AN737
Maxim > Design Support > Technical Documents > Tutorials > Power-Supply Circuits > APP 737
Keywords: power supply, switching regulators, linear regulators, charge pumps, switched capacitor,
voltage converters, regulator ICs, inductorless voltage regulator, portable power, dc dc, DC-DC
converters, step-up, step-down, buck, boost, inverter, isolated p
TUTORIAL 737
Choosing the Right Power-Supply IC for your
Application
Oct 28, 2009
Abstract: This article discusses the process of choosing a power supply for a given application. It
reviews the features of the three most common types of DC-DC converting power-supply ICs. It also
provides links to other application notes and tutorials that discuss more advanced topics.
Choosing a power-supply IC can be a daunting task for the inexperienced. This application note will help
the novice engineer take the first step toward becoming a confident power-supply designer. This article
discusses how to choose these ICs at the most fundamental level. First, it explains why power-supply
ICs are necessary. It then shows how to choose among the three most common power-supply ICs
powered with DC voltages: linear regulators, switching regulators, and charge pumps. Links to supportive
application notes and tutorials will address more advanced power-supply topics.
Why Power-Supply ICs?
The chief purpose of most power-supply ICs is to regulate. These devices take an unregulated input
voltage and provide a regulated output voltage. Restated most simply, these ICs provide an output
voltage that remains steady despite varying input voltage or output current. This accounts for the names
linear regulator and switching regulator. The exception to this simple definition is the charge pump.
Depending on the specific device, a charge pump's output can be either regulated or unregulated.
Sometimes regulators create a regulated output voltage from a regulated input voltage. In that case, the
regulator's function is to change the input voltage to another voltage level, without necessarily improving
the voltage regulation.
You might be tempted to power a circuit without a regulator. In fact, in some cases you can be
successful with this. You could, for example, power portable equipment directly from a battery. But this
approach usually leads to problems. The circuitry within portable equipment typically operates correctly
only within a certain narrow voltage range. This is especially true with microprocessors and memory,
particularly if high speed is needed. For microprocessors, memory, and many other types of circuitry, the
voltage range over which the battery operates could extend beyond acceptable levels. Therefore, adding
a regulator ensures that your circuitry receives the appropriate voltage.
The battery's internal resistance can also present a problem if a regulator is not used. This difficulty
arises because circuitry within portable equipment often demands a varying level of supply current. This
varying current, when drawn from the battery, creates a varying battery voltage due to the battery's
Page 1 of 8
internal resistance. Portions of the circuitry can "object" to these variations in battery voltage because the
PSRR (power-supply rejection ratio) of the circuitry's various components might not be sufficient to reject
these voltage variations. To combat this problem, a regulator maintains a steady output voltage despite
these varying load currents. A regulator provides this steady voltage, because its active circuitry
maintains an output resistance that is significantly lower than the battery's series resistance.
Any attempt to design without a regulator challenges the necessity of the steady voltage that regulators
provide, despite varying input voltage or output current. But there is even more to consider. In most
cases switching and linear regulators, as with regulated and unregulated charge pumps, serve an
additional purpose: they create a voltage with a different magnitude from the voltage that powers them.
This explains for the name DC-DC converter. Technically speaking, all three types of power-supply ICs
discussed here (e.g. linear regulators, switching regulators, and charge pumps) are DC-DC converters,
However, the term is normally reserved for switching regulators.
Somewhat limited when converting voltage levels, a linear regulator can only produce a voltage lower
than the voltage supplying it. Much more versatile, a switching regulator can step up (boost), step down
(buck), or invert (change the polarity of) its supply voltage. Charge pumps perform these same three
operations, but with limited output-current capability.
Please see application note 269, "Trading Performance for Cost in Portable Power Supplies."
There are exceptions to the idea that power-supply ICs create voltages of a different magnitude than the
voltages fed to them. The exceptions are most often found in transformer-coupled converters. It is not
unusual to find a transformer-coupled converter whose output voltage equals its input voltage. See
Figure 1. In most cases, the sole purpose of a converter configured this way is to provide isolation,
which prevents a DC connection between the input voltage supplied to the converter and the output
voltage created by the converter. See Figure 2. Isolation is often used for safety reasons. The power for
circuitry connected to a patient through electrodes, for example, is isolated from the power derived from
a wall socket to protect against the risk of shock. But you do not need to be in a hospital to require
isolation. Nearly all consumer electronic products isolate the AC line from the operating circuitry.
Figure 1. This transformer-coupled switching-regulator circuit isolates the input voltage from the output
voltage. Power-supply designers often use an optocoupler to isolate the circuit's output from the
regulator's feedback pin.
Page 2 of 8
Figure 2. Circuit has an isolated 5V to 5V at 400mA power supply. The MAX668 step-up controller is
featured.
See the Isolated-Power Supply Reference Guide. Maxim's application notes for isolated power supplies
are categorized for convenient reference.
The advantages and disadvantages of linear regulators, switching regulators, and charge pumps will now
be explained.
Linear Regulators
Linear regulators are often the smallest, usually the least expensive, and always the least noisy of the
various types of power-supply ICs. See Figure 3. Use a linear regulator if it is appropriate for your
application, especially because a linear-regulator circuit is more likely to work correctly the first time than
circuits built around other types of power-supply ICs.
Figure 3. A linear regulator both steps down and regulates the voltage supplied to it with a minimal
number of external components. Because these devices contain no switching elements, they generate
little noise. Also, the circuit-board layout of linear regulators is less critical than for switching regulators
and charge pumps.
Why would you use any power-supply IC other than a linear regulator? There are two reasons. First, a
linear regulator can only provide an output voltage that is smaller than its input voltage. As mentioned
above, if you wanted to create a voltage that is higher than the input voltage or of opposite polarity, you
Page 3 of 8
have no choice but to use a switching regulator or a charge pump.
The second reason is efficiency. Converting a voltage to another voltage always wastes power. In the
ideal situation, a regulator wastes no power; its efficiency rating would be 100%. If half the power
supplied to a regulator found its way to the regulator's load, its efficiency would be 50%.
A linear regulator is usually, although not always, less efficient than a switching regulator. You can
calculate a linear regulator's efficiency by dividing its output voltage by its input voltage. (This formula is
sufficiently accurate if the current that powers the regulator, the regulator's supply, or quiescent current,
is a small percentage of the current drawn from the regulator's output. In most cases, it is.) Thus, with
LDOs (low dropout linear regulators), where the voltage of the source powering a linear regulator is near
the regulator's output voltage, efficiency is high, In that case, an LDO might be a better choice than a
switching regulator because the LDO has less noise. The MAX1725/MAX1726 are ultra-low supply
current LDOs with a maximum 4.5µA quiescent current.
A high-efficiency regulator provides a distinct advantage in portable equipment, as less wasted power
results in longer battery life. You may need a high-efficiency regulator for another reason: wasted power
is dissipated as heat. Thus, a high-efficiency power supply often suits wall-powered equipment as well as
portable equipment. This power supply can reduce the temperature within an enclosure to a tolerable
level in either case.
In automotive applications, usually a higher input voltage and wider temperature range are required. For
example, the MAX6765–MAX6774 operate from 4V to 72V over the -40°C to +125°C temperature range.
These linear regulators feature control inputs which make them "automotive friendly."
See low-dropout linear regulators.
Switching Regulators
Switching regulators share none of the advantages of linear regulators. Switching regulators consume
more board area (except, perhaps, when a linear regulator requires a heatsink to dissipate the power
lost within it); cost more; and generate more noise than their linear counterparts. Yet, for years switching
regulators have been enormously popular with power-supply designers. Why?
Switching regulators remain popular because these devices boast excellent efficiency when subjected to
many combinations of input voltage and load current. (The levels can be as high as 96% for both stepup and step-down switchers, although a step-down is typically more efficient, and up to 90% for an
inverter). Also, if you need to step up, step down, or invert a voltage, switching regulators are the only
devices capable of these operations for load currents above approximately 125mA. You can, admittedly,
use charge pumps to perform these operations, but the load currents that these devices allow are limited.
It is simply too expensive to integrate switches large enough to handle load currents above the 125mA
level into charge pumps, although a few charge pumps do supply several hundred milliamps.
Switching regulators are so named because they switch a power transistor, which, when used in
conjunction with an inductor, efficiently converts one voltage to another. See Figure 4. When these
power transistors switch, they do so very quickly, as fast transitions improve the regulator's efficiency. To
understand why, first consider the power transistor's power dissipation when it is not transitioning. When
the transistor is off, voltage appears across it, but no current flows through it. Thus, no power is lost.
When the transistor is on, a small voltage appears across it while appreciable current may flow through
it. Thus, typically, a small amount of power is lost. When the power transistor transitions from an OFF
state to an ON state, or vice versa, voltage appears across the transistor while current flows through it.
Therefore, appreciable power can be lost. Speeding up the switching process reduces these transition
Page 4 of 8
losses.
To minimize the power loss associated with the rectifier diode in a switching power supply, a
synchronous configuration can be used. In a synchronous configuration, the rectifier diode is replaced
with a MOSFET switch. This approach increases the efficiency of the switching converters even further.
The MAX15023 is an example of a synchronous buck controller.
See application note 2031, "DC-DC Converter Tutorial."
Figure 4. Switching regulators can (a) step down (buck), (b) step up (boost), or (c) step down/up and
invert (buck boost) the voltage that powers them. The external transistor switches pictured for each of
these three switching regulator types are often included within the switching regulator, usually when the
device is specified for moderate-load currents. Occasionally, step-down and step-up switching-regulator
ICs comprise the rectifier shown here.
These fast transitions, along with the heavy currents that often flow in these circuits, make circuit-board
layout critical. Switching-regulator circuits require a well-thought-out layout and the components external
to the switching-regulator IC must be specified correctly. Considering the various types of power
supplies, switching regulators require the most careful design.
Fortunately, there is a way around these layout concerns. For applications with lower output-current
requirements, switching regulators with integrated MOSFETs are offered. The MAX17083 is a step-down
regulator with integrated switches and it supports continuous load current up to 5A. The internal switch
facilitates the circuit-board layout significantly. However, this limits the output power to the power
dissipation limits and design specifications of the IC.
Page 5 of 8
Maxim also supplies EV (evaluation) kits for most switching regulators. These kits demonstrate a working
layout of the power supply, which in many cases can be adapted to a particular circuit board. Also,
multiple sources for the various external components are usually listed in the regulator's data sheet.
Deciding which external components to use is simply a matter of choosing from among several devices
already specified, each of which is capable of working well with the particular switching-regulator IC.
See application notes for power-supply circuits, and the Maxim's Power-Supply Cookbook.
Maxim also provides an on-line simulation tool (EE-Sim) to design and simulate the power-supply
circuits.
Charge Pumps
Charge pumps constitute the least understood category of power-supply ICs discussed here. These
devices perform the same functions as switching regulators, but without an inductor. Instead, charge
pumps use capacitors to step down, invert, or boost the voltages that power them. See Figure 5.
Figure 5. Like switching regulators, charge pumps step down, step up, and invert voltages, but use
capacitors instead of an inductor to perform these operations. In most cases, charge pumps handle lower
output currents than switching regulators. Although the topologies vary somewhat, the circuit shown here
is representative of both regulated and unregulated charge pumps, regardless of whether they step up,
step down, or invert voltages.
Charge pumps come with both unregulated and regulated outputs. Consider a circuit that is connected to
an unregulated charge pump's output, As this circuit draws more and more current, its output voltage
drops proportionately. The charge pump's output impedance is thus essentially a fixed resistance. When
used in the inverting mode, unregulated charge pumps provide an output voltage equal to the voltage
powering the device, but with opposite polarity (e.g., the MAX828). As load current increases, the
magnitude of this voltage drops, as discussed above. When used in the doubling mode, these charge
pumps precisely double the applied voltage (e.g., the MAX1682) and the output voltage also drops as
load current increases.
Regulated charge pumps can step up, step down, or invert the applied voltage. Unlike unregulated
charge pumps, these devices provide output-voltage levels that are not strictly dependent on the voltage
level fed to them. Thus, these devices could, for example, create a 5V output from a 3.3V input. Also,
because they are regulated, as the output current increases, the output voltage remains essentially
constant. As mentioned above, the amount of current that can be drawn from these devices, as well as
from unregulated charge pumps, is limited. The upper limit is about 125mA (e.g., the MAX1595),
although there are a few parts that handle several hundred milliamps (e.g., the MAX889). It is not
economical to build charge pumps that supply large load currents. Instead, inductor-based switching
regulators are well suited for this situation.
Page 6 of 8
A charge pump switches the capacitors connected to it, and thus creates noise. This noise is usually of
smaller magnitude than a switching regulator's noise. There are three primary reasons for this. First, load
currents are lighter. Second, because these circuits do not include an inductor, no magnetic noise is
created. Finally, when a charge pump interrupts the current flowing through a capacitor connected to it, a
voltage spike is not created. A switching regulator interrupting the current flowing through an inductor
usually creates a voltage spike.
Charge-pump data sheets provide you with the information needed to select the only external
components needed when using a charge pump: namely, capacitors. Although EV kits are not as
necessary and are, thus, less available for charge pumps, any available EV kit can furnish a quick way
to determine whether or not the part is right for a particular application.
See application notes for charge pumps.
Related Parts
MAX15023
Wide 4.5V to 28V Input, Dual-Output Synchronous Buck
Controller
Free Samples MAX1595
Regulated 3.3V/5.0V Step-Up/Step-Down Charge Pump
Free Samples MAX1682
Switched-Capacitor Voltage Doublers
Free Samples MAX17083
Low-Voltage, Internal Switch, Step-Down Regulator
Free Samples MAX1725
12V, Ultra-Low-IQ , Low-Dropout Linear Regulators
MAX1726
12V, Ultra-Low-IQ , Low-Dropout Linear Regulators
MAX6765
Automotive Micropower Linear Regulators with
Supervisor
Free Samples MAX6766
Automotive Micropower Linear Regulators with
Supervisor
Free Samples MAX6767
Automotive Micropower Linear Regulators with
Supervisor
Free Samples MAX6768
Automotive Micropower Linear Regulators with
Supervisor
Free Samples MAX6769
Automotive Micropower Linear Regulators with
Supervisor
Free Samples MAX6770
Automotive Micropower Linear Regulators with
Supervisor
Free Samples MAX6771
Automotive Micropower Linear Regulators with
Supervisor
Free Samples MAX6772
Automotive Micropower Linear Regulators with
Supervisor
Free Samples MAX6773
Automotive Micropower Linear Regulators with
Supervisor
Free Samples MAX6774
Automotive Micropower Linear Regulators with
Page 7 of 8
Supervisor
MAX828
Switched-Capacitor Voltage Inverters
Free Samples MAX889
High-Frequency, Regulated, 200mA, Inverting Charge
Pump
Free Samples More Information
For Technical Support: http://www.maximintegrated.com/support
For Samples: http://www.maximintegrated.com/samples
Other Questions and Comments: http://www.maximintegrated.com/contact
Application Note 737: http://www.maximintegrated.com/an737
TUTORIAL 737, AN737, AN 737, APP737, Appnote737, Appnote 737
Copyright © by Maxim Integrated Products
Additional Legal Notices: http://www.maximintegrated.com/legal
Page 8 of 8
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement