The minimum requirements for a suitable low dropout regulator are:

• The current drive capability must slightly exceed the specified load current of the application to prevent the LDO from dropping out of regulation. Therefore, for a load current of 550mA, choose a 600mA to 700mA LDO. While regulators with higher drive capabilities are acceptable, these regulators also typically possess higher dropout voltages that reduce the overall converter efficiency.
• The internal dropout voltage, V_DO, at the specified load current must be as low as possible to maintain efficiency. For a low-cost 700mA LDO, a V_DO of 600mV at 700mA is common. Be aware; however, that this lower value is typically specified at room temperature and can increase by a factor of 2 over temperature, which in turn raises 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

  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 (that is, 600mA) and add these values together. The user must 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 the output voltage is not, the LDO loses line-regulation and any variations at the input passes straight through to the output. Hence, below V_I-min the output voltage follows 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 the 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. Table 8-3 lists the maximum secondary voltages for various turns ratios commonly applied in push-pull converters.