AN-1132: How to Apply DC-to-DC Step-Up (Boost) Regulators (Rev. 0)

AN-1132
APPLICATION NOTE
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How to Apply DC-to-DC Step-Up (Boost) Regulators
by Ken Marasco
BOOST REGULATORS
Power for portable electronic devices such as smartphones, GPS
navigation systems, and tablets can come from low voltage solar
panels, batteries, or ac-to-dc power supplies. Battery-powered
systems often stack cells in series to achieve higher voltages, but
this is not always possible due to a lack of space. Switching converters use an inductor’s magnetic field to alternately store energy
and release it to the load at a different voltage. With low losses,
they are a good choice for high efficiency. Capacitors connected
to the converter’s output reduce output voltage ripple. Boost, or
step-up, converters provide higher output voltage; buck, or stepdown, converters, described in the AN-1125 Application Note,
How to Apply DC-to-DC Step-Down (Buck) Regulators, provide
lower output voltage. Switching converters that include internal
FETs as switches are called switching regulators, whereas devices
requiring external FETs are called switching controllers.
Figure 1 shows a typical low power system powered from two
series-connected AA batteries. The battery’s usable output
varies from about 1.8 V to 3.4 V, whereas the ICs require 1.8 V
and 5.0 V to operate. Boost converters, which can step up
(increase) the voltage without increasing the number of cells,
MICROPROCESSOR
power the WLED backlights, micro hard disk drives, audio,
and USB peripherals, whereas a buck converter powers the
microprocessor, memory, and display.
The tendency of the inductor to resist changes in current enables
the boost function. When charging, the inductor acts as a load
and stores energy; when discharging, it acts as an energy source.
The voltage produced during the discharge phase is related to
the current’s rate of change, not to the original charging voltage,
thus allowing different input and output voltage levels.
Boost regulators consist of two switches, two capacitors, and an
inductor, as shown in Figure 2. Nonoverlapping switch drives
ensure that only one switch is on at a time to avoid unwanted
shoot-through current. In Phase 1 (tON), Switch B is open and
Switch A is closed. The inductor is connected to ground, so that
current flows from VIN to ground. The current increases due to
the positive voltage across the inductor, and energy is stored in
the inductor. In Phase 2 (tOFF), Switch A is open and Switch B is
closed. The inductor is connected to the load, so that current
flows from VIN to the load. The current decreases due to the
negative voltage across the inductor, and energy stored in the
inductor is discharged into the load.
MEMORY
LCD
DISPLAY
1.8V
BUCK
REGULATOR
ADP2138
BOOST
REGULATOR
ADP8866
AUDIO
1.8V TO 3.4V
BATTERY PACK
SPEAKERS
5.0V
BOOST
REGULATOR
ADP1612
LOAD
SWITCH
ADP195
10290-001
USB
TRANSCEIVER
MICRO HDD
Figure 1. Typical Low Power Portable System
Rev. 0 | Page 1 of 4
AN-1132
Application Note
PWM
MODULATION
VSW
VIN
+
–
CIN
VIN
ION
PWM ON
L
tON
VSW B
+
–
VOUT
ILOAD
ION
LOAD
IOFF
IOFF
CIN
tOFF
T
COUT
A
VIN
VOUT
L
A
ILOAD
B
VSW
VOUT
COUT
ILOAD
LOAD
∆ILOAD
10290-002
PWM OFF
Figure 2. Boost Converter Topology and Operating Waveforms
+
CIN
–
IOFF
PWM
CONTROL
CURRENT
LIMIT
OSCILLATOR
FB
R2
B
VOUT
A
COUT ILOAD
LOAD
VSW
R1
10290-003
VIN
Figure 3. Boost Regulator Integrates Oscillator, PWM Control Loop, and Switching FET
L1
ADP1612/
ADP1613
Q1 A
R3
10kΩ
CIN
Q1
B
1.3MHz
650kHz
(DEFAULT)
ON
CSS
OFF
6
VIN
3
EN
D1
VOUT
SW 5
R1
FB 2
7
FREQ
8
SS
R2
COMP 1
GND
RCOMP
4
CCOMP
COUT
10290-004
VIN
Figure 4. ADP1612/ADP1613 Typical Applications Circuit
Note that the switching regulator operation can be continuous
or discontinuous. When operating in continuous conduction
mode (CCM), the inductor current never drops to zero; when
operating in discontinuous conduction mode (DCM), the
inductor current can drop to zero. The current ripple, shown as
ΔILOAD in Figure 2, is calculated using
ΔILOAD = (VIN × tON)/L
The average inductor current flows into the load, while the
ripple current flows into the output capacitor.
Regulators that use a Schottky diode in place of Switch B are
defined as asynchronous (or nonsynchronous), whereas regulators that use a FET as Switch B are defined as synchronous. In
Figure 3, Switch A and Switch B have been implemented with an
internal NFET and an external Schottky diode, respectively, to
create an asynchronous boost regulator. For low power applications requiring load isolation and low shutdown current, external
FETs can be added, as shown in Figure 4. Driving the device’s
EN pin below 0.3 V shuts down the regulator and completely
disconnects the input from the output.
Rev. 0 | Page 2 of 4
Application Note
AN-1132
Modern low power synchronous buck regulators use pulse-width
modulation (PWM) as the primary operating mode. PWM
holds the frequency constant and varies the pulse width (tON)
to adjust the output voltage. The average power delivered is
proportional to the duty cycle, D, making this an efficient way
to provide power to a load.
D=
V
− VIN
t ON
= OUT
t ON + t OFF
VOUT
KEY BOOST REGULATOR SPECIFICATIONS AND
DEFINITIONS
Input Voltage Range
The input voltage range of a boost converter determines the
lowest usable input supply voltage. The specifications may show
a wide input voltage range, but the input voltage must be lower
than VOUT for efficient operation.
Ground or Quiescent Current
As an example, for a desired output voltage of 15 V and an
available input voltage of 5 V,
D = (15 – 5)/15 = 0.67 or 67%
Energy is conserved; therefore, the input power must equal the
power delivered to the load minus any losses. Assuming very
efficient conversion, the small amount of power lost can be
omitted from the basic power calculations. The input current
can thus be approximated by
IIN = (VOUT/VIN) × IOUT
For example, if the load current is 300 mA at 15 V, IIN = 900 mA
at 5 V—three times the output current. Therefore, the available
load current decreases as the boost voltage increases.
Boost converters use either voltage or current feedback to regulate
the selected output voltage; the control loop enables the output
to maintain regulation in response to load changes. Low power
boost regulators generally operate between 600 kHz and 2 MHz.
The higher switching frequencies allow use of smaller inductors,
but the efficiency drops by approximately 2% with every doubling
of the switching frequency. In the ADP1612 and ADP1613 boost
converters (see the ADP1612 and ADP1613 section), the
switching frequency is pin-selectable, operating at 650 kHz for
highest efficiency or at 1.3 MHz for smallest external
components. Connect FREQ to GND for 650 kHz operation or
to VIN for 1.3 MHz operation.
The inductor, a key component of the boost regulator, stores
energy during the on time of the power switch and transfers
that energy to the output through the output rectifier during the
off time. To balance the trade-offs between low inductor current
ripple and high efficiency, the ADP1612/ADP1613 data sheet
recommends inductance values in the 4.7 μH to 22 μH range. In
general, a lower value inductor has a higher saturation current
and a lower series resistance for a given physical size, but lower
inductance results in higher peak currents that can lead to reduced
efficiency, higher ripple, and increased noise. It is often better to
run the boost in discontinuous conduction mode to reduce the
inductor size and improve stability. The peak inductor current
(the maximum input current plus half the inductor ripple current)
must be lower than the rated saturation current of the inductor,
and the maximum dc input current to the regulator must be less
than the inductor’s rms current rating.
This is the dc bias current that is not delivered to the load (IQ).
The lower the IQ is, the better the efficiency, but IQ can be
specified under many conditions, including switching off, zero
load, PFM operation, or PWM operation; therefore, it is best to
look at operating efficiency at specific operating voltages and
load currents to determine the best boost regulator for the
application.
Shutdown Current
Shutdown current is the input current consumed when the
enable pin (EN) has been driven low and the device is off. Low
ISD is important for long standby times when a battery-powered
device is in sleep mode.
Switch Duty Cycle
The operating duty cycle must be lower than the maximum duty
cycle or the output voltage will not be regulated. For example,
D = (VOUT − VIN)/VOUT
With VIN = 5 V and VOUT = 15 V, D = 67%. The ADP1612 and
ADP1613 have a maximum duty cycle of 90%.
Output Voltage Range
This is the range of output voltages that the device supports.
The output voltage of the boost converter can be fixed or
adjustable, using resistors to set the desired output voltage.
Current Limit
Boost converters usually specify peak current limit, not load
current. Note that the greater the difference between VIN and
VOUT is, the lower the available load current becomes. The peak
current limit, input voltage, output voltage, switching frequency,
and inductor value all set the maximum available output current.
Line Regulation
Line regulation is the change in output voltage caused by a
change in the input voltage.
Load Regulation
Load regulation is the change in output voltage for a change in
the output current.
Soft Start
It is important for boost regulators to have a soft start function
that ramps the output voltage in a controlled manner on startup
to prevent excessive output voltage overshoot at startup. The
soft start of some boost converters can be adjusted by an external capacitor. As the soft start capacitor charges, it limits the
Rev. 0 | Page 3 of 4
AN-1132
Application Note
peak current allowed by the part. With adjustable soft start, the
start-up time can be changed to meet system requirements.
Thermal Shutdown (TSD)
If the junction temperature rises above the specified limit, the
thermal shutdown circuit turns the regulator off. Consistently
high junction temperatures can be the result of high current
operation, poor circuit board cooling, or high ambient temperature. The protection circuit includes hysteresis so that the device
does not return to normal operation until the on-chip temperature
drops below the preset limit after thermal shutdown occurs.
Undervoltage Lockout (UVLO)
If the input voltage is below the UVLO threshold, the IC
automatically turns off the power switch and goes into a lowpower mode. This prevents potentially erratic operation at low
input voltages and prevents the power device from turning on
when the circuitry cannot control it.
devices and liquid crystal displays. The adjustable soft start
circuit prevents inrush currents, ensuring safe, predictable startup conditions. The ADP1612 and ADP1613 consume 2.2 mA in
the switching state, 700 μA in the nonswitching state, and 10 nA
in shutdown mode. Available in 8-lead MSOP packages, they
are specified from −40°C to +125°C
CONCLUSION
Low power boost regulators take the worry out of switching dcto-dc converter design by delivering a proven design. Design
calculations are available in the ADP1612/ADP1613 data sheet,
and the ADIsimPower™ design tool simplifies the task for the
end user. For additional information, contact the application
engineers at Analog Devices, Inc., or visit EngineerZone™ at
ez.analog.com for help. Analog Devices boost regulator selection guides, data sheets, and application notes can be found at
www.analog.com/power-management.
REFERENCES
ADP1612 AND ADP1613
Lenk, John D. 1996. Simplified Design of Switching Power
Supplies. Elsevier.
Step-up dc-to-dc switching converters operate at 650 kHz or
1300 kHz.
The ADP1612 and ADP1613 step-up converters are capable of
supplying over 150 mA at voltages as high as 20 V, while operating
with a single 1.8 V to 5.5 V and 2.5 V to 5.5 V supply, respectively.
Integrating a 1.4 A/2.0 A, 0.13 Ω power switch with a current
mode, pulse-width modulated regulator, their output varies less
than 1% with changes in input voltage, load current, and temperature. The operating frequency is pin-selectable and can be
optimized for high efficiency or minimum external component
size. At 650 kHz, they provide 90% efficiency; at 1.3 MHz, their
circuit implementation occupies the smallest space, making
them ideal for space constrained environments in portable
Marasco, Ken. 2011. AN-1125 Application Note, How to Apply
DC-to-DC Step-Down (Buck) Regulators. Analog Devices, Inc.
(September).
Marasco, Ken. 2009. “How to Successfully Apply Low-Dropout
Regulators.” Analog Dialogue. Volume 43, Number 3.
www.analog.com/power-management
www.analog.com/switching-regulators
www.analog.com/switching_controllers
www.analog.com/ADIsimPower
©2011 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN10290-0-11/11(0)
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