You may have better luck with 2 layers One expanding the other revealing.
You could use Shatter with a gradient map and animate the threshold. The gradient map would be letters on a black BG.
Particle Playground is best controlled with a gradient map, it is a great tool but is complex and a bit difficult to explain in a short reply.
To control it you’ll need to set the persistent and ephemeral property mappers using another layer.
The brightness of the pixels in the other layer can control various properties like scale, life, speed, opacity etc.
You can use a simple Gray Ramp as the controlling layer or have different values in each of the RGB channels and select which channel controls which property.
Most properties have min and max values (which you can animate). There is a lot of power in this particle system.