DN351 - Versatile Micropower Voltage Reference Provides Resistor Programmable Output from 0.4V to 18V

Versatile Micropower Voltage Reference Provides Resistor
Programmable Output from 0.4V to 18V – Design Note 351
Jon Munson
Introduction
Voltage reference integrated circuits are widely used to
establish accurate and stable voltages in analog circuits.
While calibration-grade references are based on buried
Zener diode technology (or even more exotic methods),
the ubiquitous “band-gap” technique is the workhorse
of the general purpose reference offerings. Band-gap
references have historically offered fixed 1.2V to 10V
outputs, along with a few adjustable models. The highly
miniaturized LT®6650 extends the scope of band-gap
technology to offer the guaranteed ability to operate on
a single supply down to 1.4V in ThinSOT™ packaging
and with an output voltage as low as 0.4V. The LT6650
may also be powered by or produce reference voltages
up to 18V and operate in either shunt mode or in a low
dropout (LDO) series mode. The LT6650 is easy to use,
sporting micropower dissipation (about 6μA of quiescent
current) and simple 2-resistor voltage programming.
Easy Output Voltage Programming
Figure 1 shows the basic connection for developing a
fixed 400mV ±1% reference voltage from any supply voltage in the range of 1.4V to 18V. The internal noninverting
op amp input port is always driven by a 400mV band-gap
derived signal and the inverting op amp port is pinned
out as a user connection. In this circuit, the op amp is
simply provided with 100% negative feedback, thereby
forming a unity-gain buffer for the reference source.
In applications where a reference potential greater than
0.4V is required, the simple addition of a feedback voltage divider programs the buffer op amp to provide gain.
Figure 2 shows the typical connections for developing a
reference voltage above 0.4V with the added feedback
components. This configuration provides programmable reference voltages anywhere up to 0.35V below
the supply potential used, the dropout voltage. Resistor
RG is chosen in the range from 10k to 100k to set the
quiescent loading of the reference, then resistor RF is
simply selected for the required gain. While this illustration indicates fixed component values, the introduction
of a variable element can provide a means of dynamically varying the reference output if desired. Figure 2
also shows additional input RC filtering which improves
rejection of supply noise and a feedback capacitor that
serves to both reduce noise gain and improve damping
of the load response. The low operating current of the
LT6650 and the input series resistor do not impair the
low dropout performance significantly.
Create a Virtual Ground for Unipolar Processing
of Bidirectional Signals
The LT6650 often finds use in single supply data acquisition circuits where a low voltage offset is needed to
provide a shifted “virtual ground.” Most ADC inputs can
digitize right down to 0V input, but a single-supply input
amplifier will not retain its accuracy at that low level,
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VIN
1.4V TO 18V
IQ ≈ 6μA
LT6650
VR = 400mV
REFERENCE
OUT 5
OUT
LT6650
GND
FB 1
2
1μF
Figure 1. Simple 400mV Series Reference
11/04/351_conv
5
IN
1μF
GND
2
4
VOUT
0.400V
–
1μF
VOUT7tRF/RG)
1k
VS
+
FB
1nF
1
RF
150k
VOUT
1V
1μF
RG
100k
DN351 F02
Figure 2. Typical Series Connection for
Output Voltages Greater than 0.4V
VAUTOMOTIVE
5V
LT1990
R5
900k
R7
100k
8
7
–+
2
R1
1M
3
R2
1M
RS
IL
R6
100k
+
6
VREF = 1.5V
R10
100k
4
IN
OUT
LT6550
GND FB
–12 ≤ VCM ≤ 73V
VOUT = VREFt*Lt3S)
1nF
54.9k
VOUT
–
R3
40k
R4
40k
R8
900k
5
R9
100k
30k
1
FOR RS = 1mΩ:
VOUT = 0.5V for IL = 100A
VOUT = 1.5V for IL = 0A
VOUT = 2.5V for IL = –100A
1μF
DN351 F03
Figure 3. Offsetting a Bidirectional Signal for Unipolar Processing
since the output is “saturating” (even with rail-to-rail
types). A design solution is to have a voltage reference
circuit drive the REF port of input instrumentation
amplifiers (IA), thereby introducing a controlled offset
mapping that allows the ADC to accurately capture the
“zero input” signal level, or even provide a controlled
negative signal conversion range within a positive-only
input window. Figure 3 shows a single supply powered
LT1990 difference amp sensing a bidirectional motor
current. The LT6650 reference is configured to provide
an optimal REF input level for the circuit (1.5V in this
example) which both establishes the working common
mode input range and introduces an output offset that
maps the desired bidirectional signal span into a single
supply ADC conversion range. In high accuracy applications, the offset voltage itself may also be digitized so
that software algorithms can accurately “auto-zero”
the measurements. In multichannel data acquisition
systems, a single LT6650 can generally provide offset
signaling to an entire IA array.
Shunt Mode Operation Works Like Precision
Zener Diode
The LT6650 can easily be configured to behave much
like a traditional Zener reference diode, but with far
better regulation characteristics and the flexibility to
be set to any voltage between 1.4V and 18V. This mode
of operation allows the LT6650 to form simple negative
references or other precision biasing functions. Figure
4 shows a simple negative reference circuit configuration. The programming is done just as with series mode
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operation, only the load capacitor is increased in value
to optimize transient response. The Zener “knee” of the
shunt configuration is about 10μA (with RG set to 100k)
and accurate regulation to 200μA is provided.
Conclusion
The LT6650 is an extremely flexible voltage control
element, able to form accurate positive, negative or
even floating reference voltages. With micropower
operation over a wide 1.4V to 18V supply range and
miniature ThinSOT packaging, the LT6650 offers design
solutions for both portable and industrial applications.
For single supply data-acquisition circuitry, the low
400mV output capability offers a simple virtual ground
offsetting means that doesn’t unduly sacrifice dynamic
range. Thanks to simple resistor-based programming,
references of various arbitrary voltages may be produced using a single LT6650 bill-of-material item, thus
reducing procurement and inventory costs.
4
5
IN
OUT
LT6650
GND
FB
1nF
1
RF
1.15M
10μF
RG
100k
2
RB
VOUTo7tRF/RG)
–VS
VOUT
–5V
DN351 F04
Figure 4. Typical Configuration as
– 0.4V to –18V Shunt Reference
For applications help,
call (408) 432-1900
dn351f_conv LT/TP 1104 305K • PRINTED IN THE USA
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© LINEAR TECHNOLOGY CORPORATION 2004
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