SPVA032 September   2025 LM5152-Q1 , LP8866-Q1 , LP8866S-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Expected System Challenges Without Pre-Boost
    1. 2.1 Undervoltage Protection of High-Side Switch
    2. 2.2 System Bias Power Interruption
    3. 2.3 Unexpected Lower Input Voltage on LED Driver
    4. 2.4 Unexpected Higher Input Current Condition in the System
  6. 3Design Suggestion to Resolve the System Challenges by Using Pre-Boost
    1. 3.1 Block Diagram and Test Results
    2. 3.2 Key Design Considerations
  7. 4Summary
  8. 5References

1 Introduction

The large size of automotive displays is getting more popular in the market. Figure 1-1 shows the 30 inch display from GM which has integrated cluster and CID as an example. Some OEMs use pillar-to-pillar display which is an ultra-wide automotive display that spans the entire width of the dashboard of the vehicle. As the automotive panel size increases in size, more LEDs are used in the backlight. This means the power requirement increases and more LED drivers are required. The loss across long PCB pattern increases since the PCB size increases.

GM 30" Automotive DisplayFigure 1-1 GM 30" Automotive Display

This trend can challenge tier-1s and panel makers to deal with low battery conditions according to the battery profile. The LED driver must light up normally even under 6V depending on the requirements of the OEM. The traditional block diagram of the display module is shown in Figure 1-2 as an example focusing on LED driver power design excluding peripherals. Table 1-1 lists the design requirements for one piece of the LED driver considering the LED specifications.

Traditional Block Diagram of a Display ModuleFigure 1-2 Traditional Block Diagram of a Display Module
Table 1-1 LED Specifications for Each LED Driver
ParameterValueUnitNote
VIN(MIN)6V
VIN(MAX)26.5V
Maximum Forward Voltage, VF(MAX)3.6VLED Characteristic
Forward Current in one LED String Group, IF(MAX)80mA
Number of LEDs in one string10S
Total number of LED sink channels used6Ch
Maximum Required Vout, VOUT(MAX)37VConsidering 1V margin due to headroom voltage
Max Output Power, POUT_MAX17.76W
Max input current, IIN_MAX3.48A
Max input power, PIN_Max20.9WNeed to consider system worst efficiency
System worst efficiency85%

In this system example, input current increases significantly under a low voltage condition since the LED driver operates as a boost converter. The large input current causes a substantial voltage drop across the power cable, high-side switch, ideal diode due to losses. Eventually input voltage on PCB becomes smaller which requires even more current. This vicious cycle can make the system vulnerable to unexpected protection features such as UVLO even though LED driver can even support low input voltage condition. The pre-boost can help designers address this challenge in the system level.