BAV20W 查看數據表（PDF） - Linear Technology
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BAV20W Datasheet PDF : 24 Pages
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LT8300
Applications Information
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 equation below to
calculate the output capacitance:
COUT
=
2
LPRI • ISW2
• VOUT • ∆VOUT
Example:
Design for output voltage ripple less than 1% of VOUT,
i.e., 120mV.
COUT
=
300µH • (0.21A)2
2 • 12V • 0.12V
=
4.6µF
Remember ceramic capacitors lose capacitance with ap-
plied voltage. The capacitance can drop to 40% of quoted
capacitance at the maximum voltage rating. So a 10uF, 16V
rating ceramic capacitor is chosen.
Step 5: Design Snubber Circuit.
The snubber circuit protects the power switch from leakage
inductance voltage spike. A DZ snubber is recommended
for this application because of lower leakage inductance
and larger voltage margin. The Zener and the diode need
to be selected.
The maximum Zener breakdown voltage is set according
to the maximum VIN:
VZENER(MAX) ≤ 150V – VIN(MAX)
Example:
VZENER(MAX) ≤ 150V – 72V = 78V
A 68V Zener with a maximum of 72V will provide optimal
protection and minimize power loss. So a 68V, 0.5W Zener
from On Semiconductor (MMSZ5266BT1G) is chosen.
Choose a diode that is fast and has sufficient reverse
voltage breakdown:
VREVERSE > VSW(MAX)
VSW(MAX) = VIN(MAX) + VZENER(MAX)
Example:
VREVERSE > 144V
A 150V, 0.6A diode from Diodes Inc. (BAV20W) is chosen.
Step 6: Select the RFB Resistor.
Use the following equation to calculate the starting value
for RFB:
RFB
=
NPS
• (VOUT +
100µA
VF )
Example:
RFB
=
2
•
(12V + 0.3V)
100µA
=
246k
Depending on the tolerance of standard resistor values,
the precise resistor value may not exist. For 1% standard
values, a 243k resistor in series with a 3.01k resistor
should be close enough. As discussed in the Application
Information section, the final RFB value should be adjusted
on the measured output voltage.
8300f
18
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