Datasheet

1 Fµ
V
IN
MBR0520L
MBR0520L
T1
3
1
SN6501
5
2
4
TP1
0.1µF
D2
D1
V
CC
GND
GND
1 5
43
10µF0.1 µF
VIN
10µF
MBR0520L
MBR0520L
T1
10µF
3
1
SN6501
5
2
4
LDO
2
TP 1 TP 2
V
OUT
0.1µF
D2
D1
Vcc
GND
GND
IN
EN NC
OUT
GND
min
0.2V + 0.2V + 5.175 V
n = 1.031 = 2
3.234 V 3 Ω 150 mA
´
- ´
F-max DO-max O-max
min
IN-min DS-max D-max
V + V + V
n = 1.031
V R I
´
- ´
SN6501
SLLSEA0F –FEBRUARY 2012–REVISED AUGUST 2013 www.ti.com
Then calculating the available minimum primary voltage, V
P-min
, involves subtracting the maximum possible drain-
source voltage of the SN6501, V
DS-max
, from the minimum converter input voltage V
IN-min
:
V
P-min
= V
IN-min
– V
DS-max
(5)
V
DS-max
however, is the product of the maximum R
DS(on)
and I
D
values for a given supply specified in the SN6501
data sheet:
V
DS-max
= R
DS-max
× I
Dmax
(6)
Then inserting Equation 6 into Equation 5 yields:
V
P-min
= V
IN-min
- R
DS-max
x I
Dmax
(7)
and inserting Equation 7 and Equation 4 into Equation 3 provides the minimum turns ration with:
(8)
Example:
For a 3.3 V
IN
to 5 V
OUT
converter using the rectifier diode MBR0520L and the 5 V LDO TPS76350, the data
sheet values taken for a load current of 100 mA and a maximum temperature of 85°C are V
F-max
= 0.2 V,
V
DO-max
= 0.2 V, and V
O-max
= 5.175 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 SN6501 data sheet with R
DS-max
= 3 Ω and I
D-max
= 150 mA.
Inserting the values above into Equation 8 yields a minimum turns ratio of:
(9)
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%.
Recommended Transformers
Depending on the application, use the minimum configuration in Figure 67 or standard configuration in Figure 68.
Figure 67. Unregulated Output for Low-Current Figure 68. Regulated Output for Stable Supplies
Loads with Wide Supply Range and High Current Loads
The Wurth Electronics Midcom isolation transformers in Table 2 are optimized designs for the SN6501, providing
high efficiency and small form factor at low-cost.
The 1:1.1 and 1:1.7 turns-ratios are designed for logic applications with wide supply rails and low load currents.
These applications operate without LDO, thus achieving further cost-reduction.
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