3V3 to 28V

[kcs]

I want to drive 28Vpp, 50mA rated buzzer by uC. uC's PVM output is 3V3 rated. What would be easiest way of doing this?

[rexxar]

The easiest way would be to use a transistor. You just drive your PWM signal into the base of the transistor, and it will switch your 28V supply at the same frequency.

Something like this crude drawing.

[kcs]

Thank you for a quick response.
I don't have 28V power source, the only source available is 3V3. What would be the easiest way to get 28V (only 50mA needed)?
Are there any specific requirements for transistor?

[c4757p]

First of all, that circuit won't work, it's an emitter follower. The output voltage will be under 3.3V no matter what's on the collector.

Now, if you don't have a 28V supply, you'll need to generate the voltage somehow. You could use a boost converter, but also, there are plenty of ways of using an inductor or transformer in the buzzer drive circuit to convert just the waveform itself.

But my recommendation would be to use a different buzzer.

[miguelvp]

or buy a boost step-up converter
http://www.prodctodc.com/lm2577-dc-booster-converter-step-up-dc-324v-to-4265v-max-3a-boost-voltage-regulator-p-180.html#.UzOmWT-wLs0
or build your own step-up converter by following the datasheet:
http://www.ti.com/lit/ds/symlink/lm1577.pdf

And learn about how to drive buzzers:
http://www.murata.com/products/sound/faq/piezo_sound/index.html
http://www.murata.com/products/catalog/pdf/p15e.pdf

But you should look at the datasheet of the buzzer you have in mind, it should have a recommended driving circuit schematic
http://www.alliedelec.com/images/products/datasheets/bm/MALLORY_SONALERT_PRODUCTS_INC/70186688.pdf

[kcs]

That's a lot of good info, I will take a time to read it.
One more question, those boost step-up converters are quite powerful (3A or so), and I only need 50mA, isn't there anything less powerful that could require less (or none) external components. Like an IC with 3 pins: 3Vin, 28Vout, GND. I am referring to LM78XX but with an opposite operation in mind.

[miguelvp]

It can only deliver the power you ask of it provided your supply can handle it.
Being capable is not equal to actually getting that current all the time.

So say you need 50mA (0.050A) at 28V that means 1.4W
If your supply provides is 3.3V then it will draw I=P/V= 1.4/3.3 = 0.42(42) Amps or 424mA to drive that buzzer so you have to consider if your supply can handle both the buzzer and the rest of your device.

Edit: Give it 10% so the buzzer will need 500mA from your 3.3V power supply to be on the safe side. (That is 500mA on top of the rest of your circuit)

Edit2: Plus the 3A is the max input current to the step up converter not what it will provide to the buzzer

[ludzinc]

28V at 0.05A = 1.4W.

Assuming you have an physically impossible 100% efficient converter, you need 1.4W at 3.3V, which requires 0.42A.

Not knowing what the efficiency of the boost converter is, I'd take a punt on using a 1A unit.

[miguelvp]

Data points tested are 81% up to 92% or so. but yeah, 1A sounds good.

750mA might be good enough. But not sure you want a buzzer that loud without adding an extra rail.

Like c4757p suggested, start with a smaller buzzer. 28V from 3.3V is a bit much

Edit: Also research the step up converters, you might need something even better than the one linked, I don't think it will step up by that much for looking at their datapoints.

[peter.mitchell]

my recommendation would be to use a different buzzer.

my recommendation would be to use a different buzzer.

[miguelvp]

What if he really wants a loud buzzer?

[qno]

You could drive both sides in opposite phase with an H-bridge.
Then you can get away with 12V.

[kcs]

buzzer will need 500mA from your 3.3V power supply to be on the safe side.

That's a problem of which I have not thought about .

my recommendation would be to use a different buzzer

There are not much options available, I want to use a loud (and I mean really loud) buzzer. The one I have selected is doing 95dB at 1 meter.

[miguelvp]

Actually at 80% efficiency on the step up converter you will need at least 600mA for the buzzer alone.
It will help if you shared the part number of your buzzer.

You should rethink using the 3.3V power supply, you can use a 12V power supply then create both rails (3.3V and 28V) from it.