It secret is that PAM8907 has a built-in synchronous boost converter and a bridge-style output.
22Vp-p can also be selected at the output – there is a logic input which sets the boost converter to produce 11 or 15.6V, resulting in the choice of 22 or 31Vp-p across the bridge.
There is no internal oscillator for the sounder waveform. Instead, a pulse waveform has to be supplied to the Din pin (see diagram).
Shut-down is automatically entered if the Din pin is left low for more than 100ms, where consumption drops below 1μA. The chip automatically wakes when the input waveform reappears. Typical start-up is 1.1ms, which also provide a soft start.
The boost converter is quite sophisticated, with two control loops which work together to maintain a constant output voltage. The inner current control loop provides cycle-by-cycle current limiting, while the output control loop provides output voltage control.
When the boost converter is turned on (through the Din input), an NMOS switch is turned on and the current in the external 1µH inductor ramps up to its peak value (nominally 1.6A ). A current comparator then turns off the NMOS switch for a fixed period of time to allow energy to be transferred to the output capacitor.
When the voltage on the output capacitor equals or exceeds the desired output voltage, the current loop is disabled until the load discharges the output capacitor to a voltage lower than the desired output voltage. Every time the output voltage falls below the desired value, the switching cycle starts and continues until the desired value is reached.
Protections against over-heating (die = 160°C), over-current, over-voltage and under-voltage are included.
Operation is across a -40°C to +125°C ambient.
Packaging is 10pin U-QFN2020.