For area reasons, NMOS transistors are preferred over PMOS for the pull-up path in gate drivers. Bootstrapping has to ensure sufficient NMOS gate overdrive. Especially in high-current gate drivers with large transistors, the bootstrap capacitor is too large for integration. This paper proposes three options of fully integrated bootstrap circuits. The key idea is that the main bootstrap capacitor is supported by a second bootstrap capacitor, which is charged to a higher voltage and ensures high charge allocation when the driver turns on. A capacitor sizing guideline and the overall driver implementation including a suitable charge pump for permanent driver activation is provided.
A linear regulator is used for bootstrap supply and it also compensates the voltage drop of the bootstrap diode. Measurements from a testchip in 180 nm high-voltage BiCMOS confirm the benefit of high-voltage charge storing. The fully integrated bootstrap circuit with two stacked 75.8 pF and 18.9 pF capacitors results in an expected voltage dip of lower than 1 V. Both bootstrap capacitors require 70% less area compared to a conventional bootstrap circuit. Besides drivers, the proposed bootstrap can also be directly applied to power stages to achieve fully integrated switched mode power supplies or class-D output stages.