DN414 - Micropower Op Amps Work Down to 1.8V Total Supply, Guaranteed over Temperature

Micropower Op Amps Work Down to 1.8V Total Supply,
Guaranteed over Temperature – Design Note 414
Glen Brisebois
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
battery (6 series cells) from full charge 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 excessive current at start-up (commonly
called carrots) or when the output hits a supply rail. These
current spikes defeat 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 supply voltage at various temperatures.
The LT6000 family eliminates carrots or at least chews
them down to stumps.
04/07/414
SUPPLY CURRENT PER AMPLIFIER (μA)
25
20
LT6000
AV = 1
VCM = 0.5V
15
TA = 125°C
TA = 25°C
10
TA = –55°C
5
0
0.4 0.6
0.8 1.0 1.2 1.4 1.6 1.8
TOTAL SUPPLY VOLTAGE (V)
2.0
2.5
SUPPLY CURRENT PER AMPLIFIER (μA)
Introduction
Micropower op amps extend the run time of battery-powered 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.
2.0
LT6003
AV = 1
VCM = 0.5V
TA = 125°C
1.5
1.0
TA = 25°C
0.5
TA = –55°C
0
0.5
0.7
0.9 1.1 1.3 1.5 1.7 1.9
TOTAL SUPPLY VOLTAGE (V)
2.1
DN414 F01
Figure 1. Clean Start-Up Characteristics
Without Current Spikes
Portable Gas Sensor
Figure 2 shows the LT6003 applied as an oxygen sensor
amplifier. 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 amplifies 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.
10M
1%
100k
1%
OXYGEN SENSOR
CITY TECHNOLOGY
4OX(2)
100k
1%
RF
10k
OXYGEN SENSOR
CITY TECHNOLOGY
40X(2)
100Ω
–VE
1.6V
–
A2
1/2 LT6001
+
LT6003
VOUT = 1V IN AIR
ISUPPLY = 0.95μA
+VE
www.citytech.com
330Ω
VOUT = 1V IN AIR,
0V WITHOUT OXYGEN
330Ω
–
DN414 F02
100Ω
1%
VS
–
VS
200k
www.citytech.com
Figure 2. Micropower Oxygen Sensor
monitoring environments for adequate human-livable
oxygen levels, 18% oxygen content translates to an
output voltage of 0.86V. Oxygen 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.
Better low value accuracy can be obtained by implementing
a transimpedance approach as shown in Figure 3. Op amp
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
RF. The sensor still sees the 100Ω termination, as the
manufacturer specifies. The output voltage is still 1V in
normal atmosphere, but note that the noise gain is not
much higher than unity so the output error due to offset is
now 500μV worst case instead of the 50mV of the previous
circuit. This considerable improvement in accuracy exacts
some price in supply current, because the oxygen sensor
+
20k
VS = 1.8V
ISUPPLY = 145μA IN AIR,
45μA WITHOUT OXYGEN
A1
1/2 LT6001
+
DN414 F03
Figure 3. High Accuracy Oxygen Sensor
current is now provided back through RF by the op amp
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 storage 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 family 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
For applications help,
call (408) 432-1900, Ext. 3755
www.linear.com
Linear Technology Corporation
dn414f LT/TP 0407 305K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
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