PRODUCT OF THE MONTH
Buck-Boost Converter Enables Three Modes of Operation
By Ashok Bindra, Editor, Power Electronics Technology
odern portable applications require batteries with higher power densities. Battery
chemistries such as nickel metal hydride
(NiMH), Lithium-ion (Li-ion), and
Lithium-polymer (Li-poly) have adequately
served the needs of these applications. However, in the practical world, there’s often a need to maintain system voltage
at a constant value, whether the battery is charged or discharged; therefore, a single converter is needed to provide
both buck and boost operations. This device also must function efficiently in the midrange when the output voltage is
close to the input.
Foreseeing this need, Linear Technology Corp. unveiled
a monolithic solution. Now, Allegro MicroSystems joins
the fray with its respective buck-boost topology to offer an
additional source. Using its proprietary control scheme,
Allegro MicroSystems has crafted a buck-boost converter
5.5V, letting users set an output voltage above, below or equal
to an input voltage as supplied by a rechargeable battery.
Allegro MicroSystems’ A8440 is pin and function compatible with Linear’s LTC3440. Meanwhile, Linear has expanded the current capability with its LTC3441, which offers 1A continuous output current. However, LTC3441
comes in a 12-lead plastic DFN package.
The A8440 incorporates two low on-resistance n-channel and two p-channel power switches, and uses synchronous rectification to maximize efficiency with minimum
external components (see the figure). It employs three basic operating states, depending on the input/output relationship. In boost mode, VIN << VOUT, a standard boost configuration is achieved, leaving switch P1 permanently on,
switch N1 permanently off, and switching N2 and P2 in
antiphase as normal. Similarly, in buck mode, VIN >> VOUT,
P2 is permanently on, switch N2 is permanently off, and
N1 and P1 are switched in antiphase. In the midrange, VIN
close to VOUT, a third mode is introduced. Note that in normal boost mode or buck mode, as the input voltage approaches the output voltage, the time spent per cycle in
the “direct-connection” switch configuration (i.e., P1 and
P2 closed, N1 and N2 open) increases. This state is maximized for midrange buck-boost operation because this
provides the most efficient energy transfer.
For higher loads, the unit offers a fixed-frequency PWM
mode. For light loads, where an ultralow quiescent current
is desired, the A8440 can be tailored for a pulse-frequency
mode (PFM) or burst mode. Maximum output-current capability in the burst mode is (0.3VIN)/(VOUT + VIN) amperes.
These modes of operation are selectable. The PWM frequency is programmable up to 2MHz using a single external resistor or can be synchronized to an external clock
source. It can be synchronized to external frequencies 25%
above or below programmed level. Although the part is
rated for high efficiency (about 94%) at low- and highoutput currents, it’s currently being evaluated and qualified. The qualification process should be finished before
2004.
To complete a solution, the A8440 needs a single inductor and a few resistors and capacitors. Its output tolerance
is 2%. Slated for production early next year, the A8440 is
housed in a standard 10-lead MSOP package, and is priced
at $0.85 each in 10,000-piece quantities.
PETech
M
LX1
VIN
LX2
P1
VOUT
P2
N1
N2
Crossover protected
gate drive
+
Input
current limit
+
PWM/PFM logic
and
operating mode decode
Reverse
- current
limit
Error
Amp + VBG
-
RT
CMode
Osc
PLL
PWM/
PFM
Enable
Clock
timeout
Soft
start
FB
VE
EN
GND
A noninverting buck-boost converter incorporates n- and p-channel
power MOSFETs.
that enables three modes of operation. With this novel control scheme, Allegro MicroSystems’ noninverting buckboost dc-dc converter A8440 provides automatic and
smooth transition through boost, buck-boost and buck
modes. The A8440’s input and output ranges from 2.5V to
Power Electronics Technology
October 2003
For more information visit www.allegromicro.com
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