LT8301MP 查看數據表（PDF） - Analog Devices
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LT8301MP Datasheet PDF : 24 Pages
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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:
( ) LPRI
≥
tOFF(MIN)
•NPS • VOUT
ISW(MIN)
+
VF
LPRI
≥
tON(MIN) • VIN(MAX)
ISW(MIN)
tOFF(MIN) = 450ns
tON(MIN) = 170ns
ISW(MIN) = 290mA (typ)
Example:
LPRI
≥
450ns
• 3 •(5V +
290mA
0.3V)
=
25µH
LPRI
≥
170ns • 32V
290mA
=
19µH
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.
LPRI = 40µH is then chosen in this example.
Once the primary inductance has been determined, the
maximum load switching frequency can be calculated as:
fSW
=
1
tON + tOFF
=
LPRI •ISW
VIN
1
+
LPRI •ISW
NPS •(VOUT +
VF
)
ISW
=
VOUT
η•
•IOUT •
VIN •D
2
Example:
D
=
(5V + 0.3V)• 3
(5V + 0.3V)• 3+12V
=
0.57
ISW
=
5V • 0.5A
0.85 •12V •
•2
0.57
=
0.86A
fSW = 199kHz
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,
IDIODE(MAX) = ISW(MAX) • NPS
Example:
IDIODE(MAX) = 4.125A
Next calculate reverse voltage requirement using maxi-
mum VIN:
VREVERSE
=
VOUT
+
VIN(MAX)
NPS
Example:
VREVERSE
=
5V
+
32V
3
=
15.6V
The CMSH5-20 (5A, 20V diode) from Central
Semiconductor is chosen.
Rev. B
16
For more information www.analog.com
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