Minimum Load Impedance; Recommendations - Philips TDA1562Q Application Note

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Philips Semiconductors

5.3 Minimum load impedance

Philips strongly advises against load impedances lower than 4Ω because the output

stage was designed for 4Ω loads. The maximum repetitive output current for which the

TDA1562 output stage is designed is 8A. To ensure that no damage can be caused by

too high output currents the maximum output current protection activates at an output

current of 7.5A.

The maximum peak output voltage at a supply voltage of 14.4V is 24V. The minimum

impedance to achieve a current of 7.5A at a peak voltage of 24V is 24/7.5= 3.2Ω, which

is 4Ω - 20%.

Usually, amplifiers are designed so that they will be able to drive loads which are 20%

below the nominal load impedance without problems, so the nominal minimum load

impedance for the TDA1562 is 4Ω.

When a lower impedance than 3.2Ω is connected to the TDA1562, the current

protection will activate frequently, causing severe distortion. Also no guarantee

can be given for the long term reliability of the device, since reliability guarantees

are given for operation with the nominal load impedance. Prolonged operation with

a lower load impedance may eventually result in overstress of parts.

From the information given in the previous paragraphs the following conclusions can be

drawn:

1: The TDA1562 was not designed for operation with loads lower than 3.2Ω and should

not be used with lower load impedances.

2: Noise testing should be carried out with a signal with a crest factor of 4:1, and not 2:1

because a signal with a crest factor of 2: does not have a sufficient dynamic range to

allow the lift capacitors to recharge.

3: Especially for driving subwoofers, the capacitance of the lift capacitors should be as

high as possible, it should be at least 10000µF, but preferably 22000µF. The higher the

value of the lift capacitors, the lower the risk of distortion while driving low frequency

signals.

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Application note

Rev. 01.02 — 05 May 2006

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TDA1562Q application note

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