Hi side FET switch for low voltage

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I'd like to switch the power to a small OLED display, it draws under 50mA, could be a lot less, but depends on active pixels.

It is, however, too much for me to supply the Vdd fron GPIO pins(this is not an AVR, it's a nRF52 chip).

Therefore I'm hoping someone can recommend a cheap hi-side PFET device(in combination with an NPN transistor or integrated) that will work down to around 2V5 with a low on resistance.

 

Thanks in advance for any suggestions. 

 

Four legs good, two legs bad, three legs stable.

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Si2305 P-Mosfet has a threshold voltage between 400mV and 800mV. and can easily switch 1A.

It is just a fet though, and not an integrated high-side switch.

 

About cheap... The regular western shops want redicilous prices for such components in small quantities:

https://octopart.com/search?q=si2305

 

From Ali Direct it is a more reasonable 2ct:

https://www.aliexpress.com/wholesale?SearchText=si2305

 

You can also try LCSC. It is a sort of Chinese equivalent of Mouser / Digikey / Farnell / etc and more reliable than a random Ali shop.

https://lcsc.com/search?q=si2305

 

But there must be hundreds of SOT-23 P-Mosfets with a low threshold voltage around.

 

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

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Thanks.

Thinking about it, I probably don't need the NPN to drive it, I can use just use a GPIO pin, set as input with pullup for OFF, and low output for on. As contrary to the last few things I've done, the microprocessor is always on. Albeit it sleep mode.

I'll order a sample or three from eBay. This is destined to be a product, rather than a one-off hobby thing, so the small quantity price is not a problem.

 

[edit] I get very frustrated with eBay. The sellers all claim to be in the UK, but only offer delivery times of a week... and I've had stuff through from China in around 8 days[/edit]

 

 

Four legs good, two legs bad, three legs stable.

Last Edited: Fri. Jan 18, 2019 - 09:59 AM
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John_A_Brown wrote:
The sellers all claim to be in the UK

The seller could be in the UK, just not the product they are selling!

 

I have similar problems here in the US too!

 

Jim

 

 

 

 

 

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ki0bk wrote:

John_A_Brown wrote:
The sellers all claim to be in the UK

The seller could be in the UK, just not the product they are selling!

 

I have similar problems here in the US too!

 

Jim

 

Good point!

I did actually contact the seller, and he reckoned I could receive by Tuesday.

 

Four legs good, two legs bad, three legs stable.

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John_A_Brown wrote:
I'll order a sample or three from eBay.
It might be difficult to buy from Ebay / Ali in quantities < 50, Good news is that 50 are stil around EUR 1 :)

These can indeed be driven directly from an I/O pin.

The "high-side switches" are only mandatory if you need to switch the high side of a voltage that is higher than your supply voltage, such as a 24V rail.

The more "luxorious" high-side switches also have built in over current and voltage protection

Note that the Si2305 has an Ugs of max. 8V, so it can not switch higher voltages anyway. It seems to be specifically designed to switch periperal chips from the same voltage rail.

 

You have to think about the reset state of your AVR. During that time the I/O pins are floating. The common solution is to add an external pullup.

Alternatively, you can check what happens if your display has a to low supply voltage. It can probably tolerate it without damage. But then turn your P-Fet off for a decent amount of time before turning it on for good.

 

Rds-on is pretty low for these fets. (Down to 50mOhm) and it can source 16A peaks. There is a risk that the voltage of your uC dips too far when you turn the display on if it has (big) buffer caps. You should check this with a Scope.

 

For battery operated stuff the "Zero Gate Voltage Drain Current" may also be of importance. It can be upto 10uA (@ 55Celcius). This is a common flaw of MOSfets. bipolar transistors are better in this aspect. Saturation voltage of an PNP-transistor can also be around 50mV, which is probably OK. (or even beneficial because of lower supply  current to the display).

Bipolar transistors need a base current, but that is < 1% of the current for your display. If your display is almost always off, the leakage current might be the more important factor.

 

For fun and because of long shipping times I bought a "smd sample book" with sot-23 transistors & fets recently. It's got an assortment of 36 different components and 100 of each.

That's 3600 cheap & common transistors for EUR28. It did take a month or so to arrive, but Now it's a 4m walk, and I've got all the datasheets under my fingers.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

Last Edited: Fri. Jan 18, 2019 - 07:03 PM
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I'm certainly not an expert in this stuff, but I used the dual FET below with an Xmega (3V) to switch a Bluetooth module on and off.

I'll leave it to you to double check the parameters for adequacy within your project.

 

The configuration I used is show below, also.

 

JC

 

Last Edited: Sat. Jan 19, 2019 - 02:24 AM
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Thanks, everybody.

 

Four legs good, two legs bad, three legs stable.

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My 10 pieces of Si2305 turned up today. I soldered one in, and it works perfectly. The voltage drop is so low that I can't measure it, and it works fine with the internal pullup on the chip(which, being a Nordic nRF52, which retains its GPIO configuration while sleeping, and through a reset.

So many thanks for the advice.

 

John

 

Four legs good, two legs bad, three legs stable.

Last Edited: Wed. Jan 23, 2019 - 06:55 PM