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Failure to Start of a Power Supply Using a Linear Regulator Case 3: Startup Problems Due to Shoot-Through Currents ①

2024.02.21

Points of this article

・When large peak currents such as shoot-through currents flow in a circuit block that is fed power by a linear regulator equipped with a foldback current limiting circuit, startup problems may occur.

・In order to prevent such problems in advance, it is necessary to confirm, in the design and evaluation stages, that excessive peak currents do not flow in a circuit block that is being fed power.

Case 3: Startup Problems Due to Through-Currents ①

Fig. 1 shows a circuit current characteristic of a certain circuit block connected to the output of a linear regulator. This circuit is designed to operate when a power supply voltage of 1.8 V or higher is supplied, but a large current is flowing near 0.7 V, which is reached before the circuit begins stable operation. This can occur when circuit operation below the operating power supply voltage has not been sufficiently verified; due to instability of circuit operation or unanticipated behavior, a state may arise in which a shoot-through current flows between the power supply and ground.

Fig. 1. Circuit current characteristic for a circuit block

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

Fig. 2 shows a case in which this circuit block is connected to the output of a linear regulator equipped with the foldback current limiting circuit explained in “Case 2: Startup Problems Caused by Constant-Current Loads“. In the graph, the characteristic of Fig. 1, with the X and Y axes swapped, is superposed on the current foldback characteristic (green curve).

The circuit block begins startup from point (A), and when the power supply voltage (output of the linear regulator IC) reaches about 0.7 V, a shoot-through current suddenly begins to flow, and it is necessary to pass a current of 800 mA, as shown in Fig. 1 (point C of Fig. 2). However, due to the foldback current limiting of the linear regulator IC, the current is limited at point (B), and the required current cannot be supplied, so that the output voltage does not rise and a startup problem occurs.

In actuality, in many cases startup occurs as a result of noise occurring during startup and the effects of parasitic elements. Consequently, after mass production begins some devices that do not start may be discovered among the many products manufactured, so that issues with design and evaluations are noticed for the first time. In order to prevent such problems in advance, the current characteristics of a circuit block to which a linear regulator supplies power must be actually measured, and the absence of excessive peak currents must be confirmed.

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