P027_NELE_JUL10.QXP_Layout 1 05/07/2012 11:01 Page 27
Embedded Design Power
Small makes sense
Multichannel regulators enable smaller, more reliable
embedded power solutions. By David Pearson and John Bowman.
Physical dimensions matter
While solution size is critical for portable designs
and for embedded applications, where space is at
a premium, it is equally important for mains
powered solutions. Shrinking the power supply
cuts manufacturing costs by reducing the size
and number of components required. It lessens
the impact on the environment and cuts
transportation costs.
The recent innovation of integrating multiple
switching buck regulators, LDOs, supervisory and
watchdog functionality in to a single chip solution
enables designers to significantly decrease the
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Fig 1: A discrete buck regulator/LDO compared with a single chip multi output regulator
10.6mm
800mA
buck
sot-23-5
0806
chip
inductor
800mA
buck
sot-23-5
0806
chip
inductor
300mA
LDO
sot-23-5
6.8mm
9.3mm
10.4mm
P
ower management, commonplace in
desktop computers and battery powered
embedded systems, is often ignored in
embedded systems featuring a reliable mains
power supply. However, end users and system
designers are becoming conscious of the cost and
undesirability of uncontrolled power usage.
Fortunately, power management is relatively
easy to integrate into mains powered embedded
systems and, from a system development
perspective, efficient design also reduces thermal
issues, leading to benefits elsewhere.
The starting point is to look at the requirements
of the processor and the board architecture.
Modern mpu and fpga based systems require
increasing numbers of voltage rails to supply the
core, interface, memory and precision analogue
devices used in the system.
Typically, today’s mpu based systems use
discrete switching regulators and low drop out
regulators (LDOs) to deliver power; however, as
board area shrinks, this complicates the design
task. Combining multiple switching regulators and
LDOs into one package enables small, flexible,
efficient power management solutions. Integration
of multiple devices into one package brings four
key advantages: size reduction; greater ease of
use; higher reliability; and lower noise.
0806
chip
inductor
300mA
LDO
sot-23-5
Bowman: “Shrinking
the power supply
cuts manufacturing
costs.”
0806
chip
inductor
Four channel
micro pmu
Micro pmu implementation
0402 chip
capacitor
pcb area of a multirail power supply. A good
example of this is Analog Devices’ ADP5034: a dual
1.2A buck regulator with two 300mA LDOs in a
24LFCSP package (see fig 1).
Higher levels of integration allow the use of
fewer, smaller external components. Integrated
switching regulators operate at 3MHz, which
allows very small chip inductors to be used and,
when both switching regulators are enabled and
ADP5037/
ADP5034
0603 chip
capacitor
0402 chip
capacitor
operate in PWM mode, they are configured to run
out of phase. This also reduces the size and cost
of the input capacitors required.
Comparing the layout of a power solution
based on the single chip multioutput regulator
with the discrete approach in fig 2 shows the
discrete solution requires 22 components to be
placed in 97mm2 of board space, whereas the
integrated solution has 19 components placed in
an area of 72mm2.
Integrated solutions
As design cycles continue to shrink, device
manufacturers are responding with power
solutions that are not only easy to design, but
also simple to modify in the future. This enables
designs to be completed without the intricate
knowledge and experience usually associated
with complex power supplies; it enables design
teams to accelerate the development process and
meet tighter product release schedules.
Integrated regulators with dedicated Enable
pins allow the power supply designer to enable or
10 July 2012 27
P027_NELE_JUL10.QXP_Layout 1 05/07/2012 11:01 Page 28
Embedded Design Power
Fig 2: Multichannel regulators are smaller and easier to use
disable each regulators in hardware without
software overhead, allowing power supply rails to
be sequenced easily. The ability to set outputs of
individual regulators with an external resistor
divider is another innovation, allowing designers
to change output voltages during prototyping.
New designs requiring a different combination of
output voltages are therefore easier to implement.
Multi regulators like the micro PMU shown in
fig 2 have internal compensation designed for a
range of I/O voltages and output capacitance. In
addition, soft start and protection circuits are
integrated. These features reduce design and
troubleshooting time.
Device manufacturers are simplifying board
layout and placement with pinouts configured to
allow passive components to be placed as close
as possible to each regulator. Engineers with little
or no knowledge in power circuit design need no
longer fear using sophisticated multioutput
regulators; all that is needed is to follow the
manufacturer’s board layout and component
selection guidelines.
Higher reliability systems
Designs using a multiregulator improve early
failure reliability compared with discrete solutions,
because there are fewer components to be placed
and inspected on the pcb. There are also fewer
devices and circuit connections that may fail later
in the product lifecycle. Furthermore, by
integrating supervisory and watchdog timers
along with multiple regulators into single chip
solutions, reliability and longevity are improved at
the system level.
28
10 July 2012
Some multioutput regulators take enhanced
reliability a step further, by integrating high
accuracy power on reset circuits that monitor the
I/O voltage rails. In a typical microprocessor based
system, a power on reset is used to ensure the
core voltage rail is at the correct level before taking
the processor out of reset. Monitoring these rails
helps to ensure more reliable end products.
As core voltage rails on new mpus and fpgas
continue to fall, the need for high accuracy power
on reset has become more important. For
example, the ADP5041 provides an externally
resistor programmable power on reset with
±1.5% accuracy over the full temperature range,
enabling the low voltage core rail of the newest
generations of microprocessors, asics and fpgas
to be monitored precisely. This also has an
integrated watchdog timer, allowing the
microprocessor code execution activity to be
monitored and guaranteeing safe, reliable
processor operation.
Pearson: “Integrated
regulators [allow]
power supply rails to
be sequenced easily.”
To increase reliability and uptime of remote
systems, such as electric meters, a second
watchdog is integrated into the regulator. This
allows a remote system to be automatically
‘power cycled’ if the system does not operate or
respond correctly.
Low noise solutions
Electromagnetic noise continues to bedevil the
designer of microprocessor based embedded
systems and the problem is worsening with
design complexity. Power supplies are frequently
the source of this undesired noise.
The impact of electromagnetic noise can be
reduced through careful positioning of device pin
outs. Placing external passives as close as
possible to each regulator minimises the effect of
board parasitics and generated noise.
Another source of power supply noise is
switchers operating in burst mode. Forcing
switching regulators to operate in constant PWM
mode eliminates this problem. Providing a
dedicated MODE pin on the multiregulator allows it
to be controlled by an mpu I/O port, useful when
the supplied circuit is sensitive to wideband noise.
To further reduce noise, new integrated power
devices have been optimised by providing a low
input voltage range on the integrated LDOs. This
allows them to deliver high efficiency low noise
outputs by combining one of the buck regulators
with an LDO. For example, LDOs integrated into
Analog Devices’ µPMUs have inputs ranging from
1.7 to 5.5V. The buck regulators can be used as
preregulators to give a high efficiency drop from
a 5V input to 1.8V output on the buck regulator,
which is then applied to the LDO’s input to
provide a low noise 1.2V output for powering
sensitive analogue circuitry.
Integrated LDOs have a high power supply
rejection ratio – even with a low Vin-Vout headroom
– and low inherent noise. In addition, crosstalk
between regulators has been minimised. All these
characteristics are important when supplying
noise sensitive circuits. Meanwhile, the ADP5034
employs phase shift technology; the integrated
step down regulators are operated 180° out of
phase, reducing the need for input filtering and
allowing smaller input capacitors.
Author profiles:
David Pearson, technical director, and John
Bowman, semiconductors marketing director, are
with Anglia Components.
www.newelectronics.co.uk