TDA8920C Datasheet, PDF (20/40 Pages) NXP Semiconductors – 2 X110 W class-D power amplifier

NXP Semiconductors
TDA8920C
2 × 110 W class-D power amplifier
In the following example, a heatsink calculation is made for an 8 Ω BTL application with a
±30 V supply:
The audio signal has a crest factor of 10 (the ratio between peak power and average
power (20 dB)), this means that the average output power is ¹â„₁₀ of the peak power.
Thus, the peak RMS output power level is the 0.5 % THD level, i.e. 170 W.
The average power is then ¹â„₁₀ × 130 W = 17 W.
The dissipated power at an output power of 17 W is approximately 5 W.
When the maximum expected ambient temperature is 85 °C, the total R_th(j-a) becomes
(_---1_---4_---0_---5-–_-----8_---5_----) _{= 11 K/W}
R_th(j-a) = R_th(j-c) + R_th(c-h) + R_th(h-a)
R_th(j-c) (thermal resistance from junction to case) = 1.1 K/W
R_th(c-h) (thermal resistance from case to heatsink) = 0.5 K/W to 1 K/W (dependent on
mounting)
Based on this the thermal resistance between heatsink and ambient temperature is:
R_th(h-a) (thermal resistance from heatsink to ambient) = 11 − (1.1 + 1) = 8.9 K/W
The derating curves for power dissipation (for several R_th(j-a) values) are illustrated in
Figure 9. A maximum junction temperature T_j = 150 °C is taken into account. The
maximum allowable power dissipation for a given heatsink size can be derived or the
required heatsink size can be determined at a required power dissipation level; see
Figure 9.
13.7 Output current limiting
To guarantee the robustness of the TDA8920C, the maximum output current that can be
delivered by the output stage is limited to 9.2 A. OverCurrent Protection (OCP) is built in
for each output power switch.
If the current flowing through any of the power switches exceeds the 9.2 A threshold
current due to, for example, a short-circuit to a supply line or across the load, the
maximum output current of the amplifier is regulated to 9.2 A.
The TDA8920C amplifier distinguishes between low-ohmic short-circuit conditions and
other overcurrent conditions such as dynamic impedance drops of the loudspeakers used.
The impedance threshold (Z_th) depends on the supply voltage used.
Depending on the impedance of the short-circuit, the amplifier reacts as follows:
• Short-circuit impedance (> Z_th): The maximum output current of the amplifier is
regulated to 9.2 A but the amplifier will not shut down the PWM outputs. Effectively
this results in a clipped output signal across the load (behavior very similar to voltage
clipping).
• Short-circuit impedance (< Z_th): The amplifier limits the maximum output current to
9.2 A and at the same time discharges the capacitor on pin PROT. When the voltage
across this capacitor drops below the threshold voltage, the amplifier shuts down
completely and an internal timer is started.
TDA8920C_1
Preliminary data sheet
Rev. 01 — 29 September 2008
© NXP B.V. 2008. All rights reserved.
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