2-Terminal Current Source Boasts High Accuracy, Programmability and Stability - Power Electronics Technology May 2009

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SAM DAVIS, Editor in Chief
2-Terminal Current Source Boasts High
Accuracy, Programmability and Stability
A monolithic current source uses two external
resistors to program its output from 0.5 mA to 200
mA with 1% initial accuracy and a low temperature
coefficient.
L
tors (ROUT and RSET) to set an output current between 0.5 mA
and 200 mA. It is stable without input and output capacitors,
offering high dc and ac impedance, which allow operation in
intrinsically safe applications. Current regulation is better than
10 ppm/V for a VIN of 1.5 V to 40 V. The IC exhibits 1%
initial accuracy and a low temperature coefficient. Protection
features include reverse-voltage, reverse-current, short-circuit
and thermal shutdown with hysteresis. The LT3092 requires
a 0.5-mA minimum load or its output will not regulate.
The Table lists the possible relationships between RSET, ROUT
and the output current. Depending on the designer’s application, the RSET and ROUT resistors can be changed. If the
designer does not want to use low value resistors for ROUT,
then make RSET and ROUT larger. If there is a need for more
headroom, the resistors can be lower. If certain resistors have
a better temperature coefficient and stability with time, you
can adjust the values to work with them. The 3092 has high
enough gain and good frequency stability, so it is “user’s
choice” for resistors.
inear Technology’s recently introduced LT3092
is a 2-terminal programmable current source, typically
defined as a circuit that delivers or absorbs current.
Several applications for current source circuits include
but are not limited to:
• Voltage regulator without output capacitors
• AC current limiter
• Voltage clamp
• Active load
• Remote temperature sensor
• LED driver.
Before the LT3092, the simplest current source was
merely a voltage source with a resistor in series feeding the load resistance.
1 mA
Assuming the load resistance is much
smaller than the series resistor, the current available from this circuit is the ratio
LT1004
1.2 kΩ
of the voltage source to the series resistor.
1N457
Unfortunately, the only way to vary the
current is to change the value of the series
1 MΩ*
resistor, which doesn’t provide much cirQ1
PNP
cuit flexibility.
transistor
An improvement over the voltage
Q2
source-resistor approach is to configure
NPN
a 2-terminal current source using distransistor
crete components, as shown in Fig. 1.
1N457
Here, the temperature coefficients of the
1.2 kΩ
LT1004
LT1004 Zener and 1N457 diode don’t
provide a perfect match for the two transistors, so drift and accuracy are subject
* Insures start-up
to their parameter changes over temperature. Also, changing the output current Fig. 1. 2-terminal current source using discrete
requires a discrete circuit modification.
components does a reasonable job, but its accu0509PowStage_F1
Now, the monolithic LT3092 racy is limited to a few percent. (The top is IOUT;
(Fig. 2) requires only two external resis- the bottom is VIN.)
42 Power Electronics Technology | May 2009
PROGRAMMING THE CURRENT
The IC uses a precision “0” TC 10-μA
reference current source to program the
output current. As shown in Fig. 2, this
10-μA current source connects to the
noninverting input of a power operational amplifier. The power operational
amplifier provides a low impedance buffered output of the voltage on the noninverting input.
The 10-μA reference current from the
SET pin uses RSET resistor to generate a
voltage in the range of 100 mV to 1 V.
This voltage is applied across the ROUT
resistor that connects from the OUT pin
to the RSET resistor. Fig. 2 shows connections and the formula to calculate a
current source output.
With a 10-μA current source generating the reference, it is important to
minimize board leakage by encircling
the SET pin and circuitry with a guard
www.powerelectronics.com
powerstage
ring operated at a potential close to the
SET pin, and tying the guard ring to the
OUT pin. Guarding both sides of the
circuit board is required. Bulk leakage
reduction depends on the guard ring
width. A 10 nA of leakage into or out of
the SET pin and its associated circuitry
creates a 0.1% reference current error.
Leakages of this magnitude, coupled
with other sources of leakage, can cause
significant offset voltage and reference
current drift — especially over the possible operating temperature range.
The LT3092 does not require input
or output capacitors for stability in
many applications. Clean, tight PCB
layouts provide a low-reactance, wellcontrolled operating environment for
the IC without requiring capacitors for
frequency compensation.
Some applications require a capacitor in parallel with RSET to lower current source noise. This capacitor also
provides a soft-start for the current
source.
Although the LT3092 is stable without any capacitors over a variety of
operating conditions, it may be necessary to add capacitors because of the
input and output impedances encountered by the LT3092. These impedances may include resistive, capacitive
and inductive components, and may
be complex distributed networks. In
addition, the current source’s value
will differ between applications, and its
connection may be ground referenced,
power supply referenced or floating in a
signal line path.
If an application uses GND referred
capacitors on the input or output (particularly the input), pay attention to
the length of the lines powering and
VIN - VOUT = 1.3 V to 40 V
IN
LT3092
10 µA
+
–
SET
OUT
ROUT
RSET
ISOURCE = 10 µA ×
RSET
ROUT
Fig. 2. The LT3092 uses two resistors (ROUT and
RSET) to set an output current between 0.5 mA
0509PowStage_F2
and 200 mA.
Reference current change (%)
1
0
–1
0
25
50
75
Temperature (°C)
100
Fig. 3. Temperature change of the current source
at 1-mA output current.
Table. (below) Possible relationships between
RSET, ROUT and the output current.
RSET
ROUT
(Ω)
Output Current (mA)
20,000
1.0
200.0
10,000
1.0
100.0
5,000
1.0
50.0
2,500
1.0
25.0
1,000
1.0
10.0
500
1.0
5.0
50
1.0
0.5
(Ω)
0509PowStage_F3
www.powerelectronics.com returning ground from the circuit. In
the case where long power supply and
return lines are coupled with low ESR
input capacitors, application-specific
voltage spikes, oscillations and reliability concerns may be seen. This is not
an issue with LT3092 stability, but
rather the low ESR capacitor forming
a high-Q resonant tank circuit with the
inductance of the input wires. Adding
series resistance with the input of the
LT3092, or with the input capacitor,
often solves this. Resistor values of
0.1 Ω to 1 Ω are often sufficient to
dampen this resonance.
Give extra consideration to the use
of ceramic capacitors. The X5R and
X7R dielectrics result in more stable
characteristics and are more suitable for
use as the output capacitor. The X7R
type has better stability across temperature, while the X5R is less expensive
and is available in higher values.
Higher output current can be obtained
by paralleling multiple LT3092s together. The simplest approach is to run two
current sources side by side with both
of their inputs and outputs tied together. This allows the sum of the current
sources to deliver more output current
than a single device can deliver.
Another method of paralleling devices requires fewer components and helps
to share power between devices. Do
this by tying the individual SET pins
together and tying the individual IN
pins together. Then, connect the outputs in common using small lengths of
circuit board trace as ballast resistors to
promote equal current sharing.
The LT3092’s internal power and
thermal limiting circuitry protects itself
under overload conditions. For continuous normal load conditions, do not
exceed the 125°C maximum junction
temperature. Account for all thermal
resistance sources from junction-toambient. Furthermore, consider all adjacent heat generating sources on the PCB
within proximity of the LT3092.
The LT3092 is housed in the 8-lead
TSOT-23, 3-lead SOT-223 and 8-lead
3-mm × 3-mm DFN packages.
MAy 2009 | Power Electronics Technology
43
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