LT3759 datasheet(17/24 Pages) AD

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LT8301

Rev. B

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 •ISW2

2• VOUT • ΔVOUT

Example:

Design for output voltage ripple less than 1% of VOUT,

i.e., 50mV.

COUT =

40µH•(0.86A)2

2•5V •0.05V

= 60µF

Remember ceramic capacitors lose capacitance with

applied voltage. The capacitance can drop to 40% of

quoted capacitance at the maximum voltage rating. So

a 100µF, 10V rating ceramic capacitor is chosen X5R or

X7R type is recommended.

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) ≤ 65V – VIN(MAX)

Example:

VZENER(MAX) ≤ 65V – 32V = 33V

APPLICATIONS INFORMATION

A 20V Zener with a maximum of 21V will provide optimal

protection and minimize power loss. So a 20V, 0.25W

Zener from Central Semiconductor (CMDZ5250B) 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 > 53V

A 100V, 0.25A diode from Central Semiconductor

(CMHD4448) 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 =

3•(5V

+0.3V)

100µA

= 159k

Depending on the tolerance of standard resistor values,

the precise resistor value may not exist. For 1% standard

values, a 158k resistor should be close enough. As dis-

cussed in the Application Information section, the final

RFB value should be adjusted on the measured output

voltage.

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