Datasheet

LT8300
14
8300f
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 performance.
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 leak-
age inductance spike. Choose a diode that has a reverse-
voltage rating higher than the maximum SW pin voltage.
The Zener diode breakdown voltage should be chosen to
balance 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
ZENER(MAX)
≤ 150V – V
IN(MAX)
For an application with a maximum input voltage of 72V,
choose a 68V Zener diode, the V
ZENER(MAX)
of which is
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
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 V
ZENER
is chosen.
Figure 5. Maximum Voltages for SW Pin Flyback Waveform
Figure 6. Snubber Circuits
8300 F05
V
SW
t
OFF
> 350ns
V
LEAKAGE
t
SP
< 250ns
V
SW
V
SW
TIME
No Snubber with DZ Snubber with RC Snubber
t
OFF
> 350ns
V
LEAKAGE
t
SP
< 250ns
TIME
t
OFF
> 350ns
V
LEAKAGE
t
SP
< 250ns
TIME
<150V
<120V
<150V
<120V
<150V
<120V
8300 F06b8300 F06a
DZ Snubber RC Snubber
L
ℓ
Z
D
C
R
•
•
L
ℓ
•
•