LT8300
applicaTions inForMaTion
V
SW
V
V
SW
SW
<150V
<120V
<150V
<120V
<150V
<120V
V
V
V
LEAKAGE
LEAKAGE
LEAKAGE
t
> 350ns
t
> 350ns
t
> 350ns
OFF
OFF
OFF
t
SP
< 250ns
t
SP
< 250ns
t
SP
< 250ns
TIME
TIME
TIME
8300 F05
No Snubber
with DZ Snubber
with RC Snubber
Figure 5. Maximum Voltages for SW Pin Flyback Waveform
L
L
ℓ
ℓ
Z
•
•
C
•
•
R
D
8300 F06a
8300 F06b
DZ Snubber
RC Snubber
Figure 6. Snubber Circuits
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)snubberandtheRC(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 performance.
Figure 5 shows the flyback waveforms with the DZ and
RC snubbers.
The Zener diode breakdown voltage should be chosen to
balancepowerlossandswitchvoltageprotection.Thebest
compromise is to choose the largest voltage breakdown.
Use the following equation to make the proper choice:
V
≤ 150V – V
IN(MAX)
ZENER(MAX)
For an application with a maximum input voltage of 72V,
choose a 68V Zener diode, the V of which is
ZENER(MAX)
around 72V and below the 78V maximum.
The power loss in the clamp will determine the power rat-
ing of the Zener diode. Power loss in the clamp is highest
at maximum load and minimum input voltage. The switch
currentishighestatthispointalongwiththeenergystored
in the leakage inductance. A 0.5W Zener will satisfy most
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 leak-
age inductance spike. Choose a diode that has a reverse-
voltage rating higher than the maximum SW pin voltage.
applications when the highest V
is chosen.
ZENER
8300f
14
Linear [ Linear ]
