Piezo ceramic driver from 3.3V and microcontroller

[Tweepy]

Hello everyone,
I'm posting here to get some feedback about my new alarm circuit:
I need only to output a fairly strong frequency, used as an audio indicator, nothing related to music.
The frequency is from few Hz to 5KHz, and needs to be heard let say when driving a bike.
The volume has to be ajustable (from CPU)
Average current drawn from battery must be less than 1mA as the application run with a coin cell battery.
Since piezo ceramic buzzer need from 10 to 30V, extra circuitery are requiered.

From here, they are 3 solutions:
The all in one, made for it:
http://www.ti.com/product/tpa2100p1
But consumes quite a lot (5mA)

The one with a choke coil



And the one I tried on breadboard:



A capacitor booster with diode, using a PWM at 100KHz and changing the duty to change the volume.
The consumption is less 1mA at full volume, and nothing when, no output as I can switch down the PWM (signal and booster)

The problem is the board space used with this solution.

So I wonder if any one can submit me a new design (like using push pull or so) to reduce the number of capacitor/diode while still being cheap.

Thanks,

I will keep you updated on my investigation.