
TDF8546
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NXP B.V. 2011. All rights reserved.
Product data sheet
Rev. 3 — 30 August 2011
15 of 61
NXP Semiconductors
TDF8546
I2C-bus controlled 4
45 W best efficiency amplifier
best efficiency mode can be selected in either 2 2 channel- or 4 channel-mode via
I2C-bus commands
If 4 or 2 loads are used, they can be selected via the I2C-bus. When 2 loads
are selected, the output voltage when the best efficiency switch is opened is lower
than when 4
is selected, to avoid switching distortion. This gives a slightly higher
efficiency with 4
loads
If the supply voltage drops below 10 V, for example during an engine start, the best
efficiency mode is switched off
In legacy mode the amplifier is set to 4-channel best efficiency mode with the best
efficiency switch levels set to 2
7.4 Power-on reset and supply voltage spikes
If in I2C-bus mode the supply voltage drops below 4.5 V, the content of the I2C-bus latches
cannot be guaranteed and POR is activated at a typical VP level of 3.1 V. All latches are
reset, the amplifier is switched off and pin DIAG is pulled LOW to indicate that a POR has
occurred; see DB2[D7]. If IB1[D0] is set, the power-on flag is reset, pin DIAG is released
and the amplifier starts. In legacy mode a supply voltage drop below 6 V switches off the
amplifier. When the supply voltage is above 6 V the amplifier restarts if pin STB is still
enabled.
7.5 Protection
7.5.1 Output protection and short-circuit protection
If a short-circuit to ground, to VP or across the load occurs on one or more amplifier
outputs, only the channel with the short will be switched off. The channel that has a
short-circuit and the type of short-circuit can be read via the I2C-bus. If pin DIAG is
enabled for load fault information (IB2[D4] = 0) pin DIAG is pulled LOW. The window
protection prevents a restart of the channel with a short to ground or VP. With a short
across the load the channel is switched on again after 15 ms to check if the short across
the load is still present. If the short-circuit conditions are still present, the channel is
switched off. If several channels have a short across the load at the same time, the
channels are switched on one by one to prevent high supply current switching with four
shorts across the load at the same time. The 15 ms cycle reduces power dissipation. To
prevent audible distortion, the channel with the short can be disabled via the I2C-bus.
7.5.2 Loss-of-ground/loss of VP
Loss-of-ground/loss of VP is a double fault condition: the ground (or VP) wire of the set is
not connected and the ground (or VP) wire is connected to one of the loudspeaker
outputs. In this situation the supply capacitor in the set is charged through the body diodes
of the output power transistors. This body diode (between the drain and source of the
power transistor) is always present in amplifiers with MOS output stages. The capacitor
charge current depends on the series impedance of the supply lines, the output
impedance of the loss-of-ground tester and the value of the capacitor; see
Figure 11. To
simulate a worst-case condition, the loss-of-ground tester is equipped with a buffer
capacitor of 116 mF to simulate a very low output impedance. With an RS of 63 m, peak
currents of more than 70 A have been measured.