Every linear voltage regulator has an integrated control loop regulating output voltage. Different concepts of control loop can be implemented depending on the application. However, every regulation loop has a certain reaction time to adapt to load current variations. In a short period of time, the control loop is not able to react. It takes a minimum time for the voltage regulator to react and to set the output voltage back to its nominal value by adjusting the output current. In other words, voltage variations at the regulator’s output are inevitable for a short time during current transient.
Typical application case: supply for a microcontroller
The current consumption of a microcontroller is usually less than 1mA in standby mode and from several 10mA up to a few 100mA in normal operating mode. In its application, the microcontroller is triggered from standby mode to normal operating mode or vice versa. A fast current transient is respectively rising or falling in 1µs at the voltage regulator’s output.
The typical behavior of a linear voltage regulator at these current transients is shown in the figures below.
|Output voltage deviation at load transient|
Potential risks of big voltage variations are:
- Triggering an unwanted reset.
- Malfunction of the supplied microcontroller by exceeding its operating range.
- Damage of load by exceeding its maximum ratings.
To avoid big output voltage variations, basically two solutions are possible:
- Avoid big load current transients whenever possible. The designer should first of all try to avoid big current transients within the application.
- Increase the value of the output capacitor to buffer the voltage regulator’s output voltage.
In case big load current transients are not avoidable, increasing the output capacitance can lower the voltage variations at load current transients and avoid the risks.
The following pictures show the output voltage deviation of the TLE42754 at a load current transient from 1mA to 200mA with 22µF and 100µF output capacitors. Whereas a voltage drop of 180mV has been recorded with a 22µF output capacitor, the drop is reduced to only 85mV with a 100µF output capacitor.
TLE42754 output voltage deviation at load transient with a 22µF output capacitor
TLE42754 output voltage deviation at load transient with a 100µF output capacitor
To dimension the output capacitor reasonably, the following steps are recommended:
- Check for worst-case current transients within the application.
- Define max. allowed voltage variation ΔV max during current transient.
- Determine the voltage variation ΔV of the voltage regulator at the worst-case current transient with the minimum output capacitance fulfilling the requirement for stability.
- If ΔV is higher than ΔV max, try with a bigger output capacitance.
- Choose an output capacitor which ensures the voltage variation ΔV is within the allowed range.
- Verify the selected output capacitor on the application hardware.