How to Apply DC-to-DC

How to Apply DC-to-DC
How to Apply DC-to-DC
Step-Up (Boost)
Regulators Successfully
Switch A is closed. The inductor is connected to ground, so
current flows from V IN 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 current flows
from V IN 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.
By Ken Marasco
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—covered here—provide higher voltage; buck, or stepdown converters—covered in a previous article1—provide lower
output voltage. Switching converters that include internal FETs
as switches are called switching regulators,2 while devices requiring
external FETs are called switching controllers.3
VIN
L
B
VOUT
VSW
COUT
A
VIN
+
L
IL
B
VSW
COUT
A
VSW
LOAD
IL
IOFF
CIN
PWM OFF
PWM
MODULATION
VOUT
LOAD
VOUT
VIN
tON
t
tOFF
ION
The inductor’s tendency 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.
MICROPROCESSOR
ION
CIN
PWM ON
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 the voltage without
increasing the number of cells, power the WLED backlights,
micro hard disk drives, audio, and USB peripherals, while a buck
converter powers the microprocessor, memory, and display.
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
+
IOFF
IL
𝚫IL
Figure 2. Buck converter topology and operating waveforms.
MEMORY
LCD
DISPLAY
1.8V
BOOST
REGULATOR
BUCK
REGULATOR
ADP8866
ADP2138
1.8V TO 3.4V
BATTERY PACK
AUDIO
SPEAKERS
USB
TRANSCEIVER
5V
BOOST
REGULATOR
ADP1612
LOAD
SWITCH
ADP195
MICRO HDD
Figure 1. Typical low-power portable system.
Analog Dialogue 45-09 Back Burner, September (2011)
www.analog.com/analogdialogue
1
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 ΔIL in Figure 2,
is calculated using ΔIL = (V IN × tON)/L. The average inductor
current flows into the load, while the ripple current flows into the
output capacitor.
L
VIN
+
IOFF
CIN
OSCILLATOR
PWM
A
CONTROL
CURRENT
LIMIT
FB
R2
B
VOUT
COUT
IL
LOAD
VSW
R1
Figure 3. Boost regulator integrates oscillator,
PWM control loop, and switching FETs.
Regulators that use a Schottky diode in place of Switch B are
defined as asynchronous (or nonsynchronous), while regulators that
use a FET as Switch B are defined as synchronous. In Figure 3,
Switches A and 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.
L1
VIN
Q1
ADP1612/
ADP1613
A
6 VIN
R3
10kΩ
R1
FB 2
1.3MHz
7 FREQ
650kHz
(DEFAULT)
Q1
B
8 SS
OFF
ON
CSS
VOUT
SW 5
3 EN
CIN
D1
R2
COMP 1
GND
4
RCOMP
COUT
CCOMP
Figure 4. ADP1612/ADP1613 typical applications circuit.
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.
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, so 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
2
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 Appendix), 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.
Key Boost Regulator Specifications and Definitions
Input Voltage Range: A boost converter’s input voltage
range 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: The dc bias current not
delivered to the load (Iq). The lower the Iq the better the efficiency,
but Iq can be specified under many conditions, including switching
off, zero load, PFM operation, or PWM operation, so 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: The input current consumed when the
enable pin has been set to OFF. Low Iq 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 – V IN)/VOUT. With V IN = 5 V
and VOUT = 15 V, D = 67%. The ADP1612 and ADP1613 have a
maximum duty cycle of 90%.
Output Voltage Range: The range of output voltages the device
will support. The boost converter’s output voltage 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
V IN and VOUT, the lower the available load current. 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.
Analog Dialogue 45-09 Back Burner, September (2011)
Load Regulation: Load regulation is the change in output voltage
for a change in the output current.
3
www.analog.com/en/power-management/switching-controllersexternal-switches/products/index.html.
Soft Start: It is important for boost regulators to have a soft-start
function that ramps the output voltage in a controlled manner
upon 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 peak current allowed by the part. With adjustable soft start,
the start-up time can be changed to meet system requirements.
4
http://designtools.analog.com/dtPowerWeb/dtPowerMain.aspx
Lenk, John D. Simplified Design of Switching Power Supplies.
Elsevier/Newnes. 1996.
Marasco, K. “How to Apply DC-to-DC Step-Down (Buck)
Regulators Successfully.” Analog Dialogue. Volume 45. June 2011.
Marasco, K. “How to Apply Low-Dropout Regulators Successfully.”
Analog Dialogue. Volume 43, Number 3. 2009.
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 will not return to normal operation until the
on-chip temperature drops below the preset limit after thermal
shutdown occurs.
Author
Ken Marasco [[email protected]] is
a system applications manager. Responsible
for the technical support of portable power
products, he has been a member of the Analog
Devices Portable Applications Team for three
years. He graduated from NYIT with a degree
in applied physics and has 35 years of system
and component design experience.
Undervoltage Lockout (UVLO): If the input voltage is below the
UVLO threshold, the IC automatically turns off the power switch
and goes into a low-power mode. This prevents potentially erratic
operation at low input voltages and prevents the power device from
turning on when the circuitry cannot control it.
Appendix
Step-Up DC-to-DC Switching Converters Operate at
650 kHz/1300 kHz
Conclusion
Low-power boost regulators take the worry out of switching
dc-to-dc converter design by delivering a proven design. Design
calculations are available in the applications section of the data
sheet, and the ADIsimPower4 design tool simplifies the task
for the end user. For additional information, please contact the
applications engineers at Analog Devices, or visit the 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.
The ADP1612 and ADP1613 step-up converters are capable of
supplying over 150 mA at voltages as high as 20 V, while operating,
respectively, with a single 1.8-V to 5.5-V and 2.5-V to 5.5-V
supply. 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 devices and
liquid-crystal displays. The adjustable soft-start circuit prevents
inrush currents, ensuring safe, predictable start-up 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 +85°C and priced at $1.50/$1.20 in 1000s.
References
(Information on all ADI components can be found at www.analog.com.)
1
www.analog.com/library/analogDialogue/archives/45-06/
buck_regulators.html.
2
www.analog.com/en/power-management/switching-regulatorsintegrated-fet-switches/products/index.html.
L1
VIN
>1.6V
CIN
VIN
<0.3V
6
7
+
VIN
D
COMPARATOR
VOUT
R1
FB
ERROR
AMPLIFIER
PWM
COMPARATOR
RCOMP
CSS
D1
COUT
VOUT
5𝛍A
S
R
TSD
COMPARATOR
5𝛍A
8
SW
DREF
UVLOREF
VSS
SS
CURRENT
SENSING
UVLO
COMPARATOR
VIN
1
CCOMP
5
OSCILLATOR
VBG
COMP
A
+
2
R2
FREQ
TSENSE
SOFT
START
TREF
1.1M𝛀
RESET
DRIVER
Q
BG
ADP1612/ADP1613
N1
BAND GAP
AGND
AGND
EN
3
4
>1.6V
GND
<0.3V
Figure A. ADP1612/ADP1613 functional block diagram.
Analog Dialogue 45-09 Back Burner, September (2011)
3
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