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Failure to Start of a Power Supply Using a Linear Regulator Case 4: Startup Problems Due to Shoot-Through Currents ②

2024.03.13

Points of this article

・When the circuit current in a circuit block that is a load differs greatly at the rising and at the falling of the power supply voltage, it may be that startup problems are occurring.

・It is necessary to thoroughly evaluate the foldback current limiting circuit characteristics and the output current (load) characteristics.

Case 4: Startup Problems Due to Shoot-Through Currents ②

This article, while similar to “Case 3: Startup Problems Due to Shoot-Through Currents ①“, considers cases in which the circuit current of a circuit block is different at the rising and at the falling of the power supply voltage. A circuit current example is shown in Fig. 1.

Fig. 1. Example of differences in the circuit current at the rise (red arrows) and fall (blue arrows) of the power supply voltage

Fig. 2. Power supply to and foldback current limiting of the circuit block with a shoot-through current

Fig. 2 shows the power supply voltage rise (the red arrow in Fig. 1) when this circuit block is connected to the output of a linear regulator equipped with a foldback current limiting circuit explained in “Case 2: Startup Problems Caused by Constant-Current Loads “. Operation begins from point Ⓐ, and transitions to points Ⓑ and Ⓒ to startup normally, and it can be seen that, with respect to the voltage rise, there is no longer a problem.

Next, we consider a case in which, upon linear regulator startup, an inrush current flows to charge an output capacitor and multiple capacitors in the circuit block. Fig. 3 shows the assumed power supply configuration. Fig. 4 shows the startup waveform; we first explain the case of a circuit block in which such a current does not flow when connected, instead of a circuit block in which a large shoot-through current flows when the supply voltage falls, such as shown in Fig. 1.

Fig. 3. The assumed power supply configuration

Fig. 4. Startup waveform for a case in which a circuit block is connected in which a large shoot-through current does not flow when the supply voltage falls

　　
　　　　　　　　　　
When the input VIN of the linear regulator rises, the output VOUT follows and likewise rises. When VOUT reaches 1.8 V, the connected circuit block begins to operate. When the linear regulator VOUT begins to rise, in addition to the output capacitor connected to VOUT, inrush currents also flow to the other capacitors (point ⓐ in Fig. 4). The linear regulator output current IOUT increases, and the foldback current limiting circuit is actuated, so that VOUT initially falls to 0.6 V (point ⓑ in Fig. 4), but when capacitor charging ends, the required IOUT can be supplied, so that the output voltage once again begins to rise, finally reaching the preset voltage (point ⓒ). When the current in the connected circuit block does not increase upon power supply fall unlike shown in in Fig. 1, startup occurs normally.

Next, linear regulator startup operation is explained for a case in which a circuit block is connected in which the circuit current increases greatly upon supply voltage fall as shown in Fig. 1. Here again, we assume that in addition to the output capacitor, several other capacitors must be charged. Fig. 5 is a graph that superimposes on the current foldback curve the operation of the linear regulator in this case; Fig. 6 shows the operation waveform.
　　

Fig. 5. Startup and foldback current limiting for a case of a circuit block in which a large shoot-through current flows upon supply voltage fall

Fig. 6. Operation waveform for the case of a circuit block in which a large shoot-through current flows upon supply voltage fall (resulting in startup failure)

　　　　　　　

The linear regulator begins operation from point Ⓐ, and when VOUT reaches 1.8 V, circuit block operation begins. When the linear regulator VOUT begins rising, inrush currents flow to multiple capacitors in addition to the output capacitor connected to VOUT, the output current IOUT of the linear regulator increases, and at point Ⓑ the foldback current limiting circuit actuates.

Through this actuation, VOUT is folded back to point Ⓒ (approx. 0.6 V). At this voltage, the circuit block requires a current of about 800 mA, as shown in Fig. 1 (point Ⓓ), but because the current is limited to 500 mA by the foldback current limiting circuit, at point Ⓒ (approx. 0.6 V) VOUT cannot rise, and is latched, which results in startup failure.

In this way, when the circuit current characteristic of a circuit block connected to the output of a linear regulator does not increase monotonically with the power supply voltage, or when the current differs drastically at power voltage rise and at voltage fall, it is necessary to understand that even if a prototype circuit operates normally, problems related to startup may occur depending on the combination of the foldback current limiting circuit characteristics and inrush current values.

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