The minimum requirements for a suitable low dropout regulator are:

• Its current drive capability should slightly exceed the specified load current of the application to prevent the LDO from dropping out of regulation. Therefore for a load current of 100 mA, choose a 100 mA to 150 mA LDO. While regulators with higher drive capabilities are acceptable, they also usually possess higher dropout voltages that will reduce overall converter efficiency.
• The internal dropout voltage, V_DO, at the specified load current should be as low as possible to maintain efficiency. For a low-cost 150 mA LDO, a V_DO of 150 mV at 100 mA is common. Be aware however, that this lower value is usually specified at room temperature and can increase by a factor of 2 over temperature, which in turn will raise the required minimum input voltage.
• The required minimum input voltage preventing the regulator from dropping out of line regulation is given with:
  Equation 1. V_I-min = V_DO-max + V_O-max

  This means in order to determine V_I for worst-case condition, the user must take the maximum values for V_DO and V_O specified in the LDO data sheet for rated output current (i.e., 100 mA) and add them together. Also specify that the output voltage of the push-pull rectifier at the specified load current is equal or higher than V_I-min. If it is not, the LDO will lose line-regulation and any variations at the input will pass straight through to the output. Hence below V_I-min the output voltage will follow the input and the regulator behaves like a simple conductor.

• The maximum regulator input voltage must be higher than the rectifier output under no-load. Under this condition there is no secondary current reflected back to the primary, thus making the voltage drop across R_DS-on negligible and allowing the entire converter input voltage to drop across the primary. At this point the secondary reaches its maximum voltage of
  Equation 2. V_S-max = V_IN-max × n

with V_IN-max as the maximum converter input voltage and n as the transformer turns ratio. Thus to prevent the LDO from damage the maximum regulator input voltage must be higher than V_S-max. #GUID-CC1C0779-8D1A-47D5-B9FB-BB132E1824F4/SLLSEA05367 lists the maximum secondary voltages for various turns ratios commonly applied in push-pull converters with 100 mA output drive.