Behavior when Vin Falls Below Vout

Step-down switching regulators convert an input voltage Vin into a voltage lower than Vin. However, due to fluctuations in Vin, there can be cases in which Vin falls below the Vout setting value. Behavior that could occur under these conditions is here explained.

Behavior when a Step-down Converter Vin Falls Below Vout

In a step-down switching regulator, the normal relationship between the voltages is Vin > Vout. Strictly speaking, Vin during normal operation is the voltage equal to Vout plus the voltage drop between the input and output. For example, for a Vout of 3.3 V, the minimum operating voltage Vin might be 3.8 V.

However, for some reason Vin may fall to a voltage below Vout for a short time. For example, another device which shares the same Vin may draw a large current suddenly, so that the voltage drops momentarily.

Normally, when Vin becomes lower than Vout, Vout cannot be maintained at the preset voltage, and so falls. In general, the voltage resulting by subtracting the amount of the voltage drop across the switch (transistor) from a somewhat lower Vin is output as Vout; but if Vin falls further, anomalous operation may occur, and Vout may enter an indeterminate state. Not only could this state cause the operation of the device or system being powered to be indeterminate, but damage could be imparted as well. In order to avoid such events, in recent years nearly all step-down switching regulator ICs have been provided with a function for protection in the event of an input voltage drop called UVLO (Under Voltage Lock Out). When Vin falls below a preset UVLO threshold value, the IC shuts down, and when Vin is restored the IC restarts, thereby protecting the system.

What we will discuss here is the behavior that could occur when Vin falls below Vout, but not sufficiently far below Vout to activate UVLO, and Vin is then restored. This will be explained using figures.

The figures below are a diagram of a feedback circuit that governs output regulation of a synchronous rectifying step-down converter, and a schematic diagram indicating the behavior of Vout when there is a drop in Vin.

For output regulation, the Error Amp compares a feedback voltage from voltage dividing resistors connected to Vo in the circuit diagram with a reference voltage to control a switch. If the output voltage is lower than the setting value, the Error out output of the Error Amp goes to high, and a switching controller circuit turns the high-side switch (transistor) on and the low side switch off. Feedback control is executed in which power is supplied from Vin, Vo rises, and upon reaching the preset voltage operation is reversed such that Vo is maintained at the preset voltage.

The waveform diagram on the left indicates the behavior when the input voltage Vin falls in a range so as not to drop below the output voltage Vout (shown as Vo in the circuit diagram). In general, fluctuations in Vin have almost no effect on Vout.

The center waveform diagram illustrates the behavior that can occur when Vin falls below the Vout setting value, which is the subject of this article. The condition for this behavior is that Vin falls below the Vout setting value, but does not fall to the level (indicated by a red line) at which UVLO is activated. The behavior of Vout when Vin falls below the Vout setting value is as described above, with Vout falling to approximately the level of Vin minus the amount of the voltage drop across the switch. Seen from the feedback circuit, in this state Vout has fallen below the setting value, the Error Amp detects this state and sends the Error out output to high, the high-side switch is turned on and the low side is turned off, and power is supplied to the output.

When in this state Vin is suddenly restored, the power supplied from the high-side switch suddenly increases, Vout also rises suddenly, and there is the possibility that Vout may exceed the setting value for a short length of time. Thereafter the Error Amp, which has detected this occurrence, turns off the high-side switch to cause Vout to fall, returning it to the setting value. This phenomenon is related to the suddenness of the restoration of Vin, the response speed of the Error Amp, and other factors. If the rise in Vout exceeds the rating of a device being powered by the converter, damage may be caused, and so the possibility of occurrence under actual conditions of use should be studied carefully. Should this behavior pose a problem, Vin and the response characteristics of the power supply IC will need to be reevaluated.

The waveform diagram on the right is for a case in which Vin falls below the UVLO threshold value, so that the UVLO action causes the power supply IC to shut down, and upon restoration of Vin, Vout is restarted in conjunction with a soft start.