SLLSEP9H September 2015 – July 2019 SN6505A , SN6505B

PRODUCTION DATA.

- 1 Features
- 2 Applications
- 3 Description
- 4 Revision History
- 5 Pin Configuration and Functions
- 6 Specifications
- 7 Parameter Measurement Information
- 8 Detailed Description
- 9 Application and Implementation
- 10Power Supply Recommendations
- 11Layout
- 12Device and Documentation Support
- 13Mechanical, Packaging, and Orderable Information

- DBV|6

Assume the rectifier diodes and linear regulator has been selected. Also, it has been determined that the transformer chosen must have a V-t product of at least 11 Vμs. However, before searching the manufacturer web sites for a suitable transformer, the user still needs to know its minimum turns ratio that allows the push-pull converter to operate flawlessly over the specified current and temperature range. This minimum transformation ratio is expressed through the ratio of minimum secondary to minimum primary voltage multiplied by a correction factor that takes the transformer’s typical efficiency of 97% into account:

Equation 5. V_{P-min} = V_{IN-min} - V_{DS-max}

V_{S-min} must be large enough to allow for a maximum voltage drop, V_{F-max}, across the rectifier diode and still provide sufficient input voltage for the regulator to remain in regulation. From the *LDO Selection* section, this minimum input voltage is known and by adding V_{F-max} gives the minimum secondary voltage with:

Equation 6. V_{S-min} = V_{F-max } + V_{DO-max} + V_{O-max}

Then calculating the available minimum primary voltage, V_{P-min}, involves subtracting the maximum possible drain-source voltage of the device, V_{DS-max}, from the minimum converter input voltage V_{IN-min}:

Equation 7. V_{P-min} = V_{IN-min} – V_{DS-max}

V_{DS-max} however, is the product of the maximum R_{DS(on)} and I_{D} values for a given supply specified in the data sheet:

Equation 8. V_{DS-max} = R_{DS-max} × I_{Dmax}

Then inserting Equation 8 into Equation 7 yields:

Equation 9. V_{P-min }= V_{IN-min} - R_{DS-max} x I_{Dmax}

and inserting Equation 9 and Equation 6 into Equation 5 provides the minimum turns ration with:

Equation 10.

**Example:**

For a 3.3 V_{IN} to 5 V_{OUT} converter using the rectifier diode MBR0520L and the 5 V LDO, the data sheet values taken for a load current of 600 mA and a maximum temperature of 85°C are V_{F-max} = 0.2 V,

V_{DO-max} = 0.5 V, and V_{O-max} = 5.1 V.

Then assuming that the converter input voltage is taken from a 3.3 V controller supply with a maximum ±2% accuracy makes V_{IN-min} = 3.234 V. Finally the maximum values for drain-source resistance and drain current at 3.3 V are taken from the data sheet with R_{DS-max} = 0.31 Ω and I_{D-max} = 1 A.

Inserting the values above into Equation 10 yields a minimum turns ratio of:

Equation 11.

Most commercially available transformers for 3-to-5 V push-pull converters offer turns ratios between 2.0 and 2.3 with a common tolerance of ±3%.