Using a power switch is must in many applications. For example, take the case of a USB port where VBUS need to be switched only when device is plugged in. Also, in circuits, where monitoring is required, in circuits which need less power consumption and in hot swap circuits power switches are used. Here are some of the design considerations while selecting a power switch.
- If the power rail need to be switch, HIGH side MOSFET shall be used
- The circuit must be able to switch the power supply rails (voltage)
- Under voltage and over voltage protection - Any voltage below and above the required range must not be allowed
- Reverse voltage protection
- Low quiescent current is critical for low power applications
- Power switch must have as low ON resistance as possible such that there is less power loss in the circuit
- Maximum Current carrying capability of the switch
- Short circuit protection must be required to ensure that switch turns OFF when there is short circuit at the load side
- Thermal protection turns OFF the switch when the temperature exceeds the operating temperature limit
- Operating temperature of the application
- Package selection base don the thermal requirements. For example, a bigger package has a good thermal resistance value than a miniature package
- Fast switching capability
- Feedback required like a flag to indicate if the switch is ON or OFF for easy monitoring
- Output latching is one of the feature in the modern day power switches. This comes into picture when the fault condition occurs. Only when the fault condition is removed or device is reset, power switch comes to normal operation, else it stays in fault mode disabling the output
- Active HIGH or Active LOW enable option to be selected such that there is no need for additional circuit to drive the switch from host
- ESD protection inbuilt
Power dissipation within the switch is determined by - PD = RDS(ON)*I*I
RDS(ON) is the MOSFET ON Resistance
I is the current to the load from supply
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