As shown in Figure 1-2, there is a high-side switch to supply system power including LED drivers. Most high-side switch ICs have UVLO protection. This usually covers the voltage range considering cranking voltage such as 6V or 4.5V. However, the actual voltage level on the high-side switch can be lower than the acceptable voltage level since the long PCB pattern, power cable, ideal diode and others cause the voltage drop.

Because LED drivers work as boost converter, a lower input voltage results in a larger input current. This is worse in this system example which requires two LED drivers since the drivers draw much larger current as described in the Section 1. In such cases, the high-side switch can trigger unexpected UVLO protection which can cause power supply issues since the high-side switch usually shutdown under protection. Figure 2-1 and Figure 2-2 shows the waveforms under VIN transient condition from 12V to 6V considering cranking event. In the plots, VIN on LED driver is the red line, inductor current is the pink line and VDD on LED driver is the green line. When VIN drops to 6V, IL increases dramatically and causes the VIN on the LED driver to drop well below 6V.

Figure 2-1 LED_VIN and LED_VDD and I_L During a VIN Transient 12V to 6V

Figure 2-2 LED_VIN and LED_VDD and I_L During a VIN Transient 12V to 6V (Zoomed-in)

As expected, the voltage at high-side switch is already much lower than 6V due to losses. The high-side switch triggered UVLO, which causes shutdown. Therefore, the inductor current is reduced even though the LED driver is still switching with reduced input voltage as shown in Figure 2-2. If this happened in the real system, users experience a display shutdown during the cranking condition. To avoid this unexpected risk, designer must use the high-side switch which has much bigger margin on protections such as UVLO. However, this can increase cost and makes achieving precise protections difficult.