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From: Winfield Hill
Subject: Re: TI's low-cost 20W "filterless" class-D amplifer IC
Date: 22 Dec 2002 17:49:26 -0800
Organization: Rowland Institute
References: <email@example.com> <3E064DFC.A1D68C83@ieee.org>
X-Newsreader: Direct Read News 4.20
> 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 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
> filter configuration).
I can think of a simple analog way to implement TI's "filterless" PWM
modulation, but going beyond that to pure digital methods, involving
sigma-delta conversion with noise shaping or whatever is another scene.
> 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.
You've got our attention, tell us more.
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