DN41 - Switching Regulator Allows Alkalines to Replace NiCads

Switching Regulator Allows Alkalines to Replace NiCads
Design Note 41
Brian Huffman
This circuit is a step-up boost type switching regulator.
It maintains a constant 6V output as battery voltage
fails. The inductor accumulates energy from the battery when the LT®1270 switch pin (VSW ) switches to
ground and dumps its stored energy to the output
when the switch pin (VSW ) goes off. The feedback pin
(VFB) samples the output from the 6.19k-1.62k divider.
The LT1270’s error amplifier compares the feedback
pin voltage to its internal 1.24V reference and controls
the VSW pin switching current, completing a control
loop. The output voltage can be varied by changing
the resistor divider ratio. The RC damper on the VC pin
provides loop frequency compensation. The minimum
start up voltage for this circuit is 3V. If a 3.3V start up
voltage is permissible R1 and Q1 can be removed with
D2 replaced by a short.
In many applications it is desirable to substitute nonrechargeable batteries for chargeable types. This capability is necessary when the NiCads can’t be recharged
or long charge times are unacceptable. Alkaline batteries are an excellent choice in this situation. They are
readily available and have reasonable energy density.
Compared to Alkalines, NiCads provide a more stable
terminal voltage as they discharge. NiCads decay from
1.3V to 1.0V, while Alkalines drop from 1.5V to 0.8V.
Replacing NiCads with Alkalines can cause unacceptable low supply voltage, although available energy is
adequate. A boost type switching regulator obviates this
problem, allowing Alkaline cells to replace NiCads. The
circuit shown in Figure 1 accommodates the Alkaline
cells widely varying terminal voltage while providing a
constant output voltage.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
D1
MBR330
L1
15μH
R1
2.3k
BATTERY
3 CELLS
MINIMUM
+
VIN
3V-6V
+
Q1
2N3906**
D2
1N4148**
L2
5μH
+
VOUT
6V
1A
C2
1000μF
10V
R1
6.19k*
VIN
C1
100μF
10V
VSW
LT1270
GND
+
VFB
VC
1k
C3
220μF
10V
R2
1.62k*
0.47μF
DN041 F01
* = 1% FILM RESISTORS
** = OPTIONAL – FOR 0.3V LOWER START UP VOLTAGE
D1 = MOTOROLA – MBR330
C1 = NICHICON – UPL1A101MRH
C2 = NICHICON – UPL1A102MRH6
C3 = NICHICON – UPL1A221MRH
VOUT = 1.24V 1 + R1
L1 = COILTRONICS – CTX15-8-52
R2
L2 = COILTRONICS – CTX5-1-FR
Figure 1. Low Voltage Circuit Provides Constant Output Voltage as Battery Discharges
11/90/41_conv
100
7
MANUFACTURER 1
90
4 CELLS
MANUFACTURER 2
6
BATTERY VOLTAGE (V)
BATTERY LIFE (MIN)
80
70
60
50
4
3
CELLS CELLS
40
30
20
10
4
3
4
CELLS CELLS
CELLS
AA
C
C
CELL TYPE
4
4D
CELLS
3
2
0
AA
5
D
1
D
0
30
60
TIME (MIN)
90
DN041 F02
Bootstrapping the VIN pin off the output voltage allows the battery voltage to drop below the minimum
start up voltage, while maintaining circuit operation.
For example, with three C cells the battery voltage is
initially 4.5V and operates down to 2.4V. With this
bootstrapped technique the circuit provides a constant
output voltage over the battery’s complete operating
range, maximizing battery life.
Battery life characteristics are different for various cell
types. Figure 2 compares battery life between AA, C,
and D cells with a 6W load. In this application the power
drain from the battery remains relatively constant.
As the battery voltage decreases the battery current
increases. The AA types discharge quicker than the C
or D cells. They are physically smaller than the other
cells, and therefore store less energy. The AA cells are
3 times smaller than the C cells and 6 times smaller
than the D cells.
Current drain also influences cell life. Battery life significantly decreases at high current discharge. Slightly
higher battery stack voltages permit surprising battery
life increases. The higher voltage means lower current
drain for a constant power load. Operating at just 33%
less current the four C cells last 5 times longer than
three C cells.
Battery life characteristics vary widely between manufacturers. Some manufacturers’ cells are optimized to
operate more efficiently at lower current levels, making
it wise to consult the battery manufacturer’s discharge
characteristics.
DN041 F03
Figure 3. Alkaline Battery Discharge Characteristic with
6W Load
Figure 3 shows Alkaline battery discharge characteristics for four D cells. A fresh cell measures 1.5V and
operates down to 0.8V before the cell dies. The battery
stack voltage drops quickly and then stabilizes until it
reaches 3.2V; 0.8V per cell. There is no usable battery
life beyond this point.
Figure 4 shows efficiency exceeding 85%. The diode
and LT1270 switch are the two main loss elements. The
Schottky diode introduces a relatively constant 7% loss,
while the LT1270 switch loss varies with battery voltage.
As battery voltage decreases, switch current and duty
cycle increase. This has a dramatic effect on switch
loss, because switch loss is proportional to the square
of switch current multiplied by duty cycle. Therefore, at
low input voltages efficiency is degraded because this
loss is a higher percentage of the battery power drain.
If lower output current is desired, an LT1170, LT1171,
or LT1172 can be used.
100
90
EFFICIENCY (%)
Figure 2. Battery Life Characteristics for Different
Batteries for a 6W Load
120
80
70
60
50
2
3
5
6
4
BATTERY VOLTAGE (V)
7
DN041 F04
Figure 4. Efficiency for Various Battery Voltages
Data Sheet Download
www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900
dn41f_conv IM/GP 1190 • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1990
Similar pages