The low pass filter reduces the differential mode noise which is caused by the following switch mode power supplies. It is built as a PI filter with a corner frequency of approximately one tenth of the switching frequency of the subsequent power supplies. This results in a theoretical ripple rejection of around 40 dB, which means it’s lower due to the interwinding capacitance of the inductor and other parasitic elements on the board.
As previously mentioned, the battery voltage can break down significantly when the engine is started, especially at colder temperatures. To maintain the power for the infotainment system, a pre-boost converter is used to provide a high enough input voltage for the dual buck converter. To reduce size and cost of the pre-boost, several loads like audio amplifiers are reduced or switched off during cranking. Therefore the output power of the pre-boost can be significantly lowered compared to the overall output power of the infotainment power tree. In this design, the pre-boost only needs to support around 40% of the regular power for the two output rails. The output voltage of the LM5150-Q1 boost controller can be set to 6.8, 7.5, 8.5 or 10.5 V. This design uses 10.5V output voltage to provide a high margin for the buck inputs. The device is optimized for pre-boost applications. As a result, it starts up quickly as soon as the battery voltage drops below the programmed output voltage.
Figure 2 and Figure 3 show the 10.5 V output voltage of the boost at a 1.5 A load with the Volkswagen E-11 start test pulse “severe” charged. Even though the boost reacts quickly, the output voltage drops down to slightly above 6.0 V before it recovers – as seen in Figure 2 (5 ms per div). The reason is that this design uses only ceramic capacitors for reliability reasons. The four 10 µF (50 V, X7R, 1210) ceramic capacitors on the boost output / buck input might be too low for some applications, but for this specific design the +7.5V rail does not need to be maintained during cranking. As a result, a relatively high break down of the buck input voltage can be tolerated. If, however, a significant break down of the output voltage is not acceptable, higher output capacitance is needed. A good choice for this kind of application is to use hybrid capacitors which offer high capacitance, high ripple current capability and low ESR at the same time. Choosing an output capacitance in the range of several hundred µF reduces the break down in the worst case to only a few hundreds of millivolts.