Linear Technology Analog Devices LT8301 Datasheet page 16

LT8301
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:
t • N
OFF(MIN) PS
L
≥
PRI
I
SW(MIN)
t • V
ON(MIN) IN(MAX)
L
≥
PRI
I
SW(MIN)
t = 450ns
OFF(MIN)
t = 170ns
ON(MIN)
I = 290mA (typ)
SW(MIN)
Example:
450ns • 3 • (5V + 0.3V)
L
≥
PRI
290mA
170ns • 32V
L = 19µH
≥
PRI
290mA
Most transformers specify primary inductance with a tol-
erance of ±20%. With other component tolerance consid-
ered, choose a transformer with its primary inductance
30% larger than the minimum values calculated above.
L = 40µH is then chosen in this example.
PRI
Once the primary inductance has been determined, the
maximum load switching frequency can be calculated as:
1
f
= =
SW
L
t + t
PRI
ON OFF
V •I • 2
OUT OUT
I
=
SW
η • V • D
IN
16
( )
• V + V
OUT F
= 25µH
1
•I L •I
SW PRI SW
+
V N • (V + V
IN
PS OUT
For more information
Example:
(5V + 0.3V) • 3
D =
(5V + 0.3V) • 3 + 12V
5V • 0.5A • 2
I
=
SW
0.85 • 12V • 0.57
f = 199kHz
SW
The transformer also needs to be rated for the correct
saturation current level across line and load conditions.
A saturation current rating larger than 2A is necessary
to work with the LT8301. The 750313974 from Würth 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)
Example:
I
DIODE(MAX)
Next calculate reverse voltage requirement using maxi-
mum V :
IN
V = V
REVERSE
Example:
)
F
V = 5V +
REVERSE
The CMSH5-20 (5A, 20V diode) from Central
Semiconductor is chosen.
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= 0.57
= 0.86A
= I • N
SW(MAX) PS
= 4.125A
V
IN(MAX)
+
OUT
N
PS
32V
= 15.6V
3
Rev. A