DN63 - 2 AA Cells Replace 9V Battery, Extend Operating Life

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2 AA Cells Replace 9V Battery, Extend Operating Life
Design Note 63
Steve Pietkiewicz
Operating life is an important feature in many portable
battery-operated systems. In many cases the power
source is the ubiquitous 9V “transistor” battery. 5V
generation is accomplished with a linear regulator. Significant gains in battery life can be obtained by replacing
the 9V/linear regulator combination with 2 AA cells and
a step-up switching regulator. Two (alkaline) AA cells
occupy 1.3 cubic inches, the same as a 9V battery, but
contains 6WH of energy, compared to just 4WH in an
alkaline 9V battery. Two AA cells also cost less than a 9V
battery.1 The additional energy in the AA cells provides
longer operating life when compared to a 9V battery
based solution.
An evaluation of the three approaches with a 30mA load
illustrates the differences in battery life. An HP7100B
strip chart recorder provides a nonvolatile record of
circuit performance. The linear regulator circuit shown
in Figure 1 uses an LT®1120 micropower low dropout
regulator IC. A minimum of external components are required. No inductors or diodes are needed; however, the
linear step-down process is inherently inefficient. The
step-down switcher shown in Figure 2 uses an LT1173
configured in step-down mode driven from an alkaline
9V battery. In Figure 3 the step-up circuit uses an LT1173
configured in step-up mode driven from a pair of alkaline
AA cells. The two switching circuits require an external
inductor, diode and output capacitor in addition to the IC.
Circuit operation of the switching step-down regulator
is straightforward. A comparator inside the LT1173
senses output voltage on its “sense” pin. When VOUT
drops below 5V, the on-chip switch cycles. As current
ramps up and ramps down in L1, it flows into C1 and the
load, raising output voltage. When VOUT rises above 5V,
the cycling action stops and the regulator goes into a
standby mode, pulling 110μA from the supply. C1 is left
to supply energy to the load. These “bursts” of cycles
occur as needed to keep the output voltage at 5V. 50mV
of hysteresis at the sense pin eliminates the need for frequency compensation. The step-up regulator operates
in a similar fashion, although in this case the inductor
current flows into the load only on the discharge half of
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respective owners.
470Ω
IL
VIN
SW1
SENSE
+
9V
BATTERY
+
LT1173-5
82μH*
10μF
VOUT
5V OUTPUT
SW2
GND
+
100μF
IN5818
BATTERY = 9V DURACELL ALKALINE #MN1604
*TOKO 262LYF-0091K
%/t'
Figure 2. 9V to 5V Step-Down Regulator
VIN
VOUT
LT1120
+
9V
BATTERY
5V
OUTPUT
+
1M
82μH*
˜'
220Ω
'#
˜'
GND
1M
BATTERY = 9V DURACELL ALKALINE #MN1604
VIN
IL
+
˜'
+
2 w AA
CELLS
+
SW1
10μF
1. A quick check at the local drugstore yielded $2.99 for a 4-pack of alkaline AA cells
and $2.49 for a single 9V battery (after $1.00 mail-in rebate).
VOUT
5V OUTPUT
SENSE
%/t'
Figure 1. 9V to 5V Linear Regulator
IN5818
LT1173-5
GND
SW2
+
100μF
BATTERY = 2w DURACELL "AA" ALKALINE #MN1500
*TOKO 262LYF-0091K
Figure 3. 3V to 5V Step-Up Regulator
10/92/63_conv
%/t'
Efficiency curves for the three circuits are shown in Figures 4 and 5. The linear regulator circuit has efficiency of
52% with a fresh battery. As the input-output differential
decreases, the efficiency increases and at end of battery life exceeds 90%. Regulator ground current limits
efficiency at drop-out. The switch-mode step-down
circuit has almost constant efficiency, ranging from
84% at 6.3V input to 82% at 9.5V input. Minimum VIN
is set by the drop of the emitter follower switch inside
the LT1173. Performance for the step-up converter is
shown in Figure 5. At higher inputs, the switch drop is a
lower percentage of supply, resulting in higher efficiency.
The three regulators show substantial differences in operating life. The linear regulator operates for 16.5 hours,
as shown in Figure 6. Figure 7 shows a 19 hour operating
life for the step-down switching circuit. The step-up
regulator circuit’s performance, detailed in Figure 8,
yields an operating life of 26 hours. This is an increase
of 58% over the linear step-down approach at less cost
and 37% over the switching step-down approach.
10
8
VOLTAGE (V)
the switch cycle. Output voltage is regulated in a similar
manner.
INPUT
6
OUTPUT
4
2
0
0
2
4
6
8 10 12 14 16 18 20 22 24
TIME (HOURS)
%/t'
Figure 6. 9V to 5V Step-Down Linear –
LT1120, 30mA Load
10
9
100
LT1120
LT1173-5
8
VOLTAGE (V)
EFFICIENCY (%)
INPUT
90
80
70
6
OUTPUT
5
4
3
2
60
1
50
0
6
5
8
9
7
BATTERY VOLTAGE (V)
2
0
4
8 10 12 14 16 18 20 22 24
TIME (HOURS)
%/t'
6
10
%/t'
Figure 4. Step-Down Conversion Efficiency –
5V Output, 30mA Load
Figure 7. 9V to 5V Step-Down Switcher –
LT1173-5, 30mA Load
6
90
VOLTAGE (V)
88
86
&''*$*&/$:
OUTPUT
5
84
4
3
INPUT
2
82
80
1
78
0
76
1.8
2.0
2.8
2.2 2.4 2.6
BATTERY VOLTAGE (V)
3.0
0
4
12
16
20
TIME (HOURS)
24
%/t'
3.2
%/t'
Figure 5. Step-Up Conversion Efficiency –
5V Output, 30mA Load
Data Sheet Download
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Linear Technology Corporation
Figure 8. 3V to 5V Step-Up Switcher –
LT1173-5, 30mA Load
For applications help,
call (408) 432-1900
dn63f_conv LT/GP 1092 190K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
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●
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© LINEAR TECHNOLOGY CORPORATION 1992
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