I'm reading the datasheet for the NCP81151B-D MOSFET gate driver / half bridge. It has a standard bootstrap configuration where the switched note (drain + source of the totem pole N-FETs) is tied through a diode and acts as a charge pump to provide drive voltage for the high-side FET.
What I don't understand is this specification:
BST Bootstrap Supply Voltage
35 V wrt/ GND
40 V 50 ns wrt/ GND
6.5 V wrt/ SW
7.7 V < 50 ns wrt/ SW
−0.3 V wrt/SW
(taken from http://www.onsemi.com/pub/Collateral/NCP81151B-D.PDF )
Consider switching a 20V load.
When the gate driver is in the low-on configuration, the SW node would presumably be close to GND, but that means that the SW/BST voltage difference will be 20V minus whatever charge voltage is built up in the capacitor. This will clearly be higher than 7.7V by a rather large margin when the capacitor is empty.
The only way these specifications would make sense for the application, in my mind, is if the maximum voltage was the HIGHER of (SW + 6.5V) or (35V.) However, it doesn't specify that in the data sheet, and thus my engineering reading of this is "the device will fail if you exceed either of these voltages."
How should I know what the correct reading is? ONSemi doesn't list a mail address in their contact section. Has anyone used this part and know for sure that the "higher of" reading is correct?