AN5964

AN5964
Keywords:
power, supply, ground, star, decoupling, capacitor, current, loop, noise, radio frequency, interference,
RFI, EMI, filtering
APPLICATION NOTE 5964
POWER SUPPLIES BEGIN THE CIRCUIT
FOUNDATION, TAMING SWITCHING POWERSUPPLY LAYOUT
By: Bill Laumeister, Strategic Applications Engineer
© Oct 08, 2014, Maxim Integrated Products, Inc.
Abstract: This article outlines power, ground PCB, and system layout hints and kinks. It includes an overview of
using power, ground star points, and multifrequency power decoupling. It also shows how to control the component
placement to minimize the size of the high-current switching loop.
Just like a high-rise building in earthquake-prone areas, great circuit design depends on a good foundation for its
longevity and usefulness.
Perfect electronics power supply transforms the alternating current (AC) distributed to our homes and factories into
direct current (DC) that electronics need. In a perfect world, the direct current would be noise-free or free of ripple or
harmonics from the AC. In addition, the ground, which is the reference for our DC power, would be clean and
pristine. As engineers, we quickly learn our world is far from perfect. That means we have to use our skill and
knowledge to obtain the desired results despite the limitations.
One must first understand the operating environment and realize there is often radio frequency interference from
external transmitters as well as the digital circuits operating on the same printed circuit board (PCB). Let’s start by
examining the AC plug. In most cases, a powerline filter such as the one depicted in Figure 1 is desirable.
Figure 1. A common-mode choke powerline filter.
The filter serves a central purpose to symmetrically protect the device from both sides. It protects emissions from the
the powerline into the device and from the device back into the powerline. It is necessary to test for emissions
coming out of the box as well as testing for the susceptibility of signals entering into it.
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General Layout Errors at the Board Level
In general, multilayer boards with solid ground planes and power planes produce the highest level of signal integrity.
Choosing ground star points on the chassis and on each board is a good starting point. Some inexperienced users
view ground as a magical place where all bad things and interference disappears. Sometimes users start out with
the ground star, but then do not bring the return currents back separately from each type of circuit to that star.
Figure 2 illustrates that error.
Figure 2. Ground bounce errors in a system.
We start out with a 5V power supply with a star ground. The noisy digital circuits add more noise to both the +5V
power supply and the ground. We sense that an analog circuit requires a clean 3.3V voltage, but rather than running
a separate wire back to the power supply for both +5V and ground to their star points, we get lazy. An LDO is
needed to make a clean 3.3V power supply, or so we think. In actuality, the linear regulator always keeps its output
exactly 3.3V above its reference or ground. Therefore, if the ground is bouncing up and down like the red arrow
indicates and the LDO is doing its job, the +3.3V output will go up and down following the ground bounce. The
question we have to ask ourselves is how many hours we will spend troubleshooting because we did not run
separate analog and digital wires back to the star points? Figure 3 illustrates a better way to connect the analog
circuit.
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Figure 3. Proper use of the ground and power star points. This implies that power and ground are clean at the star
points.
The implication that power and ground are clean at the star point (shown in Figure 3) means that power and ground
are homogenous at the star points. There is no differential noise between power and ground. Ideally, the powersupply output would have a near-zero impedance or have decoupling capacitors with low equivalent series
resistance (ESR) at the frequencies of interest. The individual conductors connecting the various circuits to the
ground and power star points have series resistance and inductance. We rely on that resistance and inductance to
isolate the noisy circuits from the clean circuits. The series resistance and inductance, as well as the decoupling
capacitors at the output of each circuit block, form lowpass filters. If the wiring to the different circuit blocks is
relatively short, it can become necessary to add discrete resistance and inductance.
Decoupling is complicated because capacitors have parasitic inductance. In practice, a capacitor is represented as a
series RLC circuit as shown in Figure 4. The capacitance will dominate at low frequencies, but above the selfresonant frequency (SRF), the dip in each plot as shown in Figure 5, is the point where the impedance of the
capacitor will begin to look inductive. Thus, a capacitor is only useful for decoupling purposes over a frequency
range that is near or below its SRF when the capacitor presents a low impedance at the frequency of interest.
Figure 4. A capacitor with inherent parasitic components.
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Figure 5. Capacitors with six values and their respective self-resonance points.
1
Figure 5 shows the typical SRF performance for various capacitor values. From these plots, the SRFs can clearly
be seen by the dip in the graph. It also shows that the larger capacitances provide better decoupling (appear lower
impedance) at lower frequencies than the smaller valued capacitors. Free SPICE programs are available for
2, 3, 4
capacitor self-resonance plotting.
Specific Layout Errors at the Switching Power-Supply-Integrated Circuit Level
Figure 6. A simplified schematic diagram of the MAX17501 switching power supply.
In Figure 6, note the two different ground symbols (the triangles) that indicate the high-current fast-rise-time
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switching pulses. It is important to isolate the high-frequency, high-current pulses from the analog small-signal or
reference ground.
5
In Figure 6, a ceramic input filter capacitor (C1) is placed close to the VIN pin of the device. This capacitor acts as a
power reservoir to smooth the power pulses that would have been propagated back to the DC source supply if the
capacitor was not present. Depending on the rise time of the switching pulses, this capacitor can be made up of
several capacitors of different sizes to cover the large frequency range. The bypass capacitor for the VCC pin
should also be placed as close as possible to the VCC pin. This capacitor might also need to be made of multiple
capacitors. For the most efficient heat dissipation, a number of thermal vias should be provided under the exposed
pad of the device.
Figure 7. Isolation between grounds and ground star point and the high current loop (indicated inside the dotted red
line).
The current loop of Figure 7 is the most important area of the switching power supply. Isolating the two grounds is
critical for stable operation since small changes here can have devastating effects on efficiency, noise, radiated
electromagnetic and radio interference. This loop contains pulsed currents, so the PCB traces must be very short
and as wide as possible to reduce stray inductance. Simple changes in this current loop are the difference between
a good and bad layout. By striving for the smallest loop, a bad layout will improve by 20% and become a better
layout by simply rotating the inductor 90°. To reduce the series inductor inherent in a via, parallel two, four, or more
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vias as needed.
Looking at Figure 7, we note the via circle containing a small circle. This is the power ground plane connection (the
ground triangles on the schematic) that connects to a power ground plane on the bottom side of the board and to
the ground star point. The via circles containing an X are the ground for the reference and signal stability. These
connect to a separate ground plane on the bottom of the board and connect to the power plane at the ground star
point. The analog small-signal or reference ground and the power ground for switching currents must be kept
separate. They should be connected together at a point where switching activity is at a minimum (star point),
typically the return terminal of the VCC bypass capacitor.
The via circle containing the plus symbol connects the output voltage to the feedback pin. This needs to be routed
away from the inductor and the power loop as quickly as possible. The series resistor (R4) must be as close as
possible to the feedback pin as it, along with the input capacitance of the feedback pin, produces a lowpass filter as
shown in Figure 8.
Figure 8. With R4 near the output pin, the long lead to the feedback pin (FB/VO) acts as an antenna.
An inexperienced layout person may look at the schematic and place R4 near the power output, as in Figure 8.
Because the inductor is an unshielded coil of wire with a ferrite core it intensifies the electromagnetic fields that
crosstalks into the feedback pin (orange dotted circles). This results in unstable operation as the wire between the
feedback pin and R4 becomes an antenna, picking up the switching edges.
Figure 9. Crosstalk between wires may be capacitive, magnetic, electrostatic or a combination of these.
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In Figure 9, wire A is the high level source and wire B is the high-impedance receiver. Crosstalk can be reduced by
moving wire B away or decreasing its impedance.
Figure 10. Proper signal routing minimizes crosstalk. The combination of R4 and “C internal” acts as a lowpass filter
to reduce crosstalk.
Remember, the power switching may only run a few tens of kilohertz, however, it is the switching-edge harmonics
that crosstalk and radiate. The harmonics can extend for many hundreds of megahertz and need to be controlled.
This is why Figure 10 is a better way to connect the power output pin to the feedback pin. We keep the trace away
from the high current loop (Figure 7) and the inductor L1. R4 will attenuate any interference shown by the orange
circles. Having R4 near the MAX17501 feedback pin improves the lowpass filtering of R4 and the internal
capacitance.
We have covered the simplest layout of a device that contains internal switching transistors to explain the basic
concepts. Refer to other Maxim Integrated tutorials and application notes that cover devices with external
6, 7, 8, 9
transistors.
Conclusion
The time spent in carefully laying out the power supply will bring multiple dividends and result in an efficient quiet
supply. That in turn acts as a good foundation for the rest of the circuitry by providing good clean power and ground
as necessary in every circuit. Another advantage to the design engineer is that good power and ground will make
the troubleshooting of other circuits relatively easy. Tracking down sporadic or intermittent errors in the presence of
noisy power and ground is a real nightmare. Experienced engineers never ignore the importance of the powersupply layout by minimizing it or assigning it to the least experienced engineer. It is just that important.
References:
1. Maxim Integrated tutorial 3630, Power Supply and Ground Design for a WiFi Transceiver, Figures 2 and 3.
®
2. KEMET SPICE Software, Kemet , The Capacitance Company, (Capacitor Self-Resonance) (Freeware).
www.kemet.com/Spice
®
3. Johanson Technology, JTIsoft (freeware) is comprised of two advanced design simulation software packages,
®
®
MLCsoft and MLIsoft . This software provides complete S-parameter and SPICE modeling data on
Johanson's line of RF multilayer ceramic capacitors and inductors over the frequency range of 1MHz to
Page 7 of 8
4.
5.
6.
7.
8.
9.
20GHz.
www.johansontechnology.com/modeling-software/jtisoftr-overview.html#.VCC3ZBYXPEc
AVX Corporation Spice Software website, www.avx.com/SpiApps/default.asp.
American Technical Ceramics, Circuit Designer’s Notebook, (Capacitor handbook).
www.atceramics.com/Userfiles/cdn_nb_brochure.pdf
Maxim Integrated tutorial 2997, Basic Switching-Regulator-Layout Techniques, March 25, 2004.
Maxim Integrated application note 4944, Layout Guidelines Maximize Automotive Power-Supply Performance
and Minimize Emissions.
Maxim Integrated tutorial 716, Proper Layout and Component Selection Controls EMI.
Maxim Integrated application note 3645, Correct Board Layout Lowers EMI of Switchmode Converters.
JTIsoft is a registered trademark of Johanson Technology, Inc.
Kemet is a registered trademark of KRC Trade Corporation.
MLCsoft is a registered trademark of Johanson Technology, Inc.
MLIsoft is a registered trademark of Johanson Technology, Inc.
Related Parts
MAX17501
60V, 500mA, Ultra-Small, High-Efficiency, Synchronous StepDown DC-DC Converter
Free Samples
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APPLICATION NOTE 5964,
AN5964,
AN 5964,
APP5964,
Appnote5964,
Appnote 5964
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