The fold back current is one kind of reverse current. How is this reverse current caused? That is, when the voltage of non-power pins is higher than the voltage on power pins, the reverse current is generated. Even this current may result in voice IC crashing and high Stand-by current problems. In practical application, this unexpected current happens if we don’t make a complete consideration on circuit design. In following section, we’d like to analyze the reasons and introduce a solution to it.
1. The reasons
Reason 1: The application of connecting IO port to switch (Single pole double throw). Please see the figure 1 below.
In some cases, fold back current may occur if IO is connected to switches for converting modes or sounds. To do so, IO is usually connected to the middle pin of switch; the other two sides’ pins are connected to the ground and VDD. If slide the switch, the connection of IO will be changed from ground to VDD. Meanwhile, the voltage of IO will turn into power voltage. However, the voltage fluctuation occurs when voice IC connects to power for working. This may lead to that the voltage of power pins is lightly lower than power voltage. At that time, the fold back current may occur.
Reason 2: The application of connecting IO to VDD.
In another occasions, fold back current may happen when IO are used as Bonding Option to connect to High/Low. To do so, Bonding is connected to power supply or ground. But this may mostly cause problems in voice IC. When IO connect to power supply, the voltage of IO goes up immediately. While the voltage on VDD pins cannot rise so quickly as that of IO since the decouple capacitances delay rising. Hence, fold back current does not disappear until the decouple capacitance is charged fully. This may also results in the Power on Reset fail.
2. The solution
The solution method: A 100K resistance can be used to series connected to IO. Since the voltage of IO is higher than power supply pins voltage when in above two applications, the way to solve problem is that a resistance is employed to share parts of voltage. In this way, the voltage of IO pins can be pulled down to become lower than the voltage on power supply pins. Therefore, a 100k resistance is suitable for sharing the voltage and avoid fold back current occurring.
