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Subject: Re: TI's low-cost 20W "filterless" class-D amplifer IC
NNTP-Posting-Date: Sun, 22 Dec 2002 23:42:54 GMT
Organization: AT&T Broadband
Date: Sun, 22 Dec 2002 23:42:54 GMT
Winfield Hill wrote:
> 20 watts... TI continues to improve the performance of their low-cost
> "filterless" class-D amplifer IC offerings.
> I'm impressed by the new TPA3001, a 20-watt 0.2% distortion (15W 1kHz
> 8-ohms 18V supply), no-heat-sink part in a small 24-pin TSSOP package.
> Only $2.82 qty 1k, but no inventory or samples are available just yet.
> If you're not familiar with it, TI's new "filterless" class D uses a
> novel modulation scheme different from most other class-D amplifiers.
> "The modulation scheme has very little loss in the load without
> a filter because the pulses are very short and the change in
> voltage is Vcc instead of 2Vcc. As the output power increases,
> the pulses widen, making the ripple current larger. Ripple
> current could be filtered with an LC filter for increased
> efficiency, but for most applications the filter is not needed."
> They do suggest a small EMI filter.
> This un-named scheme appears to differ somewhat from TI's Equibit
> "true digital" technology, however Equibit is a term that doesn't
> appear much in TI's new sales literature. Hmm...
They are using a bi-phase switching scheme that doubles the carrier
frequency and adds a very convenient null in switching ripple at 50
percent duty cycle that allows them to get away with minimal filtering
(although in doing so they overlooked a simple, yet much more elegant
By the way, the bi-phase approach is just the simplest of the general
approach of using n equally spaced phases in a class-d design . Each
additional phase ups the factor by which the carrier frequency is
effective multiplied and adds an additional null point where ripple
goes to zero with duty cycle (in addition to the nulls at 0 and 100
percent, of course). With an even number of phases there always will
be a null in ripple at 50 percent duty cycle.
Happy Holidays. -- analog
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