Figure 3 shows not only the critical beginning of the test pulse, but also the sinusoidal battery voltage as soon as the starter motors begin to spin. The output voltage is well regulated to 10.5 V without any disturbance.
When the battery voltage is above 10.5 V, the boost converter is not able to switch. It then makes sense to bypass the boost’s inductor and diode to reduce the conduction losses. In this design, the status pin of LM5150-Q1 is used to control a P-FET parallel to the boost’s inductor and diode. This pin is low while the device is not switching and can be used to drive the gate of a P-FET directly. When the boost converter is switching, this open drain output goes high and the switching node voltage is used to generate a voltage to turn off the P-FET.
Finally, the two output rails are generated by LM5140-Q1. For this synchronous dual current-mode buck controller, 440 kHz or 2.2 MHz can be selected for the switching frequency. To keep the switching losses low and achieve a high efficiency, 440 kHz was selected for this reference design. For accurate current sensing and overcurrent protection, a shunt resistor in series to the inductor is used. To minimize the losses of the shunt, the overcurrent threshold can be set to either 48 mV or 73 mV (typ.). The controller also offers the possibility to operate the buck converter in forced PWM mode for best transient response but with lower efficiency at light loads. If diode emulation mode is enabled, the efficiency is significantly higher under light load conditions. To minimize the overall footprint of the design, a fast switching and low resistive 60V dual N-FET (Vishay SJQ260EP) in PowerPAK SO-8L package is selected. This offers a moderate package size and a good thermal interface to distribute the losses on the board. Choosing appropriate inductors is also important for a compact solution. Therefore, Cyntec’s VCHA054T series has been chosen with a size of only 5.4 mm by 5.2 mm by 3.8 mm. The composite material has higher core losses compared to ferrite, but offers smaller size and higher saturation current, important for the pulsed loads. All capacitors are ceramic types to ensure high reliability as well as low height. Due to the high converter bandwidth of 30 to 40 kHz at 440 kHz switching frequency, 3x 47 µF (10 V, X7R, 1210) for the +3.3V rail and 2x 22 µF (16 V, X7R, 1210) for the +7.5V rail are sufficient.