Datasheet

c
c
ƒ
=
ƒ
D
d
c
ƒ
m
δ ƒ
m =
ƒ
´
( )
( )
hmfB
ffmfB
fmh
mcfm
×+××=
+D×=+××=
12
)(212
( )
)(212
mcfm
ffmfB +D×=+××=
0dBV
0dBVref
F
1
F
ENV,PEAK
Dfc
Dfc
Non-modulatedharmonic
Side-bandharmonics
windowaftermodulation
TPS82670, TPS82671, TPS82672, TPS82673, TPS82674
TPS82675, TPS82676, TPS82677, TPS826711, TPS826745, TPS826765
SLVSAI0F –OCTOBER 2010–REVISED NOVEMBER 2012 www.ti.com
For additional flexibility, it is possible to switch from power-save mode to PWM mode during operation. This type
of operation allows efficient power management by adjusting the operation of the converter to the specific system
requirements.
SPREAD SPECTRUM, PWM FREQUENCY DITHERING
The goal of spread spectrum architecture is to spread out the emitted RF energy over a larger frequency range
so that any resulting electromagnetic interference (EMI) is similar to white noise. The end result is a spectrum
that is continuous and lower in peak amplitude. Spread spectrum makes it easier to comply with EMI standards.
It also makes it easier to comply with the power supply ripple requirements in cellular and non-cellular wireless
applications. Radio receivers are typically susceptible to narrowband noise that is focused on specific
frequencies.
Switching regulators can be particularly troublesome in applications where electromagnetic interference (EMI) is
a concern. Switching regulators operate on a cycle-by-cycle basis to transfer power to an output. In most cases,
the frequency of operation is either fixed or regulated, based on the output load. This method of conversion
creates large components of noise at the frequency of operation (fundamental) and multiples of the operating
frequency (harmonics).
The spread spectrum architecture varies the switching frequency by approximately ±10% of the nominal
switching frequency, thereby significantly reduces the peak radiated and conducting noise on both the input and
output supplies. The frequency dithering scheme is modulated with a triangle profile and a modulation frequency
f
m
.
Figure 41. Spectrum of a Frequency Modulated Figure 42. Spread Bands of Harmonics in
Sin. Wave with Sinusoidal Variation in Time Modulated Square Signals
(1)
Figure 41 and Figure 42 show that after modulation the sideband harmonic is attenuated when compared to the
non-modulated harmonic, and when the harmonic energy is spread into a certain frequency band. The higher the
modulation index (mf) the larger the attenuation.
(1)
With:
f
c
is the carrier frequency (i.e. nominal switching frequency)
f
m
is the modulating frequency (approx. 0.016*f
c
)
δ is the modulation ratio (approx 0.1)
(2)
(1) Spectrum illustrations and formulae (Figure 41 and Figure 42) copyright IEEE TRANSACTIONS ON ELECTROMAGNETIC
COMPATIBILITY, VOL. 47, NO.3, AUGUST 2005. See REFERENCES Section for full citation.
18 Submit Documentation Feedback Copyright © 2010–2012, Texas Instruments Incorporated
Product Folder Links: TPS82670 TPS82671 TPS82672 TPS82673 TPS82674 TPS82675 TPS82676 TPS82677
TPS826711 TPS826745 TPS826765