Linear Technology Analog Devices LT8301 Datasheet page 13

APPLICATIONS INFORMATION
V
SW
<65V
V
LEAKAGE
<50V
t > 450ns
OFF
t < 350ns
SP
No Snubber
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
V
SW
<65V
<50V
t
OFF
t < 350ns
SP
TIME
with DZ Snubber
Figure 5. Maximum Voltages for SW Pin Flyback Waveform
L
ℓ
•
Z
•
D
8300 F06a
DZ Snubber
Figure 6. Snubber Circuits
For more information
V
SW
<65V
V
LEAKAGE
<50V
> 450ns
TIME
L
ℓ
C
•
R
8301 F06b
RC Snubber
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:
V ≤ 65V – V
ZENER(MAX)
For an application with a maximum input voltage of 32V,
choose a 20V Zener diode, the V
around 21V and below the 33V 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 0.25W Zener
will satisfy most applications when the highest V
is chosen.
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LT8301
V
LEAKAGE
t > 450ns
OFF
t < 350ns
SP
with RC Snubber
•
IN(MAX)
of which is
ZENER(MAX)
TIME
8301 F05
ZENER
Rev. A
13