DN110 - Micropower Buck/Boost Circuits, Part 2: Converting Four Cells to 5V*

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Micropower Buck/Boost Circuits, Part 2:
Converting Four Cells to 5V* – Design Note 110
Mitchell Lee
Two combinations of cell count and output voltage are to
be strictly avoided: three cells converted to 3.3V and four
cells converted to 5V. These combinations are troublesome because no ordinary regulator (boost, buck or linear)
can accommodate a situation where the input voltage
range overlaps the desired output voltage.
500kHz converter is used for compact layouts at higher
current levels.
You can expect 100mA output from LT1303 based circuits
and 300mA from the LT1372 circuit without modification.
All of the circuits feature output disconnect; in shutdown
the outputs fall to 0V. The input range of LT1303 based
converters extends well beyond the 3-cell source shown.
These function at 1.8V, and although not fully characterized
for efficiency, can accept inputs of up to 10V. The LT1372
converter operates from 2.7V to 10V.
This design note presents four circuits capable of solving
the 4-cell dilemma. Design Note 109 discussed 3-cell, 3.3V
circuits. The LT ®1303 and LT1372 high efficiency DC/DC
converters are used throughout, giving a fair comparison
of each topology’s efficiency. The LT1303 is optimized for
battery operation and includes a low-battery detector which
is required to implement one of the topologies. The LT1372
, LTC and LT are registered trademarks of Linear Technology Corporation.
*For 3-cell to 3.3V buck/boost circuits, see Design Note 109.
95
SGND
ON
PGND
1
IOUT = 100mA
90
MBRS130T3
VOUT
5V
100mA
+
220µF*
•
20µH**
1/2 CTX20-1
85
EFFICIENCY (%)
20µH**
22µF*
1/2 CTX20-1 +
VIN = 2V TO 8V
•
6
7
VIN
SW
+
2
22µF*
LBO
4
LT1303-5 FB 4
OFF
CELLS
3
5
SHDN
LBI
80
75
70
65
60
55
8
50
*AVX TPS SERIES TANTALUM
OR SANYO OS-CON
**COILTRONICS
2
1
3
6
4
5
INPUT VOLTAGE (V)
7
8
DN110 • F01
Figure 1. 4-Cell to 5V SEPIC
+
10µF*
35V
ON
4
CELLS
4
LT1372
+16V
2
9.09k
1%
3.01k
1%
SHDN
VC
OFF
FB
95
47µF*
8µH**
1/2 CTX5-1
•
5
8
VIN
VSW
MBRS130T3
+
•
8µH**
1/2 CTX8-1
47µF*
16V
GND
1
IOUT = 300mA
80
75
IOUT = 100mA
70
65
55
50
4.7nF
1
*AVX TPS SERIES TANTALUM
**COILTRONICS
2
3
4
5
6
INPUT VOLTAGE (V)
7
DN110 • F02
Figure 2
08/95/110
85
60
6, 7
47nF
2k
90
VOUT
5V
300mA
EFFICIENCY (%)
VIN = 2.7V TO 8 V
8
The circuits in Figures 1 and 2 are based on the SEPIC
(Single-Ended Primary Inductance Converter) topology.
Although not stellar, the efficiency is quite consistent over
a wide input voltage range. Peculiar to the SEPIC topology
is its use of two inductors. These, however, are wound
together on a single core and consume no more space than
a simple 2-terminal inductor of similar rating. A wide
selection of stock 2-winding, 4-terminal inductors are
available from Coiltronics and other magnetics vendors.
the boost converter and dissipating power as would any
linear regulator.
Highest peak efficiency is obtained with the circuit in Figure
4 using a MOSFET buck/boost converter. For VIN < VOUT,
the circuit operates as a boost converter and the MOSFET,
driven by the LT1303’s low-battery detector/amplifier, is
held 100% ON. The output voltage is developed and
controlled by the boost converter.
For VIN > VOUT, the boost function can no longer control the
output voltage and it begins to rise. Staggered feedback
(R3, R4, R5) allows the low-battery detector/amplifier to
take control using the MOSFET as a linear pass element.
Because the MOSFET requires no base drive, and because
it has such a low ON resistance, the efficiency peaks at well
over 90%. Furthermore, the efficiency peak occurs in the
vicinity of a NiCd’s nominal terminal voltage of 4 × 1.25 =
5V, right where the efficiency is needed most.
Peak efficiency improves in Figure 3 using a bipolar buck/
boost topology. This circuit is essentially a boost converter with a linear post regulator. For V IN < VOUT, the
LT1303 boosts the input driving the bipolar emitter just
high enough to maintain the desired output voltage—the
transistor is saturated. For V IN > VOUT, the LT1303 drives
the emitter to a value just higher than the input voltage
sufficient to develop the base current necessary to support any load current. In this condition the transistor
serves as a linear post regulator, cascoding the output of
95
2k
6
+
VIN
22µF*
4
CELLS
LBO
OFF
LT1303-5
3
SHDN
LBI
SGND
ON
FB
2
4
5
+
85
+
22µF*
PGND
1
VOUT
5V
100mA
ZTX788B
7
SW
EFFICIENCY (%)
VIN = 2V TO 8V
IOUT = 100mA
90
10µH**
CD54-100MC MBRS130T3
80
75
70
65
60
220µF*
55
8
50
*AVX TPS SERIES TANTALUM
OR SANYO OS-CON
**SUMIDA
2
1
3
6
4
5
INPUT VOLTAGE (V)
7
8
DN110 • F03
Figure 3. 4-Cell to 5V Bipolar Buck/Boost
95
VIN = 2V TO 8V
+
7 22µF*
VIN
SW
2
22µF*
LBO
4
OFF
LT1303
FB
3
5
SHDN
LBI
6
+
4
CELLS
SGND
ON
1
100k
100Ω
75k
1%
+
220µF*
243Ω
1%
PGND
24.3Ω
1%
8
IOUT = 100mA
90
VOUT
5V
100mA
Si9405
85
EFFICIENCY (%)
10µH**
MBRS130T3
CD54-100MC
80
75
70
65
60
55
50
1
*AVX TPS SERIES TANTALUM
OR SANYO OS-CON
**SUMIDA
2
3
6
4
5
INPUT VOLTAGE (V)
7
8
DN110 • F05
Figure 4. 4-Cell to 5V MOSFET Buck/Boost
For literature on our DC/DC Converters,
call 1-800-4-LINEAR. For applications help,
call (408) 432-1900, Ext. 361
Linear Technology Corporation
LT/GP 0895 160K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977
 LINEAR TECHNOLOGY CORPORATION 1995
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