The LM5088 contains a high voltage, low-dropout regulator that provides the VCC bias supply for the controller and the bootstrap MOSFET gate driver. The input pin (VIN) can be connected directly to an input voltage as high as 75 V. The output of the VCC regulator (7.8 V) is internally current limited to 30 mA. Upon power up, the regulator sources current into the capacitor connected to the VCC pin. When the voltage at the VCC pin exceeds the upper VCC UV threshold of 4 V and the EN pin is greater than 1.2 V, the output (HG) is enabled and a soft-start sequence begins. The output is terminated if VCC falls below its lower UV threshold (3.8 V) or the EN pin falls below 1.1 V. When VIN is less than VCC regulation point of 7.8 V, then the internal pass device acts as a switch. Thereby, VCC tracks VIN with a voltage drop determined by the R_DS(ON) of the internal switch and operating current of the controller. The required VCC capacitor value is dependent on system start-up characteristics with a minimum value no less than 0.1 µF.

An auxiliary supply voltage can be applied to the VCC pin to reduce the IC power dissipation. If the auxiliary voltage is greater than 8.2 V, the internal regulator is disabled. The VCC regulator series pass transistor includes a diode between VCC and VIN that must not be forward biased in normal operation.

In high voltage applications, take care to ensure that the VIN pin does not exceed the absolute maximum voltage rating of 76 V. During line or load transients, voltage ringing on the VIN pin that exceeds the absolute maximum ratings can damage the IC. Both careful PC board layout and the use of high quality bypass capacitors located close to the VIN and GND pins are essential.