LT8300
applicaTions inForMaTion
Step 2: Determine the Primary Inductance.
Example:
Primary inductance for the transformer must be set above
a minimum value to satisfy the minimum switch-off and
switch-on time requirements:
(12V + 0.3V)• 2
(12V + 0.3V)• 2+ 48V
12V • 0.12A • 2
D =
= 0.34
ISW
=
= 0.21A
0.85 • 48V • 0.34
fSW = 260kHz
tOFF(MIN) •NPS • V
+ VF
(
)
OUT
LPRI
≥
≥
ISW(MIN)
tON(MIN) • VIN(MAX)
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.
LPRI
ISW(MIN)
t
= 350ns
= 160ns
= 52mA
OFF(MIN)
ON(MIN)
SW(MIN)
t
I
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.
Aconservativemetricisthemaximumswitchcurrentlimit
multiplied by the turns ratio,
Example:
350ns • 2 •(12V + 0.3V)
LPRI
LPRI
≥
≥
= 166µH
52mA
160ns • 72V
= 222µH
52mA
I
= I
• N
SW(MAX) PS
DIODE(MAX)
Mosttransformersspecifyprimaryinductancewithatoler-
anceof 20%.Withothercomponenttoleranceconsidered,
choose a transformer with its primary inductance 20% to
40% larger than the minimum values calculated above.
Example:
I
= 0.52A
DIODE(MAX)
Next calculate reverse voltage requirement using maxi-
mum V :
L
PRI
= 300µH is then chosen in this example.
IN
Once the primary inductance has been determined, the
maximum load switching frequency can be calculated as:
VIN(MAX)
VREVERSE = VOUT
Example:
VREVERSE = 12V +
+
NPS
1
1
fSW
=
=
LPRI •ISW
LPRI •ISW
tON + tOFF
+
VIN
NPS •(VOUT + VF )
72V
2
= 48V
VOUT •IOUT • 2
η • VIN •D
ISW
=
The SBR0560S1 (0.5A, 60V diode) from Diodes Inc. is
chosen.
8300f
17
Linear [ Linear ]
