TDF8553J NXP Semiconductors, TDF8553J Datasheet - Page 10

• Manufacturer Part Number: TDF8553J
• Description: I2C-bus controlled 4 X 50 Watt power amplifier and multiple voltage regulator
• Manufacturer: NXP Semiconductors

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TDF8553J_1
Objective data sheet
8.5.7.1 DC-load detection
8.5.7.2 AC-load detection
8.5.6 Input and AC ground capacitor values
8.5.7 Load detection
The negative inputs to all four amplifier channels are combined at pin ACGND. To obtain
the best performance of supply voltage ripple rejection and unwanted audible noise, the
value of the capacitor connected to pin ACGND must be as close as possible to 4 times
the value of the input capacitor connected to the positive input of each channel.
When DC-load detection is enabled, during the start-up cycle, a DC-offset is applied
slowly to the amplifier outputs, and the output currents are measured. If the output current
of an amplifier rises above a certain level, it is assumed that there is a load of less than
6
detected.
Because the offset is measured during the amplifier start-up cycle, detection is inaudible
and can be performed every time the amplifier is switched on.
AC-load detection can be used to detect that AC-coupled speakers are connected
correctly during assembly. This requires at least 3 periods of a 19 kHz sine wave to be
applied to the amplifier inputs. The amplifier produces a peak output voltage which also
generates a peak output current through the AC-coupled speaker. The 19 kHz sine wave
is also audible during the test. If the amplifier detects three current peaks that are greater
than 550 mA, the AC-load detection bit is set. Three current peaks are counted to avoid
false AC-load detection which can occur if the input signal is switched on and off. The
peak current counter can be reset by setting bit D1 of instruction byte IB1 to logic 0.
To guarantee AC-load detection, an amplifier current of more than 550 mA is required.
AC-load detection will never occur with a current of less than 150 mA.
Figure 3
load is detected at an output voltage of 2.5 V peak, the load is less than 4 . If no load is
detected, the output impedance is more than 14 .
and bit D5 is reset in the associated data byte register to indicate that a load is
shows which AC loads are detected at different output voltages. For example, if a
Rev. 01 — 3 December 2008
4
50 Watt power amplifier and multiple voltage regulator
TDF8553J
© NXP B.V. 2008. All rights reserved.
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