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[FAQ] Inrush Current From an LDO Perspective

Edgar Acosta1
Genius 12365 points

Modern electronic systems are widely used across different applications. Capacitors have become a key component and are used throughout a design for various reasons. However, when any given system is initially powered, the capacitors are uncharged and behave similar to a short-circuit. In order to charge these capacitors, the system will experience some peak current. This peak current is known as Inrush Current. The amount of inrush current experienced set by the amount of capacitance and the speed at which the voltage rises. This can be calculated using the following equation:  

IINRUSH=CLOAD dV/dt

Low Dropout Voltage Regulators (LDOs) are widely used across electrical systems. These LDOs almost always require an output capacitor. As in any other system that uses capacitors, some inrush current could be experienced when the capacitor is charged during startup. This instantaneous spike of current might be enough to create system level concerns depending on the application requirements.

Therefore, inrush current can raise some questions like: Can inrush current permanently damage my device? Can inrush current exceed the specified current? Why doesn’t current limit kick in instantly? Will the device enter thermal shutdown if inrush current exceeds current limit?  How to manage inrush current and prevent droop on the input voltage?

From an LDO perspective, inrush current might not be a problem.  Yet, other components and/or even the traces on a PCB design could suffer from an excessive current spike.

  • 0
    TI__Genius 12365 points
    • Can inrush current permanently damage my device?

    If the current reaches or exceeds the fusing current for the internal wire bonds, then it is possible that the LDO could be damaged. However, the maximum current that wire bonds can withstand is typically higher than those during an inrush event. Fusing current is highly dependent on material properties, length, diameter, and temperature of the wire bonds, so the actual fusing current will vary from device to device. Typically, wire bonds can withstand 5x or more of the nominally rated current of a device.

     

    • Can inrush current exceed specified current?

    Yes, as an example, an LDO that is rated with a nominal output current of 500-mA and a maximum current limit of 1.1-A, could experience a peak current close to 1.35-A of inrush current as shown below:

    It can be observed that after the inrush current, the device is still operable and continues to regulate its nominal output voltage. For this device, as an example, the maximum allowable current given a 1-ms pulse is 2.98-A. This is to avoid the fusing of the internal bond wires. Having a peak current close to 1.35-A for around 50-µs is well below the fusing current showing that the device does not show damage and will continue to perform as expected.

     

    • Why doesn’t current limit kick instantly?

    The current limit circuit is a control loop and like all control loops there is a minimum response time which is set by the bandwidth of the control loop. Additionally, during startup the reference voltage starts to rise so the error amplifier detects a low output voltage and tries to drive the Pass FET as hard as possible to bring the output voltage into regulation. During this time, all the internal circuitry is powering up as well so the current limit circuitry might not be completely on, therefore, the LDO could experience a slightly longer delay in current limit activation, resulting in a peak current above the current limit value.

     

    • Will the device enter thermal shutdown if inrush current exceeds current limit?

    There could be situations when the current is high enough that the power dissipated across the LDO is enough to reach Thermal Shutdown. However, the duration of an inrush current event typically occurs for a short period of time, which might not be enough for the LDO to reach a high enough junction temperature for Thermal Shutdown to be activated.

     

    • How to manage inrush current and prevent droop on the input voltage?

     

    There are different ways to mitigate inrush current. One way is to reduce the capacitance at the output (Cout) since inrush current is directly proportional to capacitance at the load as described by the following equation:

    IINRUSH=CLOAD dV/dt

    Additionally, by reducing the rise time of VOUT, it will also decrease the amount of inrush current as shown by the equation above. This can be done either implementing external circuitry, i.e using a PMOS FET following the output, or, nowadays, many of our newer LDOs offer a soft-start feature being either an internal circuit, or having an additional pin to support this feature.

     

    During an inrush current event, if the source powering the LDO can’t handle the amount of inrush current, the system will experience a voltage droop. For some systems, preventing this droop is of high importance, especially if the input voltage rail is shared across different devices. Different methods exist to help mitigate the droop of the input voltage.

     

    Often times, having a turn on sequence can help mitigate the voltage droop experienced. By turning on different components in a controlled manner so they do not all startup at the same time, can help reduce the maximum amount of in rush current required to bring up the system, which will reduce the voltage droop.

     

    Another method to prevent voltage droop is to minimize the impedance from the source to the LDO. This can be achieved by placing multiple low ESR capacitors in parallel as close as possible to the input pin and making the GND connections close to the GND pin. This will also improve LDO performance by reducing the PCB loop inductance between the caps and the LDO. Designing the PCB so the trace between the supply and the LDO uses the shortest and widest traces possible will reduce both the ESR and ESL of the trace.  Overall, by minimizing the input supply impedance will allow the inrush current to flow more freely minimizing the voltage drop.

     

     

    If further information is needed, please refer to the following Application Notes:

    “Managing Inrush Current”

    “Monotonic, Inrush Current Limited Start-Up for Linear Regulators”

    “Inrush Current Limit in the TPS720xx”

    “Soft-start circuits for LDO linear regulators”