LINEAR TECHNOLOGY LT6650 Technical data

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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 miniatur-

®

ized LT
to offer the guaranteed ability to operate on a single
supply down to 1.4V in ThinSOT

6650 extends the scope of band-gap technology

TM

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 unity­gain buffer for the reference source.

V

IN

1.4V TO 18V

I

≈ 6µA

Q

LT6650

V

= 400mV

1µF

R

REFERENCE

GND

2

+

OUT

5

V

OUT

0.400V

–

FB

1

1µF

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 R
chosen in the range from 10k to 100k to set the quiescent
loading of the reference, then resistor R
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, since

, LTC and LT are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.

1k

V

S

1µF

45

OUTIN

1

FB

LT6650

GND

2

V

= 0.4V • (1 + RF/RG)

OUT

R

F

1nF

150k

R

G

100k

DN351 F02

G

is simply

F

V

OUT

1V

1µF

is

11/04/351

Figure 1. Simple 400mV Series Reference

Figure 2. Typical Series Connection for
Output Voltages Greater than 0.4V

V

AUTOMOTIVE

V

+

–

I

L

–12 ≤ VCM ≤ 73V
= V

OUT

REF

R

S

+ (10 • IL • RS)

5V

IN OUT

LT6550

GND

V

= 1.5V

REF

54.9k

FB

1nF

30k

1µF

7

2

3

4

LT1990

R1
1M

R2
1M

R3
40k

1

Figure 3. Offsetting a Bidirectional Signal for Unipolar Processing

R5

900k

+

–

R8

R4

900k

40k

= 1mΩ:

FOR R

S

= 0.5V for IL = 100A

V

OUT

= 1.5V for IL = 0A

V

OUT

= 2.5V for IL = –100A

V

OUT

100k

R6
100k

100k

R9
100k

R7

R10

DN351 F03

8

6

V

OUT

5

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 operation,

only the load capacitor is increased in value to optimize
transient response. The Zener “knee” of the shunt con­figuration is about 10µA (with R

set to 100k) and

G

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.

45

OUTIN

1

FB

LT6650

GND

2

V

R

–V

S

OUT

B

Figure 4. Typical Configuration as
–0.4V to – 18V Shunt Reference

R

F

1nF

1.15M

R

G

100k

= –0.4V • (1 + RF/RG)

DN351 F04

10µF

V
–5V

OUT

Data Sheet Download

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