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TA3020 Datasheet PDF : 27 Pages
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Tripath Technology, Inc. - Technical Information
Low-frequency Power Supply Pumping
A potentially troublesome phenomenon in single-ended switching amplifiers is power supply pumping. This
phenomenon is caused by current from the output filter inductor flowing into the power supply output filter
capacitors in the opposite direction as a DC load would drain current from them. Under certain conditions
(usually low-frequency input signals), this current can cause the supply voltage to “pump” (increase in
magnitude) and eventually cause over-voltage/under-voltage shut down. Moreover, since over/under-voltage
are not “latched” shutdowns, the effect would be an amplifier that oscillates between on and off states. If a DC
offset on the order of 0.3V is allowed to develop on the output of the amplifier (see “DC Offset Adjust”), the
supplies can be boosted to the point where the amplifier’s over-voltage protection triggers.
One solution to the pumping issue it to use large power supply capacitors to absorb the pumped supply current
without significant voltage boost. The low-frequency pole used at the input to the amplifier determines the value
of the capacitor required. This works for AC signals only.
A no-cost solution to the pumping problem uses the fact that music has low frequency information that is
correlated in both channels (it is in phase). This information can be used to eliminate boost by putting the two
channels of a TA3020 amplifier out of phase with each other. This works because each channel is pumping out
of phase with the other, and the net effect is a cancellation of pumping currents in the power supply. The phase
of the audio signals needs to be corrected by connecting one of the speakers in the opposite polarity as the
other channel.
Theoretical Efficiency Of A TA3020 Amplifier
The efficiency, η, of an amplifier is:
η = POUT/PIN
The power dissipation of a TA3020 amplifier is primarily determined by the on resistance, RON, of the output
transistors used, and the switching losses of these transistors, PSW. For a TA3020 amplifier, PIN (per channel)
is approximated by:
PIN = PDRIVER + PSW + POUT ((RS + RON + RCOIL + RL)/RL)2
where: PDRIVER = Power dissipated in the TA3020 = 1.6W/channel
PSW = 2 x (0.01) x Qg (Qg is the gate charge of M, in nano-coulombs)
RCOIL = Resistance of the output filter inductor (typically around 50mΩ)
For a 125W RMS per channel, 8Ω load amplifier using STW34NB20 MOSFETs, and an RS of 50mΩ,
PIN = PDRIVER + PSW + POUT ((RS + RON + RCOIL + RL)/RL)2
= 1.6 + 2 x (0.01) x (95) + 125 x ((0.025 + 0.11 + 0.05 + 8)/8)2 = 1.6 + 1.9 + 130.8
= 134.3W
In the above calculation the RDS (ON) of 0.065Ω was multiplied by a factor of 1.7 to obtain RON in order to account
for some temperature rise of the MOSFETs. (RDS (ON) typically increases by a factor of 1.7 for a typical MOSFET
as temperature increases from 25ºC to 170ºC.)
So, η = POUT/PIN = 125/134.3 = 93%
Performance Measurements of a TA3020 Amplifier
Tripath amplifiers operate by modulating the input signal with a high-frequency switching pattern. This signal is
sent through a low-pass filter (external to the TA3020) that demodulates it to recover an amplified version of the
audio input. The frequency of the switching pattern is spread spectrum and typically varies between 200kHz
and 1.5MHz, which is well above the 20Hz – 22kHz audio band. The pattern itself does not alter or distort the
audio input signal but it does introduce some inaudible noise components.
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TA3020 – KL Rev. 3.0/09.03
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