Electronic products often utilize high side P-channel MOSFET's to manage power usage by turning on and off power to sections of circuitry. For example, in a battery-powered RF device, an RF power amplifier and associated circuitry would be energized only when transmitting RF energy. As a result of controlling power to sections of circuitry, inrush currents will occur when the circuitry is energized. Left unchecked, inrush current can detrimentally affect the quality, reliability, and performance of electronic products by causing voltage transients at the output of battery systems, switching, and linear regulators. Low power products such as battery-powered devices typically have low quiescent current linear voltage regulators that can be negatively affected by inrush currents due to their low regulation bandwidth. The output voltage of lithium battery systems can also be negatively affected by inrush currents due to their varying output resistance and possible passivation. Lower temperatures can exacerbate this effect even more due to the increased output resistance. These unwanted effects can cause erratic product behaviors such as micro-controller resets, perturbation of oscillators, and negative effects on analog and digital circuitry. The use of Dialog load switches will control the inrush currents by design thus avoiding the negative effects associated with these inrush currents.
Currents Inrush currents are caused when capacitors are charged during the initial application of voltage. The slew rate of the voltage being applied to the capacitance and the value of the capacitance will determine the inrush current as follows:
The inrush current can be expressed as:
where CLOAD is the circuit capacitance being charged, and 𝑑𝑣 / 𝑑𝑡 is the rate of change of the voltage being applied. Thus as an example, if the capacitance value equals 10 uF and the rise time of the voltage being applied is 5 V/us, then the inrush current equals:
Enhanced Product Quality due to Inrush Current Control
Unchecked large inrush currents can negatively affect the quality of electronic products. Inrush currents will vary from product to product due to varying component characteristics, aging, and temperature variations. If inrush currents are not controlled by design, then it is possible a percentage of shipped products will be problematic, resulting in field returns and unwanted associated costs. The steps necessary to control inrush current involve circuitry that controls the slew rate of the voltage being applied to sections of circuitry being energized. Dialog load switch parts incorporate circuitry that provides a programmable slew rate. For example, if the slew rate is programmed to a value of 1 V/ms, then the resulting inrush current for a capacitor with a value of 10 uF would be:
Below is an oscilloscope image showing the transient inrush current (yellow trace-2 A/div) without the use of slew rate control. Note the resulting 0.7 V voltage dip (blue trace1 V/div) at the power source which in this case is a lithium battery system (3.6 V). The peak inrush current is 9.2 amps. The high side switch was implemented with a P-channel MOSFET as the series pass element. This is a very common method used for the implementation of a high side load switch that controls the power of sections of circuitry. The white trace-2 V/div is the voltage being applied to the load which consists of a 15 uF capacitor.
CLOAD =1 5 uF, VIN = 3.6 V, RLOAD= no load
Control Inrush Current with Dialog Load Switch Parts
Dialog high side load switch solutions utilize a programmable controlled slew rate when the switch is enabled. The use of a controlled slew rate on a high side switch will substantially reduce the inrush currents that are a result of energizing sections of circuitry that utilize bulk and decoupling capacitors. Controlling these inrush...Read more »