10.2.2.2 LDO Selection
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 600 mA, choose a 600 mA to 750 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, VDO, at the specified load current should be as low as possible to maintain efficiency. For a low-cost 750 mA LDO, a VDO of 600 mV at 750 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. VI-min = VDO-max + VO-max
This means in order to determine VI for worst-case condition, the user must take the maximum values for VDO and VO specified in the LDO data sheet for rated output current (that is, 600 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 VI-min. If it is not, the LDO will lose line-regulation and any variations at the input passes straight through to the output. Hence, below VI-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 RDS-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. VS-max = VIN-max × n
with VIN-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 VS-max. Table 8 lists the maximum secondary voltages for various turns ratios commonly applied in push-pull converters.
Table 8. Required Maximum LDO Input Voltages for Various Push-Pull Configurations
|3.3 VIN to 3.3 VOUT
||1.5 ± 3%
||6 to 10
|3.3 VIN to 5 VOUT
||2.2 ± 3%
|5 VIN to 5 VOUT
||1.5 ± 3%