Notes on Connecting APA Outputs to Other Devices

We might design with multiple audio power amplifiers connected to one output circuit to multiplex different sources, or an external amplifier to extend battery life. In addition, we may inadvertently connect one amplifier output to another amplifier output or to a power supply. All these connections will force the APA output voltage to be abnormal, which will damage the APA. There are some limitations that must be observed to avoid such damage, and the reasons for these limitations are explained in this article.

This corruption occurs whether APA is on or off. When the APA is turned on, the output of most APAs is protected by a short circuit protection (SCP) circuit or an overcurrent protection (OCP) circuit, but the APA can withstand the same voltage range. In general, the voltage forced into the APA output must be limited as follows to avoid APA damage:

The APA output should not be forced to exceed the APA’s positive supply voltage (VDD or VCC) by more than 0.3V, or its negative supply voltage (reference voltage or VSS) by -0.3V.

The APA output must not be forced to exceed the absolute maximum value of the rated supply voltage of the APA data sheet.

How an APA responds to a voltage forced into its output

When off, the APA outputs have different resistances, ranging from a few ohms to thousands of ohms at high impedance. If an external audio source connected to the APA output can drive this resistor, it will push the voltage of the APA output.

Most Class AB devices have an SCP continuous current limit when turned on. This APA maintains its output at the planned output voltage level until it is forced into the SCP or OCP by another source. After that, it continues to draw its current limit, but its output voltage is controlled by another source. If the APA continues to draw its current limit, it may overheat and go into thermal shutdown. Its output voltage is completely controlled by another source. Once the APA has cooled sufficiently, it will turn back on, and the cycle will not stop as long as the external source is not disconnected.

A typical Class D APA keeps its output at the planned output voltage level until it is forced into the SCP or OCP. Then, as soon as a certain voltage limit occurs, it turns off, its output voltage is controlled by another source, and no significant current is drawn. A Class D APA with cycle-by-cycle OCP typically behaves like a continuous current limiter until it turns off.

How the damage happened

If another source is connected to an APA output when it is off, it forces the APA output to follow its voltage. If the APA is on and another source can supply enough current to force the APA into SCP or OCP, the other source will force the APA output to follow its voltage. There can be several different ways of damage.

Forward Biased Body Diode

Single-supply APAs operate between a positive supply (commonly referred to as VDD or VCC) and a reference voltage. The output devices are some FETs with body diodes that are reverse biased in normal operation. If one of these diodes becomes forward biased and carries excessive current, the body diode that is reverse biased in normal operation (see Figure 1) can be damaged. This occurs when a single-supply APA is forced to exceed 0.3V above VDD (or VCC) or -0.3V below the reference voltage.

Figure 1 Current Conduction of Forward Biased Body Diode

Notes on Connecting APA Outputs to Other Devices

Figure 2 Pushing the APA supply voltage beyond its limits

Notes on Connecting APA Outputs to Other Devices

TI DirectPath™ operates between the positive power supply (commonly called VDD) and the negative rail (commonly called VSS, typically generated from the switching circuit VDD), which is typically an order of magnitude lower than that of VDD. Some DirectPath APAs regulate the main VDD lower for their output HPVDD and generate a negative rail HPVSS from HPVDD to control the maximum output power. If the output of the DirectPath APA is forced to exceed 0.3V above VDD/HPVDD or -0.3V below VSS/HPVSS, one of the body diodes may become forward biased and carry excessive current, damaging the diode.

Power supply overvoltage

Even though external source currents do not damage the body diodes, they may flow to VDD/HPVDD or VSS/HPVSS (see Figure 2). VDD/HPVDD and VSS/HPVSS generally only draw current, so diode currents can push the supply voltage beyond its absolute maximum ratings and can therefore damage the APA and/or power supply components.

Table 1 may help to understand the different power supplies for various DirectPath APAs. By comparing its data sheet material with this table, we can determine that the device’s power supply is not covered here. Power supply IDs may differ from those shown in the table.

Table 1 APA device power supply voltage limit comparison

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