BZX100A datasheet(18/24 Pages) LINER

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LT8300

8300f

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 =

LPRI •ISW

2 • 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.

Thesnubbercircuitprotectsthepowerswitchfromleakage

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

applicaTions inForMaTion

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 + VF)

100µA

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 RFBvalueshouldbeadjusted

on the measured output voltage.

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