MOSFET controlling 5v power supply from 3.3v logic level

[skinny_c]

Hi,

Ok - kind of analog question

I'm not an "analog" expert and only dabble in it when I have to, preferring to stick to microprocessors.

I have a 3.3v logic level from a microprocessor that I need to use to switch on/off a 5v 750mA supply to an external circuit. It doesn't have to be "fast" as I'm not pulsing the supply, just switching it on or off.

Using a MOSFET "seems" desirable - high current / small package.

From what I can see I need a "high" side as the "switch" is switching the 5v supply to a circuit, not the ground.

I've tried an Infineon BSS215P, but it's always "on" regardless of the logic level (0v or 3.3v) used to drive the gate. The logic drive to the gate is pulled high (3.3v) via a resistor and I can see that this is working correctly.

Ideally I need the MOSFET to be a single component solution and a SOT23 device. This will enable me to fit the device easily into the footprint of the demo board I'm using. I can accept a part that will handle 500mA if necessary, but 750mA would be ideal.

Having looked around on the net there's lots of talk about switch 12v power supplies from 5v but nothing I can find that covers what I'm trying to do. I've also seen people talking about using a transistor to drive the gate from the logic level, but I need to keep component counts low and board space is limited.

Any thoughts? It's driving me nuts!

Cheers,

C

 

[alexan_e]

BSS215P has a max gate threshold voltage of 1.2v (turn on voltage ) so there is no way to be turned off with a voltage of 1.7v like in your case

One solution would be a smart high side switch which has the mosfet driver and input level shift into a small smd package but I can't seem to find any model that works with 3.3, the ones I've seen have a threshold of 3.5v min in the input to turn on the output.

Another solution would be to use a mosfet that has a higher gate threshold voltage so that it is not turned on when the gate is at 1.7v (5v-3v3) but it turns on with 5v (5v-0v) , the problem in this case will be that you will probably have a higher on resistance because the 5v will be close the gate threshold .

The only solution I can recommend is to add a small mosfet or NPN (smd case would be fine) and a resistor as level translator, unfortunately you don't want that kind of solution.

Alex

 

[umesh49]

If your microcontroller have some open drain GPIOs then you can try driving the gate with. You need to pulled up the gate with 5V with some resistor more than 10k. Microcontroller pins may not take more than 3V at any GPIO but still it would be safe as you are connecting 5V via a high resistacne.
I would rather prefer to have last solution suggested by Alex and it should add more cost or components if microcontrller is not capable to provide open drain.

Umesh

 

[skinny_c]

Thanks to every one for all your replies and for replying so quickly!

I had considered pulling the output of the micro to 5v, but as the pin is not 5v tolerant I am reluctant to do this.

I've come across the Vishay Si2328DS - spent some time looking through data sheets at the threshold voltage having understood that the MOSFET I had tried will be on for the 1.7v in my setup.

The Si2328DS has a threshold voltage of 2v to 4v. So if I'm reading the data sheet correctly it should be ok. I guess my only worry is that at a VGS of 5V (which if my understanding is correct will be what I'll have when the MOSFET is ON) the ON resistance is 0.35 ohms...... is this a bad thing?

 

[allanvv]

Why can't you use a low-side switch? It shouldn't matter that much for low voltage.

A 0.35 ohm on-resistance means that the mosfet will drop 0.35V across it for 1A, which could be unacceptable.

 

[skinny_c]

Doesn't using low side mean that I'd have to put the "switch" on the ground side of all the components that I'm trying to switch power on and off to?

---------- Post added at 09:26 ---------- Previous post was at 09:22 ----------

Ignore the Si2328 - it's N-channel so can't use for hi-side. Strange as the parametric search for P-channel brought it up on the Vishay web site!

 

[allanvv]

Yes, but it still will switch current on and off. The disadvantage is that the ground will be a little above the ground of the rest of your system but most of the time it's not that critical. Besides, you can get some really low on resistance NMOS devices that are logic level.

However, the open drain method mentioned above is the best way to go since you don't care about switching time.

Edit: it should be safe because a 10k pullup resistor will limit the current in the worst case (5V supply powered on before 3.3 V) to 0.5 mA which should be within the operating conditions of the device. The voltage rating shouldn't matter since the internal diode on the pin should clamp it to prevent damage, and 0.5 mA shouldn't be enough to damage the diode.

 

[skinny_c]

Having looked around there are such things as "smart switches" which do what I want. I'm off to take a look at trying one

Fairchild FPF2123 is an example. Not a "3 pin" SOT23, but it's still a SOT 23

 

[FvM]

"Intelligent" high-side switches are in fact the most comfortable solution. Current limit and thermal protection are unique features, not provided by a dumb MOSFET. If you don't need this features and don't have a suitable intelligent switch avaiable, a P-MOSFET with a single transistor level converter is still a good solution, however.

 

[alexan_e]

Yes I wrote about them in my post but I wasn't able to find a model that worked with an input below 3.5v (for example BSP452 ) but the output can be up to 40v

The one you have found works with very low input control voltage , the output voltage is limited to 6v max but works fine for the 5v you want, it has a slightly more complicated circuit because there is a current limit involved but you can also protect your load.

Alex

 

[donfrench]

"Intelligent" high-side switches are in fact the most comfortable solution. Current limit and thermal protection are unique features, not provided by a dumb MOSFET. If you don't need this features and don't have a suitable intelligent switch avaiable, a P-MOSFET with a single transistor level converter is still a good solution, however.

Did this work for you? I have the same need.

 

[sky_123]

If you're going with the pure mosfet model, then the attached is what you can use, I use it all the time. Good devices are BSS138 for the N-ch MOSFET because it can be turned on even with 3v logic. A good device for the P-ch is BSS84 if you just need low current, or alternatively FDN336P if you need around 1A or more. The specific P-ch devices in the diagram are not needed for low current requirements (diag has 2 P-ch but you only need 1). These devices are all small (SOT-23) so the circuit is compact. However, it is just dumb mosfets as mentioned above, so it is only useful in controlled circuit conditions.