Integrated 42V, 2.5A Buck Switching Regulator National’s LM25005 With Emulated Current-Mode Control Reduces Noise Sensitivity and Enables Reliable Control of Small Duty Cycles at High Input Voltages

unregulated input voltage to a regulated output voltage. In applications requiring DC-DC conversion from a high input voltage, the buck regulator dramatically improves conversion efficiency relative to linear regulator alternatives. However, applying the buck regulator to applications with high input-to-output step-down ratios creates significant challenges for the pulse-width modulation (PWM) controller. Because the duty cycle of the buck regulator switch is approximately equal to VOUT/VIN, a buck DC-DC converter with high input/output voltage ratio must control very narrow PWM pulses. The switching frequency of a buck regulator is generally set to a high level to reduce the size of the inductor and capacitors. High switching frequency and low duty cycle translates to very short pulse durations in the controller. For example, a buck regulator with an input voltage of 66V and an output voltage of 3.3V will require a buck switch duty cycle of approximately 5%. At a typical switching frequency of 300 kHz, the required PWM on-time of the buck switch is a mere 166 ns.