DN413 - Wide Input Voltage Range Buck-Boost Converter Simplifies Design of Variable Input Supplies

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Wide Input Voltage Range Buck-Boost Converter Simplifies
Design of Variable Input Supplies – Design Note DN413
John Canfield
Introduction
Many of today’s portable electronic devices require
the ability to operate from a variety of power sources
including USB, wall adapters, and alkaline and lithium
batteries. Designing a power conversion solution that
is compatible with this wide array of power sources can
be daunting. The LTC ®3530 monolithic synchronous
converter simplifies the task by operating in both buck
and boost modes over an extended input voltage range
of 1.8V to 5.5V. No complicated topology is required to
account for varying inputs that can be above, below or
equal to the output.
The LTC3530 utilizes a proprietary switching algorithm
that provides seamless transitions between buck and
boost modes while simultaneously optimizing efficiency
over all operating conditions. Using this advanced control
algorithm, the LTC3530 is capable of high efficiency, fixed
frequency operation with input voltages that are above,
below, or equal to the output voltage, while requiring only
a single inductor. This capability makes the LTC3530 well
suited for lithium-ion/polymer and 2-cell alkaline or NiMH
applications which require a supply voltage that is within
the battery voltage range. In such cases, the high efficiency
and extended input operating range of the LTC3530 offer
greatly improved battery run-time, as much as 25% in
some cases, over alternative solutions.
250mA of load current is supported when the input is
1.8V. The output voltage is user-programmable from 1.8V
to 5.25V via an external resistor divider. The LTC3530
includes a soft-start circuit to minimize the inrush current
transient during power-up. The duration of the soft-start
period can be programmed via the time constant of an
external resistor and capacitor.
The switching frequency of the LTC3530 is user programmable via a single external resistor, allowing the converter
to be optimized to meet the space and efficiency requirements of each particular application. An external resistor
and capacitor provide compensation of the feedback loop,
enabling the frequency response to be adjusted to suit a
wide array of external components. This flexibility allows
for rapid output voltage transient response regardless of
inductor value and output capacitor size.
The LTC3530 features an automatic transition to Burst
Mode® operation at a user programmable current level
to improve light load efficiency. For noise sensitive applications, the LTC3530 can be forced into fixed frequency
operation at all load currents by connecting the BURST pin
to VIN. The LTC3530 also features short-circuit protection
and overtemperature shutdown. Internal reverse current
limiting circuitry prevents damage to the part should
, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
At 3.3V output, a load current of up to 600mA can be
supported over the entire lithium-ion input voltage range;
94
SW1
VIN
FB
SHDN/SS
VOUT
3.3V
250mA
340k
LTC3530
1M
CIN
10μF
SW2
VOUT
330pF
30.1k
VC
200k
COUT
22μF
EFFICIENCY (%)
L1
4.7μH
VIN
1.8V TO 5.5V
IOUT = 150mA
92
90
88
86
0.01μF
RT
33.2k
BURST
GND
100k
84
0.01μF
2
DN413 F01
L1: TDK RLF7030T-4R7M3R4
Figure 1. 3.3V at 250mA from a 1.8V to 5.5V Input
04/07/413_conv
4
3
INPUT VOLTAGE (V)
5
DN413 F02
Figure 2. Efficiency vs Input Voltage
of the Circuit in Figure 1
the output voltage be pulled above regulation through
an external path.
output voltage, and inductor value, unlike other devices
that rely on the level of peak inductor current.
Efficiency
Figure 1 illustrates a typical LTC3530 application circuit
configured with a 1MHz switching frequency, which
represents a good compromise between the PCB area
and efficiency for most applications. The efficiency curve
versus input voltage for this application circuit is shown
in Figure 2. The LTC3530 achieves greater then 85%
efficiency with an input voltage greater than or equal to
2V. These high levels of efficiency in combination with
its wide input voltage range make the LTC3530 an attractive solution for battery-operated products and other
efficiency-sensitive applications.
In noise sensitive applications, the LTC3530 can be forced
into fixed frequency PWM operation at all load currents by
simply connecting the BURST pin to VIN. In addition, the
BURST pin can be driven dynamically in the application to
provide low noise performance during critical phases of
operation, or to reduce voltage transients during periods
of expected large load transitions.
Programmable Burst Mode Operation
The LTC3530 provides automatic Burst Mode operation,
which greatly improves efficiency at light load currents.
Burst Mode operation reduces the operating current of
the LTC3530 to only 40μA in order to improve light load
efficiency and extend battery runtime. The LTC3530 automatically transitions to Burst Mode operation when the
average output current falls below a user programmable
level set via an external resistor. When the load current
rises above the Burst Mode threshold, the part automatically returns to fixed frequency PWM operation.
The precise control circuitry of the LTC3530 allows the
Burst Mode threshold to be set at load currents as low
as 20mA. In addition, the LTC3530 directly monitors the
average load current thereby providing a Burst Mode
transition threshold that is independent of input voltage,
1.27mm Profile Li-Ion to 3.3V Regulator
The high switching frequency and advanced buck-boost
switching algorithm of the LTC3530 allow the use of small
external components. Figure 3 shows a circuit that is
optimized to reduce the total application size. The entire
converter has a maximum height of 1.27mm and occupies
a PCB area of only 0.135 square inches making it ideal
for height constrained applications such as PC cards.
Figure 4 shows the efficiency versus input voltage for
this area-optimized application circuit. This converter is
able to support a 600mA load for output voltages above
2.4V and obtains greater than 86% efficiency over the
entire Li-Ion input voltage range.
Conclusion
The LTC3530 with its high efficiency, wide input voltage
range, and tiny circuit size is well suited to a variety of
battery operated products and other efficiency-sensitive applications. With the IC’s array of programmable
features, the circuit can be customized to meet the needs
of any application, while still maintaining a compact total
solution footprint.
94
IOUT = 150mA
L1
4.7μH
SW2
VOUT
FB
SHDN/SS
OFF ON
RT
33.2k
340k
LTC3530
CIN
10μF
330pF
30.1k
VC
BURST
GND
200k
VOUT
3.3V
650mA
COUT
22μF
EFFICIENCY (%)
90
SW1
VIN
VIN
3V TO 4V
86
82
100k
0.01μF
DN413 F03
CIN: MuRATA GRM21BR60J106K (0805 PACKAGE)
COUT: MuRATA GRM21BR60J226K (0805 PACKAGE)
L1: COOPER BUSSMAN SD12-2R2
Figure 3. 1.27mm Profile and Area Optimized Application Circuit
78
3
3.2
3.6
3.8
3.4
INPUT VOLTAGE (V)
4
DN413 F04
Figure 4. Efficiency vs Input Voltage
of Figure 3 Circuit
Data Sheet Download
http://www.linear.com
For applications help,
call (408) 432-1900, Ext. 2759
Linear Technology Corporation
dn413_conv LT/TP 0407 409K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2007
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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