 3-terminal adjustable current source Datasheet

LM134/LM234/LM334
3-Terminal Adjustable Current Sources
General Description
The LM134/LM234/LM334 are 3-terminal adjustable current
sources featuring 10,000:1 range in operating current, excellent current regulation and a wide dynamic voltage range of
1V to 40V. Current is established with one external resistor
and no other parts are required. Initial current accuracy is
± 3%. The LM134/LM234/LM334 are true floating current
sources with no separate power supply connections. In addition, reverse applied voltages of up to 20V will draw only a
few dozen microamperes of current, allowing the devices to
act as both a rectifier and current source in AC applications.
The sense voltage used to establish operating current in the
LM134 is 64mV at 25˚C and is directly proportional to absolute temperature (˚K). The simplest one external resistor
connection, then, generates a current with ≈+0.33%/˚C temperature dependence. Zero drift operation can be obtained
by adding one extra resistor and a diode.
Applications for the current sources include bias networks,
surge protection, low power reference, ramp generation,
LED driver, and temperature sensing. The LM234-3 and
LM234-6 are specified as true temperature sensors with
guaranteed initial accuracy of ± 3˚C and ± 6˚C, respectively.
These devices are ideal in remote sense applications because series resistance in long wire runs does not affect accuracy. In addition, only 2 wires are required.
The LM134 is guaranteed over a temperature range of
−55˚C to +125˚C, the LM234 from −25˚C to +100˚C and the
LM334 from 0˚C to +70˚C. These devices are available in
TO-46 hermetic, TO-92 and SO-8 plastic packages.
Features
n
n
n
n
n
n
Operates from 1V to 40V
0.02%/V current regulation
Programmable from 1µA to 10mA
True 2-terminal operation
Available as fully specified temperature sensor
± 3% initial accuracy
Connection Diagrams
SO-8
Surface Mount Package
SO-8 Alternative Pinout
Surface Mount Package
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DS005697-24
Order Number LM334M or
LM334MX
See NS Package Number M08A
TO-46
Metal Can Package
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Order Number LM334SM or
LM334SMX
See NS Package Number M08A
V− Pin is electrically connected to case.
Bottom View
Order Number LM134H,
LM234H or LM334H
See NS Package
Number H03H
TO-92 Plastic Package
DS005697-10
Bottom View
Order Number LM334Z, LM234Z-3 or LM234Z-6
See NS Package Number Z03A
© 2000 National Semiconductor Corporation
DS005697
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LM134/LM234/LM334 3-Terminal Adjustable Current Sources
March 2000
LM134/LM234/LM334
Absolute Maximum Ratings (Note 1)
LM234/LM234-3/LM234-6
−25˚C to +100˚C
LM334
0˚C to +70˚C
Soldering Information
TO-92 Package (10 sec.)
260˚C
TO-46 Package (10 sec.)
300˚C
SO Package
Vapor Phase (60 sec.)
215˚C
Infrared (15 sec.)
220˚C
See AN-450 “Surface Mounting Methods and Their Effect on
Product Reliability” (Appendix D) for other methods of soldering surface mount devices.
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
V+ to V− Forward Voltage
LM134/LM234/LM334
LM234-3/LM234-6
V+ to V− Reverse Voltage
R Pin to V− Voltage
Set Current
Power Dissipation
ESD Susceptibility (Note 6)
Operating Temperature Range (Note 5)
LM134
40V
30V
20V
5V
10 mA
400 mW
2000V
−55˚C to +125˚C
Electrical Characteristics (Note 2)
Parameter
Conditions
LM134/LM234
Min
Typ
LM334
Max
Min
Typ
Units
Max
Set Current Error, V+ =2.5V,
10µA ≤ ISET ≤ 1mA
3
(Note 3)
1mA < ISET ≤ 5mA
2µA ≤ ISET < 10µA
5
8
%
8
12
%
Ratio of Set Current to
100µA ≤ ISET ≤ 1mA
Bias Current
1mA ≤ ISET ≤ 5mA
Minimum Operating Voltage
14
18
23
6
14
14
18
%
26
14
2 µA≤ISET≤100 µA
18
2µA ≤ ISET ≤ 100µA
0.8
0.8
V
100µA < ISET ≤
1mA
0.9
0.9
V
1mA < ISET ≤ 5mA
1.0
1.0
V
23
18
26
Average Change in Set Current
2µA ≤ ISET ≤ 1mA
with Input Voltage
1.5 ≤ V+ ≤ 5V
0.02
0.05
0.02
0.1
%/V
5V ≤ V+ ≤ 40V
0.01
0.03
0.01
0.05
%/V
1mA < ISET ≤ 5mA
Temperature Dependence of
1.5V ≤ V ≤ 5V
0.03
0.03
5V ≤ V ≤ 40V
0.02
0.02
25µA ≤ ISET ≤ 1mA
0.96T
T
1.04T
0.96T
T
%/V
%/V
1.04T
Set Current (Note 4)
Effective Shunt Capacitance
15
15
pF
Note 1: .“Absolute Maximum Ratings” indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional, but do not guarantee specific performance limits.
Note 2: Unless otherwise specified, tests are performed at Tj = 25˚C with pulse testing so that junction temperature does not change during test
Note 3: Set current is the current flowing into the V+ pin. For the Basic 2-Terminal Current Source circuit shown on the first page of this data sheet. ISET is determined
by the following formula: ISET = 67.7 mV/RSET ( @ 25˚C). Set current error is expressed as a percent deviation from this amount. ISET increases at 0.336%/˚C @ Tj
= 25˚C (227 µV/˚C).
Note 4: ISET is directly proportional to absolute temperature (˚K). ISET at any temperature can be calculated from: ISET = Io (T/To) where Io is ISET measured at To
(˚K).
Note 5: For elevated temperature operation, TJ max is:
LM134
150˚C
LM234
125˚C
LM334
100˚C
Thermal Resistance
TO-92
TO-46
SO-8
θja (Junction to Ambient)
180˚C/W (0.4" leads)
440˚C/W
165˚C/W
32˚C/W
80˚C/W
160˚C/W (0.125" leads)
θjc (Junction to Case)
N/A
Note 6: Human body model, 100pF discharged through a 1.5kΩ resistor.
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2
Parameter
Conditions
LM234-3
Min
Set Current Error, V+ =2.5V,
100µA ≤ ISET ≤
1mA
(Note 3)
TJ = 25˚
Typ
Equivalent Temperature Error
Ratio of Set Current to
100µA ≤ ISET ≤
1mA
14
18
LM234-6
Max
Min
Typ
Units
Max
±1
±2
%
±3
±6
˚C
26
14
18
26
Bias Current
Minimum Operating Voltage
100µA ISET ≤ 1mA
Average Change in Set Current
100µA ≤ ISET ≤
1mA
with Input Voltage
1.5 ≤ V+ ≤ 5V
0.9
0.02
5V ≤ V ≤ 30V
+
Temperature Dependence of
100µA ≤ ISET ≤
1mA
0.98T
0.9
0.05
0.01
0.03
T
1.02T
0.02
0.97T
V
0.01
%/V
0.01
0.05
%/V
T
1.03T
Set Current (Note 4) and
±2
Equivalent Slope Error
Effective Shunt Capacitance
15
3
±3
15
%
pF
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LM134/LM234/LM334
Electrical Characteristics (Note 2)
LM134/LM234/LM334
Typical Performance Characteristics
Output Impedance
Maximum Slew Rate
Linear Operation
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Start-Up
Transient Response
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Voltage Across RSET (VR)
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Current Noise
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DS005697-35
4
(Continued)
Turn-On Voltage
Ratio of ISET to IBIAS
LM134/LM234/LM334
Typical Performance Characteristics
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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 contact resistance.
where n is the ratio of ISET to IBIAS as specified in the Electrical Characteristics Section and shown in the graph. Since
n is typically 18 for 2µA ≤ ISET ≤ 1mA, the equation can be
further simplified to
CALCULATING RSET
for most set currents.
The total current through the LM134 (ISET) is the sum of the
current going through the SET resistor (IR) and the LM134’s
bias current (IBIAS), as shown in Figure 1.
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-46 package or the TO-92
leads can reduce this effect by more than 3:1.
DS005697-27
FIGURE 1. Basic Current Source
A graph showing the ratio of these two currents is supplied
under Ratio of ISET to IBIAS in the Typical Performance
Characteristics section. The current flowing through RSET is
determined by VR, which is approximately 214µV/˚K (64 mV/
298˚K ∼ 214µV/˚K).
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.
Since (for a given set current) IBIAS is simply a percentage of
ISET, the equation can be rewritten
5
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LM134/LM234/LM334
Application Hints
APPLICATION AS A ZERO TEMPERATURE
COEFFICENT CURRENT SOURCE
(Continued)
NOISE
Adding a diode and a resistor to the standard LM134 configuration can cancel the temperature-dependent characteristic of the LM134. The circuit shown in Figure 3 balances
the positive tempco of the LM134 (about +0.23 mV/˚C) with
the negative tempco of a forward-biased silicon diode (about
−2.5 mV/˚C).
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 0.7Ω 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:
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.
DS005697-28
FIGURE 3. Zero Tempco Current Source
The set current (ISET) is the sum of I1 and I2, each contributing approximately 50% of the set current, and IBIAS. IBIAS is
usually included in the I1 term by increasing the VR value
used for calculations by 5.9%. (See CALCULATING RSET.)
The first step is to minimize the tempco of the circuit, using
the following equations. An example is given using a value of
+227µV/˚C as the tempco of the LM134 (which includes the
IBIAS component), and −2.5 mV/˚C as the tempco of the diode (for best results, this value should be directly measured
or obtained from the manufacturer of the diode).
DS005697-4
FIGURE 2. Gain Adjustment
This property of the LM134 is illustrated in the accompanying
graph. Line abc is the sensor current before trimming. Line
a'b'c' is the desired output. A gain trim done at T2 will move
the output from b to b' and will simultaneously correct the
slope so that the output at T1 and T3 will be correct. This
gain trim can be done on RSET or on the load resistor used
to terminate the LM134. Slope error after trim will normally
be less than ± 1%. To maintain this accuracy, however, a low
temperature coefficient resistor must be used for RSET.
A 33 ppm/˚C drift of RSET will give a 1% slope error because
the resistor will normally see about the same temperature
variations as the LM134. Separating RSET from the LM134
requires 3 wires and has lead resistance problems, so is not
normally recommended. Metal film resistors with less than
20 ppm/˚C drift are readily available. Wire wound resistors
may also be used where best stability is required.
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With the R1 to R2 ratio determined, values for R1 and R2
should be determined to give the desired set current. The
formula for calculating the set current at T = 25˚C is shown
below, followed by an example that assumes the forward
voltage drop across the diode (VD) is 0.6V, the voltage
across R1 is 67.7mV (64 mV + 5.9% to account for IBIAS),
and R2/R1 = 10 (from the previous calculations).
6
If the estimate for the tempco of the diode’s forward voltage
drop was off, the tempco cancellation is still reasonably effective. Assume the tempco of the diode is 2.6mV/˚C instead
of 2.5mV/˚C (an error of 4%). The tempco of the circuit is
now:
(Continued)
This circuit will eliminate most of the LM134’s temperature
coefficient, and it does a good job even if the estimates of the
diode’s characteristics are not accurate (as the following example will show). For lowest tempco with a specific diode at
the desired ISET, however, the circuit should be built and
tested over temperature. If the measured tempco of ISET is
positive, R2 should be reduced. If the resulting tempco is
negative, R2 should be increased. The recommended diode
for use in this circuit is the 1N457 because its tempco is centered at 11 times the tempco of the LM134, allowing R2 = 10
R1. You can also use this circuit to create a current source
with non-zero tempcos by setting the tempco component of
the tempco equation to the desired value instead of 0.
EXAMPLE: A 1mA, Zero-Tempco Current Source
First, solve for R1 and R2:
A 1mA LM134 current source with no temperature compensation would have a set resistor of 68Ω and a resulting
tempco of
So even if the diode’s tempco varies as much as ± 4% from
its estimated value, the circuit still eliminates 98% of the
LM134’s inherent tempco.
Typical Applications
Ground Referred Fahrenheit Thermometer
The values of R1 and R2 can be changed to standard 1% resistor values (R1 = 133Ω and R2 = 1.33kΩ) with less than a
0.75% error.
If the forward voltage drop of the diode was 0.65V instead of
the estimate of 0.6V (an error of 8%), the actual set current
will be
DS005697-15
*Select R3 = VREF/583µA. VREF may be any stable positive voltage ≥ 2V
Trim R3 to calibrate
an error of less than 5%.
7
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LM134/LM234/LM334
Application Hints
LM134/LM234/LM334
Typical Applications
(Continued)
Terminating Remote Sensor for Voltage Output
Low Output Impedance Thermometer
DS005697-6
*Output impedance of the LM134 at the “R” pin is approximately
DS005697-14
where R2 is the equivalent external resistance connected from the V− pin
to ground. This negative resistance can be reduced by a factor of 5 or
more by inserting an equivalent resistor R3 = (R2/16) in series with the
output.
Low Output Impedance Thermometer
Higher Output Current
DS005697-5
DS005697-16
*Select R1 and C1 for optimum stability
Basic 2-Terminal Current Source
DS005697-1
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LM134/LM234/LM334
Typical Applications
(Continued)
Micropower Bias
Low Input Voltage Reference Driver
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Ramp Generator
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9
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LM134/LM234/LM334
Typical Applications
(Continued)
1.2V Reference Operates on 10 µA and 2V
1.2V Regulator with 1.8V Minimum Input
DS005697-20
*Select ratio of R1 to R2 to obtain zero temperature drift
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*Select ratio of R1 to R2 for zero temperature drift
Zener Biasing
Alternate Trimming Technique
Buffer for Photoconductive Cell
DS005697-51
DS005697-50
DS005697-49
*For ± 10% adjustment, select RSET
10% high, and make R1 ≈ 3 RSET
FET Cascoding for Low Capacitance and/or Ultra High Output Impedance
DS005697-21
*Select Q1 or Q2 to ensure at least 1V across the LM134. Vp (1 −
ISET/IDSS) ≥ 1.2V.
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10
LM134/LM234/LM334
Typical Applications
(Continued)
Generating Negative Output Impedance
In-Line Current Limiter
DS005697-23
*ZOUT ≈ −16 • R1 (R1/VIN must not exceed ISET)
DS005697-9
*Use minimum value required to ensure stability of protected device. This
minimizes inrush current to a direct short.
Schematic Diagram
DS005697-11
11
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LM134/LM234/LM334
Physical Dimensions
inches (millimeters) unless otherwise noted
Order Number LM134H, LM234H or LM334H
NS Package Number H03H
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12
LM134/LM234/LM334
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
SO Package (M)
Order Number LM334M, LM334MX,
LM334SM or LM334SMX
NS Package Number M08A
Order Number LM334Z, LM234Z-3 or LM234Z-6
NS Package Number Z03A
13
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LM134/LM234/LM334 3-Terminal Adjustable Current Sources
Notes
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