
105°C/W
A
J
JA DMAX
T Max = T Max - θ P
= 150 - 105 (0.4) = 108°C
Ferrite
ChipBead
Ferrite
ChipBead
1nF
OUTP
OUTN
SLOS660A
– JANUARY 2010 – REVISED MARCH 2011
EFFICIENCY AND THERMAL INFORMATION
The maximum ambient temperature depends on the heat-sinking ability of the PCB system. The derating factor
for the packages are shown in the dissipation rating table. Converting this to
θJA for the WCSP package:
(7)
Given
θJA of 105°C/W, the maximum allowable junction temperature of 150°C, and the maximum internal
dissipation of 0.4 W for 3 W output power, 4-
load, 5-V supply, from
Figure 17, the maximum ambient
temperature can be calculated with the following equation.
(8)
Equation 8 shows that the calculated maximum ambient temperature is 108
°C at maximum power dissipation
with a 5-V supply and 4-
a load. The TPA2028D1 is designed with thermal protection that turns the device off
when the junction temperature surpasses 150
°C to prevent damage to the IC. Also, using speakers more
resistive than 8-
dramatically increases the thermal performance by reducing the output current and increasing
the efficiency of the amplifier.
OPERATION WITH DACS AND CODECS
In using Class-D amplifiers with CODECs and DACs, sometimes there is an increase in the output noise floor
from the audio amplifier. This occurs when output frequencies of the CODEC/DAC mix with the Class-D
switching frequency and create sum/difference components in the audio band. The noise increase can be solved
by placing an RC low-pass filter between the CODEC/DAC and audio amplifier. The filter reduces high
frequencies that cause the problem and allows proper performance.
TPA2028D1 includes an integrated low-pass filter for this purpose. It is still possible that Class-D output noise will
be affected in extreme cases. In such a case, the RC filter may still be needed.
SHORT CIRCUIT AUTO-RECOVERY
When a short circuit event happens, the TPA2028D1 goes to low duty cycle mode and tries to reactivate itself
every 110
s. This auto-recovery will continue until the short circuit event stops. This feature can protect the
device without affecting the device's long term reliability. FAULT bit (register 1, bit 3) still requires a write to clear.
FILTER FREE OPERATION AND FERRITE BEAD FILTERS
A ferrite bead filter can often be used if the design is failing radiated emissions without an LC filter and the
frequency sensitive circuit is greater than 1 MHz. This filter functions well for circuits that just have to pass FCC
and CE because FCC and CE only test radiated emissions greater than 30 MHz. When choosing a ferrite bead,
choose one with high impedance at high frequencies, and low impedance at low frequencies. In addition, select a
ferrite bead with adequate current rating to prevent distortion of the output signal.
Use an LC output filter if there are low frequency (
< 1 MHz) EMI sensitive circuits and/or there are long leads
from amplifier to speaker.
Figure 45 shows typical ferrite bead and LC output filters.
Figure 45. Typical Ferrite Bead Filter (Chip bead example: TDK: MPZ1608S221A)
Copyright
2010–2011, Texas Instruments Incorporated
31