LINEAR TECHNOLOGY LT6000 Technical data
Micropower Op Amps Work Down to 1.8V Total Supply,
Guaranteed over Temperature – Design Note 414
Glen Brisebois
Introduction
M i c ro p o we r o p a mp s e x t en d t he r u n t i m e o f ba t t e r y - p ow ered systems and reduce energy consumption in other
energy limited systems. Nevertheless, battery voltages
change as they are depleted. To maximize a system’s
run time, op amps should operate over a wide enough
supply range to make use of the complete range of battery voltages, from fully charged to fully depleted. The
®
new LT
6000 family of 1µA and 13µA op amps operates
on supplies as high as 16V all the way down to 1.8V,
guaranteed over temperature.
NiMH and Alkaline
A NiMH battery has a nominal cell voltage of 1.2V, but
it depletes to 0.9V, below which the voltage rapidly falls
off. The LT6000 family of op amps works directly from
two series NiMH cells taking full advantage of their entire
charge discharge cycle. Likewise, an alkaline battery has
a nominal cell voltage of 1.5V, but can deliver energy
down to depletion levels of a few hundred millivolts. So,
the LT6000 can happily operate from two series alkaline
cells, and just as well operate directly from a 9V alkaline
batt ery (6 series cells) from full char ge all the way down to
very extreme depletions (300mV average cell voltage for
1.8V total). Sure, other low voltage op amps can operate
at the depleted end of this battery range, but few of those
can also tolerate a 9V supply.
Supply Friendliness
Some micropower op amps have annoying properties
such as drawing exces sive current at star t-up (commonly
called carrots) or when the ou tput hits a supply rail. The se
current spikes defe at the purpose of the micropower operation by hastening battery discharge. Worse yet, they may
altogether prevent the supply from coming up in the case
of a current limited supply, effectively crowbarring the
system. Figure 1 shows the LT6000 and LT6003 supply
current vs applied suppl y voltage at various tempera tures.
The LT6000 family eliminates carrots or at least chews
them down to stumps.
25
LT6000
= 1
A
V
= 0.5V
V
CM
20
15
TA = 125°C
10
5
SUPPLY CURRENT PER AMPLIFIER (µA)
0
0.4
2.5
2.0
1.5
1.0
0.5
SUPPLY CURRENT PER AMPLIFIER (µA)
0
0.5 0.7 0.9
0.6 1.0
LT6003
= 1
A
V
= 0.5V
V
CM
TA = 25°C
TA = –55°C
0.8
TOTAL SUPPLY VOLTAGE (V)
TOTAL SUPPLY VOLTAGE (V)
1.2
1.3 1.5 1.7
1.1 1.9 2.1
1.6
1.4
TA = 125°C
TA = 25°C
TA = –55°C
1.8
DN414 F01
2.0
Figure 1. Clean Start-Up Characteristics
Without Current Spikes
Portable Gas Sensor
Figure 2 shows the LT6003 applied as an oxygen sensor
amplifi er. The oxygen sensor acts much like an air powered battery, and generates 100µA in one atmosphere of
fresh air (20.9% oxygen). It is designed to operate into
a 100Ω resistor, for a 10mV full-scale reading. The op
amp amplifi es this voltage with a gain of 100 as shown
(101 actually), for a 1V full-scale output. In terms of
, LTC, LT and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
04/07/414
100k
1%
100k
OXYGEN SENSOR
CITY TECHNOLOGY
4OX(2)
100Ω
1%
www.citytech.com
1%
LT6003
Figure 2. Micropower Oxygen Sensor
10M
1%
1.6V
V
OUT
I
SUPPLY
= 1V IN AIR
= 0.95µA
DN414 F02
OXYGEN SENSOR
CITY TECHNOLOGY
40X(2)
–V
E
+V
E
www.citytech.com
200k
20k
–
1/2 LT6001
+
V
S
R
F
10k
100Ω
V
S
A1
DN414 F03
–
A2
1/2 LT6001
+
0V WITHOUT OXYGEN
VS = 1.8V
I
SUPPLY
45µA WITHOUT OXYGEN
330Ω
V
= 1V IN AIR,
OUT
330Ω
= 145µA IN AIR,
+
–
monitoring environments for adequate human-livable
oxygen levels, 18% oxygen content translates to an
output voltage of 0.86V. Ox ygen contents below this are
considered hazardous. Oxygen deprivation in the lungs
causes immediate loss of consciousness and bears no
resemblance to holding your breath. Total supply current
for the circuit is 950nA. The 500µV worst-case input
offset voltage at room temperature contributes a 50mV
uncertainty in the output reading.
Bet ter low value accuracy can be obt ained by implementing
a t r a n s i m p e d an c e a p p r o a c h a s s h o w n in F i g u r e 3. O p a m p
A1 provides a buffered reference voltage so the circuit
is accurate all the way down to a zero-oxygen environment without clipping at ground. Op amp A2 provides
the current-to-voltage function through feedback resistor
. The sensor still sees the 100Ω termination, as the
R
F
manufacturer specifi es. The output voltage is still 1V in
normal atmosphere, but note that the noise gain is not
much higher than unit y so the output error due to offset is
now 500µV wors t case instead of the 50 mV of the previous
circuit. This considerable improvement in accuracy exac ts
some price in supply current, because the oxygen sensor
Figure 3. High Accuracy Oxygen Sensor
current is now provided back through R
by the op amp
F
output, which necessarily takes it from the supply. The
supply current is therefore oxygen-presence dependant.
Nevertheless, this solution is still ultralow power when
monitoring environments that are oxygen-free by design,
such as environments for food s torage and those designed
to inhibit combustion. It would also be ideal for portable
sensors where the detected substance is not oxygen
but is rather a hostile substance, which is not normally
present and is therefore usually low current.
Conclusion
The LT6000 and LT6003 famil y of op amps offer 13µA and
1µA micropower operation over a wide supply range from
18V all the way down to 1.8V, guaranteed over temperature. Careful attention was paid during the design phase
to minimizing gotchas such as supply current carrots.
They are ideal for maximizing battery life in portable applications, operating over a wide range of battery charge
levels and environments.
Data Sheet Download
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
For applications help,
call (408) 432-1900, Ext. 3755
dn414f LT/TP 0407 305K • PRINTED IN THE USA
© LINEAR TECHNOLOGY CORPORATION 2007
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