TDF8544
NXP Semiconductors
I2C-bus controlled 4 50 W power amplifier
V
= (OUT+ - OUT-)
o
offset
threshold
time
reset, setting
disabled
1 second:
read → no offset,
DBx[D2] reset
V
= (OUT+ - OUT-)
o
offset
threshold
time
read →
set bit
1 second:
read → offset,
DBx[D2] set
001aam700
Fig 13. Offset detection
7.5.3 AC load detection
The AC load detection, set with IB1[D2] = 1, is used to detect if AC-coupled speakers
such as tweeters are connected correctly. The detection requires a 19 kHz sine wave to
be applied to the inputs of the amplifier. A high current AC-load detection mode can be
selected, for example during car assembly, or a low current AC-load detection mode, for
example during switch on of car radio. The output voltage over the load impedance
generates an amplifier current. If the amplifier peak current triggers 4 times a 580 mA
(peak) threshold (or 320 mA (peak) in low current mode), the AC-load detection bit is set.
The 4 ‘threshold cross’ counter is used to prevent false AC-load detection caused by
switching the input signal on or off.
An AC-coupled speaker reduces the impedance at the output of the amplifier in a certain
frequency band. The presence of an AC-coupled speaker can be determined using a high
current mode (IB4[D1] = 1, see Figure 14) or using a low current detection mode
(IB4[D1] = 0; see Figure 14.
If, for instance, a 19 kHz input signal is generated with a peak output voltage of 2 V the
I2C-bus bits are guaranteed to be set with a total AC + DC load less than 4 and are
guaranteed not set with a load of more than 9 ; see Figure 14.
The interpretation of the line driver and amplifier mode DC load bit for AC load detection is
shown in Table 6.
TDF8544
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© NXP B.V. 2011. All rights reserved.
Product data sheet
Rev. 2 — 29 August 2011
18 of 54
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