LT3757 datasheet(13/24 Pages) AD

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LT8300

Rev. A

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APPLICATIONS INFORMATION

A snubber circuit is recommended for most applications.

Two types of snubber circuits shown in Figure 6 that can

protect the internal power switch include the DZ (diode-

Zener) snubber and the RC (resistor-capacitor) snubber.

The DZ snubber ensures well defined and consistent

clamping voltage and has slightly higher power efficiency,

while the RC snubber quickly damps the voltage spike

ringing and provides better load regulation and EMI per-

formance. Figure 5 shows the flyback waveforms with the

DZ and RC snubbers.

For the DZ snubber, proper care must be taken when

choosing both the diode and the Zener diode. Schottky

diodes are typically the best choice, but some PN diodes

can be used if they turn on fast enough to limit the leakage

inductance spike. Choose a diode that has a reverse-volt-

age rating higher than the maximum SW pin voltage. The

Zener diode breakdown voltage should be chosen to bal-

ance power loss and switch voltage protection. The best

compromise is to choose the largest voltage breakdown.

Use the following equation to make the proper choice:

VZENER(MAX) ≤ 150V – VIN(MAX)

For an application with a maximum input voltage of 72V,

choose a 68V Zener diode, the VZENER(MAX) of which is

around 72V and below the 78V maximum.

The power loss in the clamp will determine the power

rating of the Zener diode. Power loss in the clamp is

highest at maximum load and minimum input voltage.

The switch current is highest at this point along with the

energy stored in the leakage inductance. A 0.5W Zener

will satisfy most applications when the highest VZENER

is chosen.

Figure 5. Maximum Voltages for SW Pin Flyback Waveform

Figure 6. Snubber Circuits

8300 F05

VSW

tOFF > 350ns

VLEAKAGE

tSP < 250ns

VSW

TIME

No Snubber

with DZ Snubber

with RC Snubber

tOFF > 350ns

VLEAKAGE

tSP < 250ns

TIME

tOFF > 350ns

VLEAKAGE

tSP < 250ns

TIME

<150V

<120V

<150V

<120V

<150V

<120V

8300 F06b

8300 F06a

DZ Snubber

RC Snubber

Lℓ

•

Lℓ

•

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