max

IN-max

min

IN-max

min

T

V

Vt

V

=

2

2

f

³

´

´

.

min

min

3.6 V

Vt

= 7.2 Vμs

for 3.3 V, and

2

250 kHz

5.5 V

Vt

= 9.1 Vμs for 5 V applications

2

300 kHz

³

´

³

´

Q

RDS

SN6501

SLLSEA0C – FEBRUARY 2012 – REVISED MARCH 2012

www.ti.com

TRANSORMER SELECTION

V-t Product Calculation

To prevent a transformer from saturation its V-t product must be greater than the maximum V-t product applied

by the SN6501. The maximum voltage delivered by the SN6501 is the nominal converter input plus 10%. The

maximum time this voltage is applied to the primary is half the period of the lowest frequency at the specified

input voltage. Therefore, the transformer’s minimum V-t product is determined through:

(1)

Inserting the numeric values from the data sheet into the equation above yields the minimum V-t products of

(2)

Common V-t values for low-power center-tapped transformers range from 22 V

μs to 150 Vμs with typical

footprints of 10 mm x 12 mm. However, transformers specifically designed for PCMCIA applications provide as

little as 11 V

μs and come with a significantly reduced footprint of 6 mm x 6 mm only.

While Vt-wise all of these transformers can be driven by the SN6501, other important factors such as isolation

voltage, transformer wattage, and turns ratio must be considered before making the final decision.

Turns Ratio Estimate

Assume the rectifier diodes and linear regulator has been selected. Also, it has been determined that the

transformer choosen must have a V-t product of at least 11 V

μs. However, before searching the manufacturer

websites 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:

(3)

provide sufficient input voltage for the regulator to remain in regulation. From the LDO SELECTION section, this

(4)

14

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