Load switch for power control

[Audette]

Hi all,

I want to use a load switch as a switch to turn on and off the power to support circuitry that I have. A microcontroller which will be powered ON at all times will provide the control signals. I have a couple of questions on load switches.

I was planning to use a Vishay Si1869dh load switch @ Vin = 15V, Von/off = 3.3V.
I want the Vout to be ~15V as well and the current draw from the load will be around 0.2A most of the time. max 0.8A can be expected at times where I will have a bulb switched ON which will be for short periods of time.

1. The Id (min) value of the load switch - will this be drawn from the power supply at all times regardless of how much current is required by the load?
2. Is the drain current the same as the load current?
3. How can I calculate the best values for slew rate controlling R and C to minimise the inrush current?

Thank you in advance.

 

[BradtheRad]

It will be important to keep an eye on power dissipation. The specs say 1W. Your stated max load will be okay, based on I * I * R. (Or .8 * .8 * .242 ohm, using an 'ON' resistance found in the table.)

Nevertheless, it's wise to put your fingers on the device while testing operation. 1 W is too hot to hold, which will tell you the device is at risk of frying.

Keep in mind that a bulb has lower resistance at turn-on when cool, compared to when the filament has become hot.

Re Q 1.

Though it's not specified on the datasheet, the device when off should draw only miniscule current. It would not make sense for it to draw substantial current, seeing it's designed for battery powered projects.

Re Q 2.

Perhaps not all load currents go through the drain terminal. If they did then drain current will be the sum of all load currents going through the device.

So it may be relevant to examine source current too, in case you have it carrying one or more load currents.