DN109 - Micropower Buck/Boost Circuits, Part 1: Converting Three Cells to 3.3V*

advertisement
Micropower Buck/Boost Circuits, Part 1:
Converting Three Cells to 3.3V* – Design Note 109
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.
the topologies. The LT1372 500kHz converter is used
for compact layouts at higher current levels.
You can expect 200mA 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 3-cell dilemma. Design Note 110 will discuss 4-cell,
5V 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 lowbattery detector which is required to implement one of
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. *For 4-cell to 5V buck/boost circuits, see Design Note 110.
95
330μF*
IOUT = 200mA
90
VOUT
3.3V
200mA
85
EFFICIENCY (%)
20μH**
33μF*
MBRS130T3
1/2 CTX20-1
VIN = 2V TO 6V
•
6
7 +
+
VIN
SW
2
22μF*
169k
LBO
1% +
4
3
OFF
LT1303
FB
CELLS
•
100k
3
5
SHDN
LBI
20μH**
1%
1/2
CTX20-1
SGND
PGND
ON
1
8
80
75
70
65
60
55
*AVX TPS SERIES TANTALUM
OR SANYO OS-CON
**COILTRONICS
50
1
2
3
4
5
6
INPUT VOLTAGE (V)
7
8
DN109 • F01
Figure 1. 3-Cell to 3.3V SEPIC
+
10μF*
35V
ON
3
CELLS
4
47μF*
16V
5μH**
1/2 CTX5-1
•
5
8
VIN
VSW
LT1372
+
2
16.9k
1%
10k
1%
SHDN
VC
OFF
FB
95
MBRS130T3
+
•
5μH**
1/2 CTX5-1
47μF*
16V
GND
1
6, 7
75
70
65
55
50
4.7nF
1
*AVX TPS SERIES TANTALUM
**COILTRONICS
2
3
4
5
6
INPUT VOLTAGE (V)
7
DN109 • F02
Figure 2
08/95/109_conv
85
80
60
47nF
2k
IOUT = 300mA
90
VOUT
3.3V
300mA
EFFICIENCY (%)
VIN = 2.7V TO 6V
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 transistor serves as a linear post regulator, cascoding the output of 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 3 × 1.25 = 3.75V, 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 VIN < VOUT,
the LT1303 boosts the input driving the bipolar emitter
just high enough to maintain the desired output voltage—the transistor is saturated. For VIN > 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
95
510Ω
6
+
VIN
33μF*
3
CELLS
LBO
OFF
LT1303
3
FB
SHDN
LBI
SGND
ON
2
5
85
169k
1% +
4
220μF*
+
33μF*
PGND
1
VOUT
3.3V
200mA
ZTX788B
7
SW
EFFICIENCY (%)
VIN = 2V TO 6V
IOUT = 200mA
90
10μH**
CD54-100MC MBRS130T3
100k
1%
80
75
70
65
60
55
8
50
*AVX TPS SERIES TANTALUM
OR SANYO OS-CON
**SUMIDA
2
1
3
4
5
6
INPUT VOLTAGE (V)
7
8
DN109 • F03
Figure 3. 3-Cell to 3.3V Bipolar Buck/Boost
95
VIN = 2V TO 6V
+
7
SW
6
+
33μF*
3
CELLS
VIN
LBO
OFF
3
LT1303
SHDN
SGND
ON
1
FB
LBI
33μF*
100k
100Ω
2
4
5
200k
1%
PGND
121k
1%
8
+
330μF*
1.96k
1%
IOUT = 200mA
90
VOUT
3.3V
200mA
Si9405
85
EFFICIENCY (%)
10μH**
CD54-100MC MBRS130T3
80
75
70
65
60
55
50
1
*AVX TPS SERIES TANTALUM
OR SANYO OS-CON
**SUMIDA
2
3
4
5
6
INPUT VOLTAGE (V)
7
8
DN109 • F04
Figure 4. 3-Cell to 3.3V MOSFET Buck/Boost
Data Sheet Download
www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900
dn109f_conv LT/GP 0895 190K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1995
Similar pages