A current-mode active clampling for boostrap circuit applied to DC/DC buck converters

An active clamping circuit that regulates the voltage V_BC across the bootstrap capacitor C_B in buck converters is presented. A current-mode arbitration circuit FLAG_GEN compares V_BC, which corresponds to the difference between boot voltage V_BOOT and switching node voltage V_SW, to GV_REF, where G is a gain factor and V_REF an internal reference voltage. FLAG_GEN then outputs a flag signal BOOT_OK that commands the charging path of C_B from input power voltage PV_IN, fixing V_BC to GV_REF, at good approach. Design specification requires 9.0 V les PV_IN les 18 V and 4.5 V les V_BC les 6.0 V. In accordance with a smart-power fabrication process, a set of simulations attests the performance of the active clamping, with V_BC confined to 4.997 V les V_BC les 5.367 V, for V_REF = 5.136 V and 4.967 V les V_BC les 5.329 V, for G = 4.2 and V_REF = 1.25 V, already accounting for load transients. The spread on BOOT_OK assertion value, embedding device mismatching, process, bias and temperature dependence has a mean of 5.162 V and a standard deviation (sigma) of 31 mV. FLAG_GEN response time is 25 ns for an initial difference of 300 mV (5.8%) above VREF across C_B.