May 2005 LDO Linear Regulators Rival Switchers for Efficiency

DESIGN IDEAS
LDO Linear Regulators
Rival Switchers for Efficiency
Introduction
Thermal Limitations
While efficiency is always quoted as a
benchmark for switching regulators,
power loss is often more important.
Power loss sets the size of the heat sink,
and the size of the heat sink is—more
than any other component—directly
related to the size of the board.
Linear regulators are about simplicity, so their advantages are clearest
in designs where no more than the
multi-layer circuit board is needed
to provide heat sinking. To a first ap-
2.5
that satisfies the requirements of most
industrial applications.
The amount of output current
that the linear regulator can deliver
depends on the input-to-output differential voltage and the power loss
limitations. For instance, in a 2.5Vto-1.5V design, the 1V differential
voltage allows for 1A of load current to
meet the 1W dissipation requirement
(see Figure 1). If the differential voltage is only 0.7V, as in a 2.5V-to-1.8V
regulator, the maximum load current
increases to over 1.4A.
Figure 1 shows that there is a wide
range of power combinations that can
be filled under these circumstances.
In surface mount designs, power loss
correlates directly to board area as
power is usually dissipated through
the metal layers. With this in mind,
Figure 1 covers a range of linear regulator applications that compare well
with switching regulators—which are
very efficient at high input-to-output
differential voltages, but rarely have
better than 75%–80% efficiency at low
input-to-output differential voltages.
For instance, consider a low dropout regulator regulating 1.8V-to-1.2V
at one amp. With an input-to-output
differential of 0.6V, the maximum
amount of output current available
increases to over 1.5A, at one watt
of power dissipation (see Figure 1).
2
VIN – VOUT (V)
Switching power supplies owe much of
their popularity to their efficiency, even
when the distinction is not necessarily deserved. For instance, when low
voltage input supplies are available,
and currents are around an amp or
so, a less complex low dropout linear
regulator can match the efficiency of
a switcher. Furthermore, if the design
is limited to all surface mount applications, with heat sinking provided by the
board, a linear regulator can provide
switcher-like efficiency over a fairly
wide range of input voltages.
For example, a linear regulator
provides excellent efficiency in a
1.8V-to-1.2V application. Even at 2A
of output current, only 1.2W of power
is dissipated. This is sufficiently low
enough for a multi-layer board to
provide adequate heat sinking.
POWER DISSIPATION = 2W
1.5
POWER DISSIPATION = 1W
POWER DISSIPATION = 0.5W
1
0.5
0
0
1
2
3 4 5 6 7
LOAD CURRENT (A)
8
9
by Tom Gross
10
Figure 1. Various power dissipation limits
shown as a function of load current and
input-to-output differential voltage.
proximation, a multi layer board can
dissipate power at 40°C per watt. If we
want to limit the regulator maximum
At low input and output
voltages, linear regulators
offer excellent regulation,
and in many cases, deliver
efficiency rivaling that of
switching regulators. In all
cases a linear regulator
circuit is simpler and less
costly.
temperature to 125°C, 1W of dissipation allows an ambient temperature
of 85°C. An ambient temperature of
85°C is a conservative design number
VBIAS = 5V
1µF
LTC3026
VIN = 1.8V
VIN
1.8V
BST
SW
IN
OUT
SHDN
ADJ
VOUT
1.5V
11k
4.02k
1µF
GND
PG
a.
COUT
10µF
Ceramic
2.2µH
4.7µF
CER
SW
VIN
SWITCHER
22pF
RUN
MODE
VFB
301k
SYNC
10µF
CER
VOUT
1.5V
432k
GND
b.
Figure 2. Two 1.5V output DC/DC converters. The first (a) is a typical linear regulator using the LTC3026 with an external
bias supply. The second (b) is a typical 1.5V switching regulator application. In circuit (a), if an external bias supply is not
present, the LTC3026 can generate its own bias with an internal boost converter and an external inductor (10 µH, 150mA).
Linear Technology Magazine • May 2005
31
DESIGN IDEAS
Compare the two different topologies
in a 1.8-to-1.5 volt application. In
this design, the power dissipation is
low enough that even three amps of
output current do not exceed our 1W
power limitation. Figure 2a shows a
1.5A application using the LTC3026
CMOS linear regulator. A comparable
step-down switching regulator circuit is shown in Figure 2b. Figure 3
compares the efficiencies and power
losses of both circuits. As shown, the
switching converter is more efficient
at low load currents, but the linear
regulator efficiency matches, then
surpasses, the switcher efficiency as
the load current increases. The same
is true for the power losses. The linear
32
EFFICIENCY (%)
96
SWITCHER
EFFICIENCY
SWITCHER
POWER
LOSS
LDO
92
500
100
400
96
300
EFFICIENCY (%)
VIN = 1.8V
VOUT = 1.5V
POWER
LOSS
200
84
LDO
EFFICIENCY
100
84
0
80
80
0
200
400
600
800
LOAD CURRENT (mA)
1k
LDO
EFFICIENCY
92
88
300
VIN = VOUT + VDROPOUT, VOUT = 1.5V
SWITCHER
EFFICIENCY
88
240
SWITCHER
POWER
LOSS
180
120
LDO
POWER
LOSS
0
200
400
600
800
LOAD CURRENT (mA)
POWER LOSS (mW)
Comparison of a Switcher
and Linear Regulator in
the Same Application
100
POWER LOSS (mW)
Increasing the maximum power dissipation to 2W, allows well over 3A
of output current. The efficiency of a
switching regulator operating under
these conditions is typically 75%. The
added complexity and cost of a switching regulator makes a linear regulator
look even better.
60
1k
0
Figure 3. Efficiency and power loss of the
LTC3026 linear regulator compare favorably
to that of a switching regulator. The LDO
maintains good efficiency to 1.5A.
Figure 4. At the lowest input-to-output
differential voltage, VIN = VOUT + VDROPOUT and
VOUT = 1.5V, the efficiency and power losses of
the linear regulator fare even better compared
to those of the switching regulator.
regulator fares better as load current
increases.
As the input-to-output differential
voltages decrease, such as occurs in
battery-powered applications, the linear regulator efficiency compares even
more favorably to the switcher (see
Figure 4). For instance, at 500mA of
load current, where the dropout voltage of the LTC3026 is only 60mV, the
linear regulator is over 97% efficient,
whereas the switcher efficiency is
around 85%. In this case, the linear
regulator beats the switcher in all
aspects—efficiency, power loss, size,
simplicity and cost.
Conclusion
At low input and output voltages, linear
regulators offer excellent regulation,
and in many cases, deliver efficiency
rivaling that of switching regulators.
In all cases a linear regulator circuit is
simpler and less costly. In applications
where the board can adequately dissipate the power, linear regulators can
handle a reasonable range of inputs
and output voltages.
Linear Technology Magazine • May 2005