The Internal BJTs are connected in Darlington configuration. This is to reduce the drive current from the chip itself and get the Vce on down. However, it means that the swiching frequency is severely limited due to carrier injection swamping the BC region.
Another manufacturer does a version of this chip which allows you to drive the internal BJT in a non darlington way, because the collector of the behind BJT is not internally connected to the collector of the forward BJT. –But even that is not that great