SNVA940 November   2021 LM5157 , LM5157-Q1 , LM51571-Q1 , LM5158 , LM5158-Q1 , LM51581 , LM51581-Q1

 

  1.   Trademarks
  2. 1LM5157 Boost Design Example
  3. 2Calculations and Component Selection
    1. 2.1  Switching Frequency
    2. 2.2  Inductor Calculation
    3. 2.3  Slope Compensation Check
    4. 2.4  Inductor Selection
    5. 2.5  Diode Selection
    6. 2.6  Output Capacitor Selection
    7. 2.7  Input Capacitor Selection
    8. 2.8  UVLO Resistor Selection
    9. 2.9  Soft-Start Capacitor Selection
    10. 2.10 Feedback Resistor Selection
    11. 2.11 Control Loop Compensation
      1. 2.11.1 Crossover Frequency (fcross) Selection
      2. 2.11.2 RCOMP Selection
      3. 2.11.3 CCOMP Selection
      4. 2.11.4 CHF Selection
    12. 2.12 Power Loss and Efficiency Estimation
  4. 3Implementation Results
  5. 4Small Signal Frequency Analysis
    1. 4.1 Boost Regulator Modulator Modeling
    2. 4.2 Compensation Modeling
    3. 4.3 Open Loop Modeling

2.8 UVLO Resistor Selection

The external under voltage lockout (UVLO) resistors set the minimum operating voltage of the regulator. Two levels must be specified: the desired start-up voltage of the converter (VSUPPLY(ON)) and the desired turn-off voltage of the converter (VSUPPLY(OFF)). In this example, VSUPPLY(ON) voltage is 2.8 V and the VSUPPLY(OFF) is 2.4 V. Using Equation 15, the top UVLO resistor (RUVLOT) is calculated.

Equation 15. R UVLOT = 0.967 ∙ V SUPPLY ON - V SUPPLY OFF 5 μA = 0.967 ∙ 2.8 V - 2.4 V 5 μA = 61.5 k Ω

A standard value of 61.9kΩ is selected for RUVLOT. Using Equation 16, the bottom UVLO resistor (RUVLOB) is calculated.

Equation 16. R UVLOB = 1.5 V ∙ R UVLOT V SUPPLY ON - 1.5 V = 1.5 V ∙ 61.9 k Ω 2.8 V - 1.5 V = 71.4 k Ω

A standard value of 71.5kΩ is selected for RUVLOB