“When the load switch Q1 is turned on, a current much larger than the steady-state current will temporarily flow. When the charge of the load capacity CL on the output side is close to zero, a large charging current flows at the moment when a voltage is applied to the output VO.
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About the inrush current when the load switch is ON
When the load switch Q1 is turned on, a current much larger than the steady-state current will temporarily flow. When the charge of the load capacity CL on the output side is close to zero, a large charging current flows at the moment when a voltage is applied to the output VO.
This phenomenon of flowing a large current is called inrush current (Flash Current).
The peak value of the inrush current can be roughly determined by the input voltage VI, the RDS(on) of the MOSFET Q1, and the ESR of the load-side load capacity CL. When the input voltage VIN increases, the current increases accordingly.
When the inrush current is significantly increased, malfunction and system problems may occur.
Also, when the maximum rated current is exceeded, there is a risk of damage. By adding a capacitor C2 in parallel with the resistance R1 between the gate and the source of the MOSFET Q1, and gradually reducing the gate voltage of Q1, RDS(on) can be gradually reduced, and inrush current can be suppressed.
■Equivalent circuit diagram of load switch
Measures against inrush current when the Nch MOSFET load switch is ON
■Nch MOSFET load switch equivalent circuit diagram
Nch MOSFET load switch: RSQ020N03
VIN=5V, IO=1A, Q1_1G=1V→12V
When Q2 is OFF, the load SWQ1 is ON. (Q1’s gate voltage is set above VO(VGSQ1).)
When Q2 is ON, the load SWQ1 is OFF.
Since inrush current flows when Q1 is ON, C2 is added as a countermeasure.
Reverse current when the load switch is OFF
Even when the load switch Q1 is turned from ON to OFF, the voltage of the output VO pin remains for a certain period of time due to the presence of the output side load capacity CL.
When the voltage of the input VI side is lower than that of the output VO side, since a parasitic diode exists between the drain and source of the MOSFET Q1, the parasitic diode may be turned on and a reverse current may flow from the output VO side to the input VIN side.
Be careful not to exceed the current rating of MOSFET Q1.
Regarding the capacitance value of the input bypass capacitor CIN, please fully consider the load side conditions and rise time before deciding.
■Equivalent circuit diagram of load switch
The Links: MDC100-16 SKIIP 24NAB126V10
