TDF8599 datasheet(15/52 Pages) NXP

15 / 52 page

TDF8599_1

Product data sheet

Rev. 01 — 13 November 2008

15 of 52

NXP Semiconductors

TDF8599

Class-D power ampliﬁer with load diagnostics

8.5 Protection

The TDF8599 includes a range of built-in protection functions. How the TDF8599 handles

the various possible fault conditions differs for each protection and is described in the

following sections:

8.5.1 Thermal foldback

Thermal Foldback Protection (TFP) is activated when the junction temperature exceeds

the threshold level (145

°C). TFP decreases ampliﬁer gain such that the combination of

dissipation and Rth(j-a) create a junction temperature around the threshold level. The

device will not completely switch off but remains operational at the lower output power

levels. If the junction temperature continues to increase, a second built-in temperature

protection threshold level shuts down the ampliﬁer completely.

8.5.2 Overtemperature protection

If the junction temperature Tj > 160 °C, the OverTemperature Protection (OTP) is activated

and the power stage immediately shuts down.

8.5.3 Overcurrent protection

OverCurrent Protection (OCP) is activated when the output current exceeds the maximum

output current of 8 A. OCP regulates the output voltage such that the maximum output

current is limited to 8 A. The ampliﬁer outputs keep switching and the ampliﬁer is NOT

shutdown completely. This is called current limiting.

OCP also detects when the loudspeaker terminals are short circuited or one of the

ampliﬁer’s demodulated outputs is short circuited to one of the supply lines. In either case,

the shorted channel(s) are switched off.

The ampliﬁer can distinguish between loudspeaker impedance drops and a low-ohmic

short across the load or one of the supply lines. This impedance threshold depends on the

supply voltage used. When a short is made across the load causing the impedance to

drop below the threshold level, the shorted channel(s) are switched off. They try to restart

every 50 ms. If the short circuit condition is still present after 50 ms, the cycle repeats. The

average dissipation will be low because of this forced reduced duty cycle.

When a channel is switched off due to a short circuit on one of the supply lines, Window

Protection (WP) is activated. WP ensures the ampliﬁer does not start-up after 50 ms until

the supply line short circuit is removed.

Table 9.

Overview of protection types

Protection type

Reference

Thermal foldback

Section 8.5.1

Overtemperature

Section 8.5.2

Overcurrent

Section 8.5.3

Window

Section 8.5.4

DC Offset

Section 8.5.5

Undervoltage

Section 8.5.6

Overvoltage

Section 8.5.6

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