A Bootstrap circuit for DC-DC converters with a wide input voltage range in HV-CMOS

Bootstrap circuits are essential parts of integrated DC-DC converters with an NMOS transistor as high-side switch, which provide voltage overdrive for the gate drivers and to drive the high-side switch's gate. This paper presents a bootstrap circuit which doesn't need an additional supply voltage for charging the bootstrap capacitor to a desired voltage level, but uses the input voltage of the converter. Other advantages of this circuit are proper operation over a wide range of duty ratios and input voltages (from 7.5 V to 45 V). The circuit is designed in 0.18 μm 50 V high-voltage (HV) CMOS technology as part of a buck DC-DC converter for a high output power. Post layout simulations show a voltage dip lower than 80 mV when the high-side NMOS transistor is switched on and the power loss of the bootstrap circuit equal to 160 mW for nominal conditions (input voltage 36 V, duty ratio 15%, bootstrap voltage 4.8 V, input power of drivers 115 mW). The layout dimensions of the bootstrap circuit are 96 μm × 251 μm.