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

3.2 Key Design Considerations

The system example used LM5152-Q1 EVM to maintain the minimum output of 8.5V at 440kHz during the automotive cranking down to 2.5V boost input. The LM5152-Q1EVM is designed to achieve the power requirement as listed in Table 3-1 which can meet most requirements. The details are described in the LM5152EVM-BST Evaluation Module. Texas Instruments also provides many tools to assist designers such as Webench or the Excel design calculator for this device on ti.com.

Table 3-1 LM5152-Q1EVM Characteristics
ParameterTest ConditionMINTYPMAXUNIT
Input voltage rangeOperation2.513.536V
Start-up voltage7
Input voltage17V
Output voltage8.5V
Output current 12.5V≤VSUPPLY≤ 4.5V4A
Output current 24.5V≤VSUPPLY≤ 36V6A
Switching frequency440kHz

One of key considerations is how to set minimum output voltage. The VOUT regulation target (VOUT-REG) is adjustable by programming the TRK pin voltage, which is the reference of the internal error amplifier. The accuracy of VOUT-REG is given when the TRK voltage is between 0.25V and 1V. The high impedance TRK pin allows users to program the pin voltage directly by a D/A converter or by connecting to a resistor voltage divider (RVREFT, RVREFB) between VREF and AGND.

This device provides a 1V voltage reference (VREF), which can be used to program the TRK pin voltage through a resistor voltage divider. When RVREFT and RVREFB are used to program the TRK pin voltage, VOUT-REG can be calculated as Equation 1.

Equation 1. VOUTREG=20 ×RVVREFB RVREFB +RVREFT

So RVREFT = 56.2K and RVREFB = 41.2K in the schematic as shown in Figure 3-5.

LM5152-Q1EVM SchematicFigure 3-5 LM5152-Q1EVM Schematic

Another key issue is dithering feature. Most automotive applications require dithering feature such as spread-spectrum. LM5152-Q1 can achieve dithering feature by connecting STATUS pin to DITHER pin through diode as shown in Figure 3-6. The dithering is required when the LM5152-Q1 switches to boost output voltage. The STATUS pin is pulled down to ground during boost operation. The diode is reversed biased and the dithering ramp mechanism can work by charging and discharging the capacitor on the DITHER pin. However, this capacitor must be connected to the same SYNC/DITHER/VH/CP pin.

If VOUT pin voltage is bigger than over voltage (VOVTH), the LM5152-Q1 enters bypass mode. During bypass mode, high-side FET must be turned on 100% and the dithering is not required. As LM5152-Q1 does not have an internal charge pump, the DITHER/CP pin must be bigger than 2V to enable internal charge pump for bypass operation. Once the device enters bypass mode, STATUS pin is pulled-up and the pin enables the charge-pump. As a result, the diode and pull-up resistor must be used to implement both dithering feature and bypass mode.

Enable Both Clock Dithering and Bypass OperationFigure 3-6 Enable Both Clock Dithering and Bypass Operation