Datasheet

Effects of Applying a Square Wave Into a Speaker
P
SPKR
+ P
SUP
–P
SUP THEORETICAL
(at max output power)
(19)
P
SPKR
+
P
SUP
P
OUT
–
P
SUP THEORETICAL
P
OUT
(at max output power)
(20)
P
SPKR
+ P
OUT
ǒ
1
h
MEASURED
*
1
h
THEORETICAL
Ǔ
(at max output power)
(21)
hTHEORETICAL +
R
L
R
L
) 2r
DS(on)
(at max output power)
(22)
TPA2006D1
www.ti.com ................................................................................................................................................. SLOS498A – SEPTEMBER 2006 – REVISED JULY 2008
If the amplitude of a square wave is high enough and the frequency of the square wave is within the bandwidth
of the speaker, a square wave could cause the voice coil to jump out of the air gap and/or scar the voice coil. A
250-kHz switching frequency, however, is not significant because the speaker cone movement is proportional to
1/f
2
for frequencies beyond the audio band. Therefore, the amount of cone movement at the switching frequency
is small. However, damage could occur to the speaker if the voice coil is not designed to handle the additional
power. To size the speaker for added power, the ripple current dissipated in the load needs to be calculated by
subtracting the theoretical supplied power, P
SUP THEORETICAL
, from the actual supply power, P
SUP
, at maximum
output power, P
OUT
. The switching power dissipated in the speaker is the inverse of the measured efficiency,
η
MEASURED
, minus the theoretical efficiency, η
THEORETICAL
.
The maximum efficiency of the TPA2006D1 with a 3.6 V supply and an 8- Ω load is 86% from Equation 22 . Using
equation Equation 21 with the efficiency at maximum power (84%), we see that there is an additional 17 mW
dissipated in the speaker. The added power dissipated in the speaker is not an issue as long as it is taken into
account when choosing the speaker.
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