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

(1)

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 (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

VI-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 VI-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

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

(2)

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 8-2 lists the maximum

secondary voltages for various turns ratios commonly applied in push-pull converters with 100 mA output drive.

Table 8-2. Required Maximum LDO Input Voltages for Various Push-Pull Configurations

PUSH-PULL CONVERTER

LDO

CONFIGURATION

VIN-max [V]

TURNS-RATIO

VS-max [V]

VI-max [V]

3.3 VIN to 3.3 VOUT

3.6

1.5 ± 3%

5.6

6 to 10

3.3 VIN to 5 VOUT

3.6

2.2 ± 3%

8.2

10

5 VIN to 5 VOUT

5.5

1.5 ± 3%

8.5

10

8.2.2.3 Diode Selection

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

as possible. When used in high-frequency switching applications, such as the SN6501 however, the diode must

also possess a short recovery time. Schottky diodes meet both requirements and are therefore strongly

recommended in push-pull converter designs. A good choice for low-volt applications and ambient temperatures

of up to 85°C is the low-cost Schottky rectifier MBR0520L with a typical forward voltage of 275 mV at 100-mA

forward current. For higher output voltages such as ±10 V and above use the MBR0530 which provides a higher

DC blocking voltage of 30 V.

Lab measurements have shown that at temperatures higher than 100°C the leakage currents of the above

Schottky diodes increase significantly. This can cause thermal runaway leading to the collapse of the rectifier

output voltage. Therefore, for ambient temperatures higher than 85°C use low-leakage Schottky diodes, such as

RB168M-40.

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SN6501

SLLSEA0I – FEBRUARY 2012 – REVISED JANUARY 2021

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