PWM power switch by N-Channel Transistor?

Hi,

Can we use a N-Channel MOSFET as a PWM power switch to give divided-down power supply to a component?

The picture shows an example from OnSemi App Note Using MOSFETs in Load Switch Applications. Because V_GS needs to be positive (depending on the threshold), there is an additional VGATE and transistor used to control the gate voltage. When ENn is low, Q1 if off and VGATE is applied to N-MOSFET’s gate, making it open.



For our application, the VIN to the N-Channel transistor is only 0.6V. Therefore, if we have GPIO which is much higher than VIN, as shown in the lower part:
Can we directly use GPIO to control voltage of G, thus saving a transistor?

The transistor we choose is Vishay Si2312BDS.

Would it work if I apply PWM signal at the GPIO, so ultimately the load gets a further divided-down (in terms of power) output as compared to the original 0.6V ? Is this a typical use of transistor?


Matt

Please note that to turn on the N-MOSFET, its Vgs voltage should be about 1.8V (or higher).
Therefore, in the lower circuit above, GPIO voltage needs to be equal to (1.8V + V_load) where:
V_load = I_load * R_load
or
V_load = V_supply - I_load * (R_load + Rds)
Rds is the on-resistance of the drain-source channel.

If, when on, V_load needs to be close to 0.6V (supply), Vgate would be about 2.4V (1.8V + 0.6V) for Rds be around 50 mR.

But in the datasheet (Si2312BDS), the VGS(th) is between 0.45V and 0.85V, not 1.8V. This is somewhat particular since I intentionally picked this low VGS(th) transistor.

With this setting, can we use it as a PWM switch?

As you know, lower Vgs means higher Rds (which is in series with the load). So, as long I have no idea about your load, it is not possible giving a sure answer.
But if the V_load for V_gate=1.8V (Vgs+V_load) is acceptable (since V_load will be lower than 0.6V due to Rds), the MOSFET could be used as a PWM switch.

mmitchell said:

But in the datasheet (Si2312BDS), the VGS(th) is between 0.45V and 0.85V, not 1.8V. This is somewhat particular since I intentionally picked this low VGS(th) transistor.

With this setting, can we use it as a PWM switch?

Click to expand...

The Vgs(th) is measured at a drain current of 250µA. For low ON resistance at higher drain currents you need at least 1.8V for Vgs as shown in the Drain-Source On-Resistance entry in the Specifications. Thus, for the bottom circuit, the gate voltage would need to be 1.8V + 0.6V = 2.4V to generate the full 0.6V load voltage across the MOSFET source load resistance.