# Load switch for power supply control

Discussion in 'General Electronics Chat' started by meththas, Feb 13, 2012.

1. ### meththas Thread Starter New Member

Feb 26, 2010
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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?

2. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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Attach relevant data sheets, so we can say something.

3. ### meththas Thread Starter New Member

Feb 26, 2010
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My apologies. I have attached the datasheet of the component.

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4. ### praondevou AAC Fanatic!

Jul 9, 2011
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Do you mean IDon MIN? That's the minimum guaranteed current you can draw at the output for the given test conditions.

Yes.

I think it will be easier to experiment a little bit with those two values.

What's the cold resistance of the light bulb?

5. ### meththas Thread Starter New Member

Feb 26, 2010
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Yes it was IDon Min. If that is the minimum current draw of the output - does that mean I will need to somehow reduce the current to the rest of my circuitry if it does not need that much. For example, when I only need 200mA and if the output is giving out 1A - will that not damage my circuit?

The bulb is a household halogen bulb rated at 12V, 10W. The resistance I believe should be around 15 ohms.

6. ### praondevou AAC Fanatic!

Jul 9, 2011
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200mA will not damage anything. 800mA neither.

I meant the resistance of the filament when it's cold. You can measure it with an ohmmeter.

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7. ### crutschow Expert

Mar 14, 2008
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The answer to your question 1 is, no you don't need to worry about Id(on). That value is somewhat confusing but it means that the load current must be no greater than 1A to guarantee that the voltage drop across the switch is no more than 0.2V. The switch will only draw the same current as the load under all conditions. It indeed acts as a simple switch (with a small amount of series resistance).

Edit: For slew rate control start with R1 = 1MΩ and R2 = 100kΩ. Then experimentally select C1 to give the desired turn on time. Its value will likely will be in the several microfarad range.

Last edited: Feb 13, 2012
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8. ### Mark_T Member

Feb 7, 2012
47
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You may have problems with such large resistor and capacitor values. The output voltage may ramp up slowly?

9. ### crutschow Expert

Mar 14, 2008
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The time constant of 100kΩ and 1μF is 100ms which is short on the human time scale.

10. ### meththas Thread Starter New Member

Feb 26, 2010
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thank you... yes Id(on) was confusing me as to what it really meant. I couldn't find any place that gave a clear explanation either.

I shall experiment with the R and C values to see what I can get. I found that Fairchild has a load-switch simulator tool. Will try the R and C values on that first to get an idea of what can be theoretically expected.

sorry about the bulb resistance value - I measured it to be around ~1.2ohms.