Datasheet LT8301 (Analog Devices) - 13

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APPLICATIONS INFORMATION
VSW VSW VSW <65V <65V <65V VLEAKAGE VLEAKAGE VLEAKAGE <50V <50V <50V tOFF > 450ns tOFF > 450ns tOFF > 450ns tSP < 350ns tSP < 350ns tSP < 350ns TIME TIME TIME 8301 F05
No Snubber with DZ Snubber with RC Snubber Figure 5. Maximum Voltages for SW Pin Flyback Waveform
Lℓ Lℓ Z • C • • R • D 8300 F06a 8301 F06b
DZ Snubber RC Snubber Figure 6. Snubber Circuits
A snubber circuit is recommended for most applications. Zener diode breakdown voltage should be chosen to bal- Two types of snubber circuits shown in Figure 6 that can ance power loss and switch voltage protection. The best protect the internal power switch include the DZ (diode- compromise is to choose the largest voltage breakdown. Zener) snubber and the RC (resistor-capacitor) snubber. Use the following equation to make the proper choice: The DZ snubber ensures well defined and consistent V clamping voltage and has slightly higher power efficiency, ZENER(MAX) ≤ 65V – VIN(MAX) while the RC snubber quickly damps the voltage spike For an application with a maximum input voltage of 32V, ringing and provides better load regulation and EMI per- choose a 20V Zener diode, the VZENER(MAX) of which is formance. Figure 5 shows the flyback waveforms with the around 21V and below the 33V maximum. DZ and RC snubbers. The power loss in the clamp will determine the power For the DZ snubber, proper care must be taken when rating of the Zener diode. Power loss in the clamp is choosing both the diode and the Zener diode. Schottky highest at maximum load and minimum input voltage. diodes are typically the best choice, but some PN diodes The switch current is highest at this point along with the can be used if they turn on fast enough to limit the leakage energy stored in the leakage inductance. A 0.25W Zener inductance spike. Choose a diode that has a reverse-volt- will satisfy most applications when the highest VZENER age rating higher than the maximum SW pin voltage. The is chosen. Rev. B For more information www.analog.com 13 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts