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TPS62684 Datasheet(PDF) 13 Page - Texas Instruments

No. de Pieza. TPS62684
Descripción  1600-mA, High-Efficiency Step-Down Converter Optimized for Smallest Solution Size
Descarga  27 Pages
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Fabricante  TI1 [Texas Instruments]
Página de inicio  http://www.ti.com
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TPS62684 Datasheet(HTML) 13 Page - Texas Instruments

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0 dBV
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F1
FENV,PEAK
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Non-modulated harmonic
Side-band harmonics
window after modulation
TPS62684
www.ti.com
SLVSAC5 – APRIL 2014
9.3 Feature Description
9.3.1 Spread Spectrum, PWM Frequency Dithering
The goal is to spread out the emitted RF energy over a larger frequency range, so that the resulting EMI is
similar to white noise. The end result is a spectrum that is continuous and lower in peak amplitude, making it
easier to comply with electromagnetic interference (EMI) standards and with 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 their 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 around ±10% of the nominal switching
frequency, thereby significantly reducing the peak radiated and conducted noise on both the input and output
supplies. The frequency dithering scheme is modulated with a triangle profile and a modulation frequency fm.
Figure 24. Spectrum Of A Frequency Modulated
Figure 25. Spread Bands Of Harmonics In
Sin. Wave With Sinusoidal Variation In Time
Modulated Square Signals (1)
The above figures show that after modulation the side-band harmonic is attenuated compared to the non-
modulated harmonic, and the harmonic energy is spread into a certain frequency band. The higher the
modulation index (mf), the larger the attenuation.
(1)
where:
fc is the carrier frequency (5.5MHz)
fm is the modulating frequency (approx. 0.008*fc)
δ is the modulation ratio (approx 0.1)
(2)
The maximum switching frequency fc is limited by the device and finally the parameter modulation ratio (δ),
together with fm , which is the side-band harmonic´s bandwidth around the carrier frequency fc . The bandwidth of
a frequency modulated waveform is approximately given by Carson’s rule and is summarized as:
(3)
(1)
Spectrum illustrations and formulae (Figure 24 and Figure 25) copyright IEEE TRANSACTIONS ON ELECTROMAGNETIC
COMPATIBILITY, VOL. 47, NO.3, AUGUST 2005. See References Section for full citation.
Copyright © 2014, Texas Instruments Incorporated
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