AN-1125: How to Apply DC-to-DC Step-Down (Buck) Regulators (Rev. 0)

AN-1125
APPLICATION NOTE
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How to Apply DC-to-DC Step-Down (Buck) Regulators
by Ken Marasco
a low dropout regulator (LDO). Unfortunately, power not delivered to the load is lost as heat, making LDOs inefficient when
VIN is much greater than VOUT. A popular alternative, the switching converter, alternately stores energy in an inductor’s magnetic
field and releases the energy to the load at a different voltage. Its
reduced losses make it a better choice for high efficiency. The
buck, or step-down, converters provide lower voltage. Switching
converters that include internal FETs as switches are called
switching regulators, whereas devices requiring external FETs
are called switching controllers. Most low power systems use
both LDOs and switching converters to achieve cost and performance objectives.
INTRODUCTION
Smartphones, tablet computers, digital cameras, navigation
systems, medical equipment, and other low power portable
devices often contain multiple integrated circuits manufactured
on different semiconductor processes. These devices typically
require several independent supply voltages, each usually
different from the voltage supplied by the battery or external acto-dc power supply.
Figure 1 shows a typical low power system operating with a LiIon battery. The battery’s usable output varies from 3 V to 4.2 V,
whereas the ICs require 0.8 V, 1.8 V, 2.5 V, and 2.8 V. A simple
way to reduce the battery voltage to a lower dc voltage is to use
BATTERY
Li-Ion
1.8V
VDD I/O
MICROPROCESSOR
MEMORY
POWER ON
RTC
VDD CORE
2.5V
SENSOR
LOW POWER RF
2.8V
0.8V
LDO
ADP151
LDO
ADP150
BUCK
REGULATOR
ADP2138
Figure 1. Typical Low Power Portable System
Rev. 0 | Page 1 of 8
10104-001
LCD DISPLAY
3.6V
BUCK
REGULATOR
ADP2120
AN-1125
Application Note
TABLE OF CONTENTS
Introduction ...................................................................................... 1 Load Transients .............................................................................6 Revision History ............................................................................... 2 Current Limit.................................................................................6 Buck Regulators ................................................................................ 3 Soft Start .........................................................................................6 Buck Regulators Improve Efficiency.............................................. 5 Start-Up Time ................................................................................6 Key Buck Converter Specifications and Definitions.................... 6 Thermal Shutdown (TSD) ...........................................................6 Input Voltage Range ..................................................................... 6 100% Duty Cycle Operation ........................................................6 Ground or Quiescent Current .................................................... 6 Discharge Switch ...........................................................................6 Shutdown Current........................................................................ 6 Undervoltage Lockout ..................................................................6 Output Voltage Accuracy ............................................................ 6 Conclusion..........................................................................................7 Line Regulation............................................................................. 6 References.......................................................................................8 Load Regulation............................................................................ 6 REVISION HISTORY
9/11—Revision 0: Initial Version
Rev. 0 | Page 2 of 8
Application Note
AN-1125
BUCK REGULATORS
Buck 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
current shoot-through. In Phase 1, Switch B is open and Switch A
is closed. The inductor is connected to VIN; therefore, current
flows from VIN to the load. The current increases due to the
positive voltage across the inductor. In Phase 2, Switch A is open
and Switch B is closed. The inductor is connected to ground;
therefore, current flows from ground 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.
In Figure 3, Switch A and Switch B have been implemented with
pFET and nFET switches, respectively, to create a synchronous
buck regulator. The term synchronous indicates that a FET is
used as the lower switch. Buck regulators that use a Schottky diode
in place of the lower switch are defined as asynchronous (or
nonsynchronous). For handling low power, synchronous buck
regulators are more efficient, because the FET has a lower voltage
drop than a Schottky diode. However, the synchronous converter’s efficiency at light load is compromised if the bottom
FET is not released when the inductor current reaches zero,
and additional control circuitry increases the complexity and
cost of the IC.
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. Low power buck converters
rarely operate in DCM. The current ripple, shown as ΔILOAD in
Figure 2, is typically designed to be 20% to 50% of the nominal
load current.
Current low power synchronous buck regulators use pulsewidth 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
t ON
≈ OUT
VIN
t ON + t OFF
PWM
MODULATION
VSW
+
–
IA
CIN
A
PWM ON
VSW
VOUT
L
–
tOFF
T
LOAD
IA
IB
CIN
A
VSW
L
ILOAD
VOUT
PWM OFF
B
COUT
LOAD
IB
ILOAD
∆ILOAD
Figure 2. Buck Converter Topology and Operating Waveforms
VIN
+
–
CIN
IA
A
OSCILLATOR
PWM
CONTROL
CURRENT
LIMIT
ILOAD
VSW
B
IB
COUT
VOUT
LOAD
10104-003
+
tON
VOUT
COUT
B
VIN
ILOAD
Figure 3. Buck Regulator Integrates Oscillator, PWM Control Loop, and Switching FETs
Rev. 0 | Page 3 of 8
10104-002
VIN
AN-1125
Application Note
The FET switches are controlled by a pulse width controller,
which uses either voltage or current feedback in a control loop
to regulate the output voltage in response to load changes. Low
power buck converters generally operate between 1 MHz and
6 MHz. Higher switching frequencies allow the use of smaller
inductors, but efficiency is decreased by approximately 2% for
every doubling of the switching frequency.
4.7µF
VOUT
SW
4.7µF
ADP2138/
ADP2139
AUTO
VOUT
EN
MODE
GND
10104-004
FORCE
PWM
Figure 4. ADP2138/ADP2139 Typical Applications Circuit
100
90
80
70
60
50
40
30
VIN = 2.3V
VIN = 3.6V
VIN = 4.2V
VIN = 5.5V
20
10
0
0.001
0.01
0.1
1
ILOAD (A)
Figure 5. ADP2138 Efficiency in Continuous PWM Mode,
Efficiency vs. Load Current, Across Input Voltage, VOUT = 0.8 V
100
90
80
60
50
40
30
VIN = 2.3V
VIN = 3.6V
VIN = 4.2V
VIN = 5.5V
20
10
0
0.001
0.01
0.1
1
ILOAD (A)
10104-006
EFFICIENCY (%)
70
Figure 6. ADP2138 Efficiency in Automatic PWM/PSM Mode,
Efficiency vs. Load Current, Across Input Voltage, VOUT = 0.8 V
Rev. 0 | Page 4 of 8
10104-005
Analog Devices, Inc., defines efficient light load operation as
power save mode (PSM). When the power save mode is entered,
an offset induced in the PWM regulation level causes the output
voltage to rise, until it reaches approximately 1.5% above the
PWM regulation level, at which point PWM operation turns
off: both power switches are off, and idle mode is entered. COUT
is allowed to discharge until VOUT falls to the PWM regulation
voltage. The device then drives the inductor, causing VOUT to
rise again to the upper threshold. This process is repeated as
long as the load current is below the power save current
threshold.
The ADP2138 is a compact 800 mA, 3 MHz, step-down dc-todc converter. Figure 4 shows a typical applications circuit.
Figure 5 and Figure 6 show the improvement in efficiency
between forced (continuous) PWM and automatic PWM/PSM
operation. Due to the variable frequency, PSM interference can
be hard to filter; therefore, many buck regulators include a
MODE pin (shown in Figure 4) that allows the user to force
continuous PWM operation or allow automatic PWM/PSM
operation. The MODE pin can be hardwired for either operating mode or dynamically switched when needed to save power.
1µH
VIN
ON
OFF
EFFICIENCY (%)
PWM operation does not always improve system efficiency at
light loads. Consider, for example, the power circuitry for a
graphics card. As the video content changes, so does the load
current on the buck converter driving the graphics processor.
Continuous PWM operation can handle a wide range of load
currents, but the efficiency rapidly degrades at light loads
because the power required by the regulator consumes a larger
percentage of the total power delivered to the load. For portable
applications, buck regulators incorporate additional power
saving techniques—such as pulse frequency modulation (PFM),
pulse skipping, or a combination of both.
2.3V TO 5.5V
Application Note
AN-1125
BUCK REGULATORS IMPROVE EFFICIENCY
switching regulator, which offers 82% operating efficiency with
a 4.2 V input and 0.8 V output, delivers more than four times
the efficiency and reduces the temperature rise of the portable
device. Such substantial improvements in system efficiency
have resulted in large numbers of switching regulators being
designed into portable devices.
Increased efficiency allows longer battery operating times
before replacement or recharging, a highly desirable feature in
new portable device designs. For example, a rechargeable Li-Ion
battery can drive a 500 mA load at 0.8 V using the ADP125
LDO, as shown in Figure 7. The LDO’s efficiency, VOUT/VIN ×
100%, or 0.8/4.2, is only 19%. LDOs cannot store the unused
energy; therefore, the 81% (1.7 W) of power not delivered to the
load is dissipated as heat within the LDO, which may cause a
handheld device to heat up quickly. Using the ADP2138
ILOAD = 500mA
VOUT = 0.8V
1
VOUT
VIN
8
VIN = 4.2V
C1
ADP125
C2
R1
2
VOUT
VIN
7
3
ADJ
NC
6
4
GND
EN
5
LITHIUM ION
BATTERY
ON
R2
OFF
Figure 7. ADP125 Low Dropout Regulator Can Drive 500 mA Loads
Rev. 0 | Page 5 of 8
10104-007
LOAD
AN-1125
Application Note
KEY BUCK CONVERTER SPECIFICATIONS AND DEFINITIONS
INPUT VOLTAGE RANGE
SOFT START
A buck converter’s input voltage range determines the lowest
usable input supply voltage. The specifications may show a wide
input voltage range, but VIN must be greater than VOUT for
efficient operation. For example, a regulated 3.3 V output
voltage requires an input voltage above 3.8 V.
It is important for buck regulators to have an internal soft start
function that ramps the output voltage in a controlled manner
upon startup to limit the inrush current. This prevents input
voltage from a battery or high impedance power source from
dropping when it is connected to the input of the converter.
After the device is enabled, the internal circuit begins the
power-up cycle.
GROUND OR QUIESCENT CURRENT
IQ is the dc bias current that is not delivered to the load. Devices
with lower IQ provide higher efficiency. IQ can be specified for
many conditions, however, including switching off, zero load,
PFM operation, or PWM operation. It is, therefore, best to look
at actual operating efficiency data at specific operating voltages
and load currents to determine the best buck regulator for an
application.
SHUTDOWN CURRENT
This is the input current consumed when the enable pin has
been set to off. This current, usually well below 1 μA for low
power buck regulators, is important during long standby times
on the battery while the portable device is in sleep mode.
START-UP TIME
Start-up time is the time between the rising edge of the enable
signal and when VOUT reaches 90% of its nominal value. This
test is usually performed with VIN applied and the enable pin
toggled from off to on. In cases where the enable pin is
connected to VIN, when VIN is toggled from off to on, the startup time can substantially increase, because the control loop
takes time to stabilize. Start-up time of a buck regulator is
important for applications where the regulator is frequently
turned on and off to save power in portable systems.
THERMAL SHUTDOWN (TSD)
Analog Devices buck converters are designed for high output
voltage accuracy. Fixed-output devices are factory trimmed to
better than ±2% at 25°C. Output voltage accuracy is specified
over the operating temperature, input voltage, and load current
ranges, with worst-case inaccuracies specified as ±x%.
If the junction temperature rises above the specified limit, the
thermal shutdown circuit turns the regulator off. Extreme junction temperatures can be the result of high current operation,
poor circuit board cooling, or high ambient temperature. Hysteresis is included in the protection circuit to prevent return to
normal operation until the on-chip temperature drops below
the preset limit.
LINE REGULATION
100% DUTY CYCLE OPERATION
Line regulation is the change in output voltage caused by a
change in the input voltage at the rated load.
With a drop in VIN or an increase in ILOAD, the buck regulator
reaches a limit where the pFET switch is on 100% of the time
and VOUT drops below the desired output voltage. At this limit,
the ADP2138 smoothly transitions to a mode where the pFET
switch stays on 100% of the time. When the input conditions
change, the device immediately restarts PWM regulation with
no overshoot of VOUT.
OUTPUT VOLTAGE ACCURACY
LOAD REGULATION
Load regulation is the change of the output voltage for a change
in the output current. Most buck regulators can hold the output
voltage essentially constant for slowly changing load current.
LOAD TRANSIENTS
Transient errors can occur when the load current quickly
changes from low to high, causing mode switching between
PFM and PWM or from PWM to PFM operation. Load
transients are not always specified, but most data sheets have
plots of load transient responses at different operating
conditions.
CURRENT LIMIT
Buck regulators such as the ADP2138 incorporate protection
circuitry to limit the amount of positive current flowing
through the pFET switch and the synchronous rectifier. The
positive current control limits the amount of current that can
flow from the input to the output. The negative current limit
prevents the inductor current from reversing direction and
flowing out of the load.
DISCHARGE SWITCH
In some systems, if the load is very light, a buck regulator’s
output can stay high for some time after the system enters sleep
mode. Then, if the system starts the power-on sequence before
the output voltage has discharged, the system may latch up or
devices can be damaged. The ADP2139 buck regulator uses an
integrated switched resistor (typically 100 Ω) to discharge the
output when the enable pin goes low or when the device enters
undervoltage lockout or thermal shutdown.
UNDERVOLTAGE LOCKOUT
Undervoltage lockout (UVLO) ensures that voltage is supplied
to the load only when the system input voltage is above the
specified threshold. UVLO is important because it allows the
device to power on only when the input voltage is at or above
the value required for stable operation.
Rev. 0 | Page 6 of 8
Application Note
AN-1125
CONCLUSION
Low power buck regulators demystify switching dc-to-dc
converter design. Analog Devices offers a family of highly
integrated buck regulators that are rugged, easy to use, cost
effective, and require minimal external components to achieve
high operating efficiency. System designers can use the design
calculations presented in the buck regulator data sheet, or use
the ADIsimPower™ design tool. Selection guides, data sheets,
and application notes for Analog Devices buck regulators can be
found at www.analog.com/power-management. For additional
information, contact an applications engineer at Analog
Devices.
Rev. 0 | Page 7 of 8
AN-1125
Application Note
REFERENCES
Information on all Analog Devices components can be found at
www.analog.com.
www.analog.com/portable_power_solutions
Lenk, John D. 1996. Simplified Design of Switching Power
Supplies. Elsevier.
www.analog.com/power-management
Marasco, Ken. 2009. “How to Successfully Apply Low-Dropout
Regulators.” Analog Dialogue. Volume 43, Number 3.
www.analog.com/linear-regulators
www.analog.com/ADIsimPower
©2011 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN10104-0-9/11(0)
Rev. 0 | Page 8 of 8
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