Datasheet

LT8302
19
Rev E
For more information www.analog.com
APPLICATIONS INFORMATION
Example:
D =
5V
+
0.3V
( )
• 3
5V + 0.3V
( )
• 3 + 12V
= 0.57
I
SW
=
5V • 1.5A • 2
0.8 • 12V • 0.57
f
SW
= 277kHz
The transformer also needs to be rated for the correct
saturation current level across line and load conditions.
A saturation current rating larger than 7A is necessary
to work with the LT8302. The 750311564 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
at the average current requirement for the output diode.
Under output short-circuit condition, the output diode
needs to conduct much higher current. Therefore, a con-
servative metric is 60% of the maximum switch current
limit multiplied by the turns ratio:
I
DIODE(MAX)
= 0.6 • I
SW(MAX)
• N
PS
Example:
I
DIODE(MAX)
= 8.1A
Next calculate reverse voltage requirement using maxi-
mum V
IN
:
V
REVERSE
= V
OUT
+
V
IN(MAX)
N
PS
Example:
V
REVERSE
= 5V +
32V
3
= 15.7V
The PDS835L (8A, 35V diode) from Diodes Inc. is chosen.
Step 4: Choose the output capacitor.
The output capacitor should be chosen to minimize the
output voltage ripple while considering the increase in size
and cost of a larger capacitor. Use the following equation
to calculate the output capacitance:
C
OUT
=
L
PRI
•I
SW
2
2 • V
OUT
• ΔV
OUT
Example:
Design for output voltage ripple less than ±1% of V
OUT
,
i.e., 100mV.
C
OUT
=
9µH • 4.5A
( )
2
2 • 5V • 0.1V
= 182µF
Remember ceramic capacitors lose capacitance with
applied voltage. The capacitance can drop to 40% of
quoted capacitance at the maximum voltage rating. So
a 220µF, 6.3V rating X5R or X7R ceramic capacitor is
chosen.
Step 5: Design snubber circuit.
The snubber circuit protects the power switch from leak
-
age inductance voltage spike. A (RC + DZ) snubber is
recommended for this application. A 470pF capacitor in
series with a 39Ω resistor is chosen as the RC snubber.
The maximum Zener breakdown voltage is set according
to the maximum V
IN
:
V
ZENNER(MAX)
≤ 60V – V
IN(MAX)
Example:
V
ZENNER(MAX)
≤ 60V – 32V = 28V
A 24V Zener with a maximum of 26V will provide optimal
protection and minimize power loss. So a 24V, 1.5W Zener
from Central Semiconductor (CMZ5934B) is chosen.
Choose a diode that is fast and has sufficient reverse volt-
age breakdown:
V
REVERSE
> V
SW(MAX)
V
SW(MAX)
= V
IN(MAX)
+ V
ZENNER(MAX)
Example:
V
REVERSE
> 60V
A 100V, 1A diode from Diodes Inc. (DFLS1100) is chosen.
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