Noty an144f Linear Technology
Application Note 144
Reduce EMI and Improve Efficiency with
Silent Switcher Designs
Christian Kueck
November 2013
Switching regulators replace linear regulators in areas
where low heat dissipation and efficiency are valued. The
switching regulator is typically the first active component
on the input power bus line, and therefore has a significant
impact on the EMI performance of the complete converter
circuit.
Modern input filter components in surface mount technology have better performance than through-hole parts.
However, this improvement is outpaced by the increase in
operating switching frequencies of switching regulators.
Higher efficiency, low minimum on- and off-times result in
higher harmonic content due to the faster switch transitions.
For every doubling in switching frequency the EMI becomes
6dB worse when all other parameters, such as switch capacity and transition times, remain constant. The wideband
EMI behaves like a first order highpass with 20dB higher
emissions if the switching frequency increases by 10×.
Savvy PCB designers will make the hot loops small and
use shielding GND layers as close to the active layer as
possible; nevertheless pinout, package construction,
thermal design requirements and package sizes needed
for adequate energy storage in decoupling components
dictate a certain minimum hot loop size.
To make layout even more challenging, on a typical planar
printed circuit board the magnetic or transformer style
coupling between traces above 30MHz will diminish all
filter efforts, since the higher the harmonic frequencies are,
the more effective unwanted magnetic coupling becomes.
The tried and true solution is to use a shielding box for
the complete circuit. Of course, this adds costs, increases
required board space, makes thermal management and
testing more difficult, and introduces additional assembly
costs. Another frequently used method is to slow down the
switching edges. This has the undesired effect of reducing
the efficiency, increasing minimum on-, off-times, as well
as the required dead times, and compromises the potential
current control loop speed.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and Silent
Switcher is a trademark of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
The LT8614 Silent Switcher™ Minimizes EMI/EMC Emissions While Delivering High Efficiency at Frequencies Up to 3MHz
V
IN
5.8V TO 42V
f
SW
0.1µF
4.7µF
10nF
41.2k
= 1MHz
V
IN1
PG
SYNC/MODE SW
TR/SS BIAS
1µF
INTV
CC
RT GND
EN/UV
LT8614
V
IN2
GND2GND1
BST
FB
0.1µF
0.1µF
10pF
2.2µH
1M
243k
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V
OUT
5V
4A
47µF
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Application Note 144
With Linear’s new LT8614 Silent Switcher regulator, you
have the effect of a shielded box without using a shield
and also eliminate the above mentioned drawbacks. See
Figure 1.
60
50
40
30
20
(dBuV/m)
10
0
–10
–20
30
FREQUENCY (MHz)
NOISE FLOOR
8614
300138.1 246.284.1 192.2
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Figure 1. Noise Floor and the LT8614 Board at CISPR25
Radiated Measurement in an Anechoic Chamber
The LT8614 has a world class low IQ of the LT861x series
of only 2.5µA operating current. This is the total supply
current consumed by the device, in regulation, with no load.
It features the same ultralow dropout of this family,
which is only limited by the internal top switch. Unlike
alternative solutions, the LT8614’s R
is not limited
DSON
by maximum duty cycle and minimum off-times. The part
skips its switch-off cycles in dropout and performs only
the minimum required off cycles to keep the internal top
switch boost stage voltage sustained, as shown in Figure 6.
sensitive applications such as automotive environments,
a good balance can be attained and the LT8614 can run
either below the AM band for even lower EMI, or above
the AM band. In a setup with 700kHz operating switching
frequency, the standard LT8614 demo board does not
exceed the noise floor in a CISPR25 measurement.
The Figure 2 measurements were taken in an anechoic
chamber 12V in 3.3V out at 2A with a fixed switching
frequency of 700kHz.
60
50
CISPR22 RADIATED CLASS B LIMITS
40
30
20
10
0
−10
−20
RADIATED NOISE LEVEL (dBV)
−30
−40
30
LT8610
LT8614
FREQUENCY (MHz)
450100
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Figure 2. LT8610 and LT8614 700kHz 14V to 3.3V 2A
Radiated EMI in GTEM Corrected for OATS
To compare the LT8614 Silent Switcher technology against
a current state-of-the-art switching regulator, the part was
measured against an LT8610. The test was performed in
a GTEM cell using the same load, input voltage and the
same inductor on the standard demo boards for both parts.
At the same time, the minimum operating input voltage is
2.9V typical (3.4V max.) and the device can supply a 3.3V
rail with the part in dropout. The LT8614 is higher efficiency
than the LT8610/LT8611 at high currents since its total
switch resistance is lower. It can also be synchronised to
an external frequency operating from 200kHz to 3MHz.
The AC switch losses are low so it can be operated at high
switching frequencies without much efficiency loss. In EMI-
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One can see that up to a 20dB improvement is made using
the LT8614 Silent Switcher technology compared to the
already very good EMI performance of the LT8610, especially in the more difficult to manage higher frequency area.
This enables simpler and more compact designs where
the LT8614 switching power supply needs less filtering
and distance compared to other sensitive systems in the
overall design.
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