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What is a transistor?

What is a transistor?

Load Switch


Inrush Current when the Load Switch is ON

A large current much greater than the steady-state current will temporarily flow the moment the load switch Q1 is turned ON. When the charge of the load capacitance CL at the output side gets close to zero, voltage is supplied to the output Vo, resulting in instantaneous large charge current flow.
This excessive current is often referred to as inrush current.
The peak of the inrush current is largely determined by the input voltage Vi, Rds(on) of the Q1 MOSFET, and the ESR of the load capacitance CL on the load side, and increases along with the input voltage Vi.
Excessively large inrush current can cause system failures or malfunctions.
Exceeding the maximum rated current can also result in destruction.
However, by adding a capacitor C2 in parallel with the resistor R1 connected between the Gate and Base of the MOSFET Q1 it is possible to slow the decrease of the Gate voltage, which will gradually reduce Rds(on) and suppress inrush current.

Load Switch Equivalent Circuit Diagram
Load Switch Equivalent Circuit Diagram


Inrush Current Countermeasure (When the Nch MOSFET Load Switch is turned ON)

Nch MOSFET Load Switch Equivalent Circuit Diagram
Nch MOSFET Load Switch: RSQ020N03
Vin=5V, Io=1A, Q1_1G=1V→12V

  • The load switch Q1 is ON when Q2 is OFF
    (the Q1 Gate voltage will be greater than Vo (Q1 Vgs))
  • The load switch Q1 is OFF when Q2 is ON
  • As a countermeasure, C2 has been added to
    minimize inrush current when Q1 is turned ON

Nch MOSFET Load Switch Equivalent Circuit Diagra
Nch MOSFET Load Switch Equivalent Circuit Diagra


Reverse Current when the Load Switch is turned OFF

Even when the load switch Q1 is switched from ON to OFF, voltage at the output Vo pin will remain for a certain period of time based on the load capacitance CL at the output side.
If the voltage at Vi is lower than Vo, reverse current may flow from the Vo output to the Vin input through the parasitic diode formed between the Q1 MOSFET Drain and Source.
Please ensure that the rated current of the Q1 MOSFET is never exceeded under any circumstances.
In addition, please consider the rise timing, taking into account the load conditions, when determining the capacitance value of the input bypass capacitor CIN.

Load Switch Equivalent Circuit Diagram

oad Switch Equivalent Circuit Diagram

Electronics
Fundamentals