LT3757 datasheet(17/24 Pages) AD

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LT8300

Rev. A

For more information www.analog.com

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 • ΔVOUT

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

applied 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 leak-

age inductance voltage spike. A DZ snubber is recom-

mended 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 volt-

age 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 RFB value should be adjusted

on the measured output voltage.

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