Emergency ballast used in LED is the emergency driver and it is different from fluorescent ballast although they have similar functions.
The function of an emergency fluorescent ballast is to generate a spike of high AC voltage that strikes an arc. The AC passes from cathode to anode inside the discharge tube to turn the lamp on. After that, fluorescent ballast regulates the current passing through the tube.
Unlike fluorescent tubes, LEDs require a DC source of low voltage. So, the emergency ballast for LED (i.e. LED driver) converts AC of high voltage into DC of low voltage. So, the LED driver acts as AC to DC converter.
LED does not require a fluorescent ballast as a driver is capable of performing many similar functions and the operation is also more efficient. Also, LEDs are much smaller than fluorescent tubes and a fluorescent ballast doesn’t fit there. The LED driver can be designed in smaller sizes and hence is suited for LEDs.