The Burst Mode efficiency at a given light load is mainly affected by the switching loss, which is a function of switching frequency and gate voltage. Because a fixed amount of energy is required to switch the MOSFET on and off, and keep the internal logic alive, a lower switching frequency reduces gate charge losses and increases efficiency. The switching frequency is primarily determined by the Burst Mode current limit, the inductor value, and the output capacitor. For a given load current, increasing the burst current limit allows more energy to be delivered during each switching cycle, and the corresponding switching frequency is lower. For a given burst current limit, a larger value inductor stores more energy than a smaller one, and the switching frequency is lower as well. For the same reason, a bigger output capacitor stores more energy and takes longer to discharge.
Figure 12 shows the ultralow IQ synchronous buck regulator LT8650S in a solution that features high efficiency over wide input voltage and load current ranges. With integrated MOSFETs, this device can deliver up to 8 A total output current at fixed output voltages of 3.3 V or 5 V. Despite the simple overall design and layout, this converter includes options that can be used to optimize the performance of specific applications in battery-powered systems.
Table 1 lists low IQ monolithic regulators that are well-suited to the automotive market, with inputs up to 42 V or 65 V. Typical quiescent current for these devices is only 2.5 µA, thanks to the low IQ technologies developed by Analog Devices. With minimum turn-on time of 35 ns, these regulators deliver 3.3 V output voltage from input 42 V at switching frequency of 2 MHz, which is common in the automotive industry.
Automotive applications demand systems that do not produce electromagnetic noise that could interfere with the normal operations of other automotive systems. For instance, switching power supplies are efficient power converters, but by nature generate potentially unwelcome high frequency signals that could affect other systems. Switching regulator noise occurs at the switching frequency and its harmonics.
Ripple is a noise component that appears at the output and input capacitors. Ripple can be reduced with the low ESR and ESL capacitors, and low-pass LC filters. A higher frequency noise component, which is much more difficult to tackle, results from the fast switching on and off of the power MOSFETs. With designs focused on compact solution size and high efficiency, operating switching frequencies are now pushed to 2 MHz to reduce the passive component size and avoid the audible band. Furthermore, switching transition times have been reduced to the nanosecond realm to improve efficiency—by reducing switching losses and duty ratio losses.