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9.2.2.2 LDO Selection

SN6507 is an open-loop transformer driver without load regulation capability. The output voltage may vary over a

wide range load current. Therefore, if a high-accuracy, load independent supply is required, the implementation

of a low dropout regulator (LDO) on the output side is strongly advised.

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 200 mA, choose a 200 mA to 300 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 300 mA LDO, a VDO of 600 mV at 300 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:

VI-min = VDO-max + VO-max

(5)

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, 200 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

VS-max = VIN-max × N

(6)

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 9-2 lists the maximum

secondary voltages for various turns ratios commonly applied in push-pull converters.

Table 9-2. Required Maximum LDO Input Voltages for Various Push-Pull Configurations

PUSH-PULL CONVERTER

LDO

CONFIGURATION

VIN-max [V]

TURNS-RATIO (N)

VS-max [V]

VI-max [V]

24 VIN to 15 VOUT

1.38:1

12 VIN to 15 VOUT

12.5

1:1.5

9.2.2.3 Diode Selection

A rectifier diode should always possess low-forward voltage to provide as much voltage to the converter output

as possible. However, when SN6507 is used in high-frequency switching applications, the diode must also

possess a low total capacitance, a short recovery time and a current rating greater than the load current.

Schottky diodes meet these requirements and are therefore strongly recommended in SN6507 push-pull

converter designs.

The necessary diode reverse voltage rating, VR, is determined by the transformer secondary side voltage plus

any voltage ringing. The voltage ringing, however, is difficult to predict, because it depends on multiple factors,

such as loop resistance, the leakage inductance of the transformer, and the diode junction capacitance. As a rule

of thumb, the diode voltage rating should be greater than 1.5 times the transformer turns ratio multiplied by the

maximum input voltage. Because the two secondary windings are connected across the rectifier bridge, a factor

of two is needed, producing the diode maximum DC blocking voltage rating:

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SN6507

SLLSFM0A – MARCH 2022 – REVISED JUNE 2022

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