TDF8553J datasheet(10/47 Pages) NXP

10 / 47 page

TDF8553J_1

Objective data sheet

Rev. 01 — 3 December 2008

10 of 47

NXP Semiconductors

TDF8553J

× 50 Watt power ampliﬁer and multiple voltage regulator

8.5.6 Input and AC ground capacitor values

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.

8.5.7 Load detection

8.5.7.1

DC-load detection

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

Ω and bit D5 is reset in the associated data byte register to indicate that a load is

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.

8.5.7.2

AC-load detection

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 shows which AC loads are detected at different output voltages. For example, if a

load is detected at an output voltage of 2.5 V peak, the load is less than 4

Ω. If no load is

detected, the output impedance is more than 14

Ω.

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