LDO Design

20 August 2019 Link

Specifications to Consider

  1. Dropout Voltage
  2. Load Regulation
  3. Line Regulation
  4. Accuracy
  5. Short Circuit Current and Output Overshoot for sudden input step.
  6. Maximum Sink Current


  1. PSRR
    1. Small Signal
      1. Different Supplies
      2. Different Loads
    2. Step Response
      1. Different Supplies
      2. Different Loads
  2. Line Regulation
    1. Step response - especially pay attention to the transient current through the pass FET when the LDO is suddenly applied a supply.
    2. DC Response
  3. Load Regulation
    1. Step Response
    2. DC Response
  4. Stability of main loop and current limit loop (if any) – Loop Gain, Phase, Gain Margin, Phase Margin, Bandwidth, Gain Bandwidth Product
    1. For different input levels
    2. For different outputs/loads
  5. Dropout voltage - Ramp the supply down until output falls by 5%. Measure dropout.
  6. Startup and Shutdown (Record time taken)
  7. Short Circuit Protection - Especially for Fast enable and fast application of supply and the output is 0.
  8. Supply and Ground Currents

Caution List

  1. When having a short circuit protection circuitry always simulate and record behavior of the short circuit protection circuit during dropout mode. Make sure that it does not turn ON during dropout and increase the dropout of the LDO especially at corners.
  2. Make sure if the LDO is enabled immediately or if the supply is applied immediately the output current still is controlled by the short circuit limit protection and does not fly uncontrollable. This means the short circuit protection circuit must always be enabled first and allowed to settle before releasing the output FET for operation.


  1. Refer General Design Guidelines
  2. Refer Opamp, Transconductor and Comparator Design
  3. Refer Opamps for notes and theory