TDF8553J datasheet(11/47 Pages) NXP

11 / 47 page

TDF8553J_1

Objective data sheet

Rev. 01 — 3 December 2008

11 of 47

NXP Semiconductors

TDF8553J

× 50 Watt power ampliﬁer and multiple voltage regulator

8.5.7.3

Load detection procedure

1. At start-up, enable the AC or DC-load detection by setting D1 of instruction byte IB1 to

logic 1.

2. After 250 ms the DC load is detected and the mute is released. This is inaudible and

can be implemented each time the IC is powered on.

3. When the ampliﬁer start-up cycle is completed (after 1.5 s), apply an AC signal to the

input, and DC-load bits D5 of each data byte should be read and stored by the

microcontroller.

4. After at least 3 periods of the input signal, the load status can be checked by reading

AC-detect bits D4 of each data byte.

The AC-load peak current counter can be reset by setting bit D1 of instruction

byte IB1 to logic 0 and then to logic 1. Note that this will also reset the DC-load

detection bits D5 in each data byte.

8.5.8 Low headroom protection

The normal DC output voltage of the ampliﬁer is set to half the supply voltage and is

related to the voltage on pin SVR. An external capacitor is connected to pin SVR to

suppress power supply ripple. If the supply voltage drops (at vehicle engine start), the DC

output voltage will follow slowly due to the affect of the SVR capacitor.

The headroom voltage is the voltage required for correct operation of the ampliﬁer and is

deﬁned as the voltage difference between the level of the DC output voltage before the VP

voltage drop and the level of VP after the voltage drop (see Figure 4).

At a certain supply voltage drop, the headroom voltage will be insufﬁcient for correct

operation of the ampliﬁer. To prevent unwanted audible noises at the output, the

headroom protection mode will be activated (see Figure 4). This protection discharges the

capacitors connected to pins SVR and ACGND to increase the headroom voltage.

(1) IoM = < 150 mA.

(2) IoM = > 550 mA.

Fig 3.

Tolerance of AC-load detection as a function of output voltage

001aai691

102

(

Ω)

VoM (V)

010

7.5

2.5

(2)

(1)

no load present

load present

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