Tripath Technology, Inc. - Technical Information
The switching outputs, OUT1 and OUT2, are not synchronized, so a common inductor may not
be used with a bridged TK2150. For this same reason, individual zobel networks must be applied
to each output to load each output and lower the Q of each common mode differential LC filter.
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 TK2150 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 TK2150 Amplifier
The efficiency, η, of an amplifier is:
η = POUT/PIN
The power dissipation of a TK2150 amplifier is primarily determined by the on resistance, RON, of
the output transistors used, and the switching losses of these transistors, PSW. For a TK2150
amplifier, PIN (per channel) is approximated by:
PIN = PDRIVER + PSW + POUT ((RS + RON + RCOIL + RL)/RL)2
where: PDRIVER = Power dissipated in the TP2150 = 1.0W/channel
PSW = 2 x (0.01) x Qg (Qg is the gate charge of MOSFET, in nano-coulombs)
RCOIL = Resistance of the output filter inductor (typically around 50mΩ)
For a 125W RMS per channel, 8Ω load amplifier using FQP13N10 MOSFETs, and an RS of
50mΩ,
PIN = PDRIVER + PSW + POUT ((RS + RON + RCOIL + RL)/RL)2
= .8 + 2 x (0.01) x (12) + 125 x ((0.05 + 0.2414 + 0.05 + 8)/8)2 = 0.8 + 0.24 + 135.9
= 136.94W
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 = 125/136.94 = 91%
Performance Measurements of a TK2150 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 TK2150) that demodulates it to
recover an amplified version of the audio input. The frequency of the switching pattern is spread
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TK2150 – Rev. 1.0/12.02
TRIPATH [ TRIPATH TECHNOLOGY INC. ]
