Datasheet

LT8300
17
8300f
applicaTions inForMaTion
Step 2: Determine the Primary Inductance.
Primary inductance for the transformer must be set above
a minimum value to satisfy the minimum switch-off and
switch-on time requirements:
L
PRI
≥
t
OFF(MIN)
•N
PS
• V
OUT
+ V
F
( )
I
SW(MIN)
L
PRI
≥
t
ON(MIN)
• V
IN(MAX)
I
SW(MIN)
t
OFF(MIN)
= 350ns
t
ON(MIN)
= 160ns
I
SW(MIN)
= 52mA
Example:
L
PRI
≥
350ns • 2 •(12V + 0.3V)
52mA
= 166µH
L
PRI
≥
160ns • 72V
52mA
= 222µH
Most transformers specify primary inductance with a toler-
ance of ±20%. With other component tolerance considered,
choose a transformer with its primary inductance 20% to
40% larger than the minimum values calculated above.
L
PRI
= 300µH is then chosen in this example.
Once the primary inductance has been determined, the
maximum load switching frequency can be calculated as:
f
SW
=
1
t
ON
+ t
OFF
=
1
L
PRI
•I
SW
V
IN
+
L
PRI
•I
SW
N
PS
•(V
OUT
+ V
F
)
I
SW
=
V
OUT
•I
OUT
• 2
η • V
IN
•D
Example:
D =
(12V
+
0.3V)• 2
(12V + 0.3V)• 2 + 48V
= 0.34
I
SW
=
12V • 0.12A • 2
0.85 • 48V • 0.34
= 0.21A
f
SW
= 260kHz
The transformer also needs to be rated for the correct
saturation current level across line and load conditions. A
saturation current rating larger than 400mA is necessary
to work with the LT8300. The 10396-T022 from Sumida
is chosen as the flyback transformer.
Step 3: Choose the Output Diode.
Two main criteria for choosing the output diode include
forward current rating and reverse voltage rating. The
maximum load requirement is a good first-order guess
as the average current requirement for the output diode.
A conservative metric is the maximum switch current limit
multiplied by the turns ratio,
I
DIODE(MAX)
= I
SW(MAX)
• N
PS
Example:
I
DIODE(MAX)
= 0.52A
Next calculate reverse voltage requirement using maxi-
mum V
IN
:
V
REVERSE
= V
OUT
+
V
IN(MAX)
N
PS
Example:
V
REVERSE
= 12V +
72V
2
= 48V
The SBR0560S1 (0.5A, 60V diode) from Diodes Inc. is
chosen.