TAS54x4C Hardware Design Guidelines
1.7.1.1
S2G and S2P Detection With and Without Loads
Softer shorts on S2G and S2P can be detected with no load as opposed to with a load. The minimum
resistance for S2G and S2P detect increases by a factor of 2 (and thereby increasing the threshold) when
no load is present than when a load, specifically 4
Ω
, is added to the output.
1.7.2
Tweeter Detect or AC Load Diagnostics
The regular load diagnostic uses DC to perform the detection functions. To detect a capacitor-coupled
tweeter a tweeter-detection circuit is needed. The TAS54x4C devices use a special detection threshold of
the current limiter circuitry and therefore required an audio signal to trigger this circuitry. This signal must
be at a frequency and signal level where the tweeter impedance is the nominal. The crossover or
capacitor increases the load impedance at lower frequencies. The output LC filter can increase the load
impedance at high frequencies.
For example, a DC connected woofer and a capacitor-coupled tweeter are in parallel. In a typical
installation the capacitor is connected within the tweeter terminal as a pretested module. The load
diagnostics test the presence of the woofer and the integrity of the speaker wires, but cannot test the
presence of the tweeter. In this case tweeter detect is used to check that the tweeter is connected
properly.
An issue in accurately measuring an unloaded capacitor-coupled tweeter can occur when a DC-coupled
woofer is not in parallel with the capacitor coupled tweeter. An unloaded LC filter can cause a resonance
to occur in the TAS54x4C and falsely trigger the tweeter detection circuitry.
1.8
Power Supply
The power supply for the TAS54x4C family of devices does not need to be regulated and therefore the
power supply can be the battery of the vehicle. These devices have feedback around the class D amplifier
to provide a fixed gain. As in many class D amplifiers the gain is dependent on the power supply voltage,
but with feedback this is not the case. Additional circuits have been added to improve the power-supply
rejection ratio (PSRR) further. The need for wideband PSRR is to remove high frequency, but audible,
noises from the speaker outputs, such as alternator whine.
The power supply or PVDD needs proper PCB layout for proper performance. The PVDD pins are in two
groups. Place a bulk capacitor of 470 µF or greater near each group of pins. Also a 1-µF bypass X7R
ceramic capacitor must be placed as close to each group of PVDD pins and the PGND pins as possible
for good bypassing. These capacitors are necessary to provide high frequency stability on the PVDD. The
placing of the capacitors at each group of pins is needed because the internal connection between the two
groups in not a high current connection. Therefore, good current carrying capability to all the PVDD pins is
needed.
1.9
Charge Pump
The charge pump, CP, is necessary so that the high-side output N-channel FETS have the proper gate
voltage. The charge pump voltage is above PVDD. The function of the charge pump is optimized for the 1-
µF / 50-V capacitors as shown in the data sheet. Changing these values is not needed. The charge pump
is a capacitor-coupled switch-mode supply that pumps up the charge on a capacitor. If either of the two 1-
µF capacitors is not connected the charge-pump voltage will not be available for the high-side FETS. A
CPUV fault will place all the amplifier output stages in hi-Z. Also, the AVDD pin will be turned off so that
the output FETS do not try to function without the charge pump voltage, so an AVDDUV fault will be
present in the I
2
C fault registers.
15
SLOA196 – June 2014
TAS54x4C Design Guide
Copyright © 2014, Texas Instruments Incorporated
