If the actual input voltage on the system is kept stable even under low battery condition, all the challenges in Section 2 can be resolved. The pre-boost is a preferred design, making sure that the minimum voltage of 8.5V on the system as shown in Figure 3-1.

Figure 3-1 Recommended Block Diagram of a Display Module Using Pre-Boost

The LM5152-Q1 device is a wide input voltage range synchronous boost controller that employs peak current mode control. The device can work as a pre-boost controller which can boost up to a specific voltage if battery voltage is low when starting up as shown in Figure 3-2 . The device also supports an ultra-low IQ deep sleep mode with bypass operation, which eliminates the requirements for an external bypass switch when the supply voltage is greater than the boost output regulation target.

Figure 3-2 LM5152-Q1 in Automotive Pre-Boost Application

As shown in Figure 3-2, LM5152-Q1 EVM was used for testing in this system example as the design is preferred. Figure 3-3 shows the waveforms during a VIN transient from 12V to 6V. The red line is VIN, the green line is VOUT of the LM5152-Q1, which supplies system including LED drivers and pink line is input current from the battery. As VIN is reduced from 12V to 6V, the LM5152-Q1 maintains a minimum VOUT 8.5V as expected, keeping the system stable. Figure 3-4 shows the waveforms from the perspective of LP8866S-Q1 compared to Figure 2-1. VIN on the LED driver is shown as the red line, inductor current is the pink line and VDD on the LED driver is the green line. Even though VIN drops to 6V, the input voltage on LP8866S-Q1 is maintained at 8.5V as expected due to pre-boost. Therefore, the LED drivers can achieve stable operation even under cranking conditions on the battery.

Figure 3-3 VBAT and VOUT of LM5152-Q1 and System Input Current During a VIN Transient 12V to 6V with Pre-Boost

Figure 3-4 LED_VIN and LED_VDD and I_L Waveforms During a VIN Transient 12V to 6V with Pre-Boost