Datasheet

Q
V
IN
V
F
R
L
R
DS
V
DS
V
DO
V
P
V
I
V
O
V
S
.
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
´
max IN-max
min IN-max
min
T V
Vt V =
2 2 f
³ ´
´
SN6501
www.ti.com SLLSEA0F –FEBRUARY 2012–REVISED AUGUST 2013
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 Vμs to 150 Vμs with typical
footprints of 10 mm x 12 mm. However, transformers specifically designed for PCMCIA applications provide as
little as 11 Vμs 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 Vμs. 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:
V
P-min
= V
IN-min
- V
DS-max
(3)
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:
V
S-min
= V
F-max
+ V
DO-max
+ V
O-max
(4)
Figure 66. Establishing the Required Minimum Turns Ratio Through n
min
= 1.031 × V
S-min
/ V
P-min
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