LT8300
applicaTions inForMaTion
Step 4: Choose the Output Capacitor.
A 68V Zener with a maximum of 72V will provide optimal
protection and minimize power loss. So a 68V, 0.5W Zener
from On Semiconductor (MMSZ5266BT1G) is chosen.
The output capacitor should be chosen to minimize the
outputvoltageripplewhileconsideringtheincreaseinsize
and cost of a larger capacitor. Use the equation below to
calculate the output capacitance:
Choose a diode that is fast and has sufficient reverse
voltage breakdown:
2
V
V
> V
SW(MAX)
REVERSE
SW(MAX)
L
•I
SW
PRI
C
=
OUT
= V
+ V
ZENER(MAX)
IN(MAX)
2 • V
• ∆V
OUT
OUT
Example:
Example:
V
> 144V
REVERSE
Design for output voltage ripple less than 1% of V
i.e., 120mV.
,
OUT
A 150V, 0.6A diode from Diodes Inc. (BAV20W) is chosen.
Step 6: Select the R Resistor.
300µH•(0.21A)2
2 •12V • 0.12V
FB
COUT
=
= 4.6µF
Use the following equation to calculate the starting value
for R :
FB
Remember ceramic capacitors lose capacitance with ap-
plied voltage. The capacitance can drop to 40% of quoted
capacitanceatthemaximumvoltagerating.Soa10uF,16V
rating ceramic capacitor is chosen.
NPS •(VOUT + VF )
RFB =
100µA
Example:
Step 5: Design Snubber Circuit.
2 •(12V + 0.3V)
RFB =
= 246k
Thesnubbercircuitprotectsthepowerswitchfromleakage
inductance voltage spike. A DZ snubber is recommended
for this application because of lower leakage inductance
and larger voltage margin. The Zener and the diode need
to be selected.
100µA
Depending on the tolerance of standard resistor values,
the precise resistor value may not exist. For 1% standard
values, a 243k resistor in series with a 3.01k resistor
should be close enough. As discussed in the Application
Informationsection, thefinalR valueshouldbeadjusted
on the measured output voltage.
The maximum Zener breakdown voltage is set according
to the maximum V :
FB
IN
V
≤ 150V – V
IN(MAX)
ZENER(MAX)
Example:
V
≤ 150V – 72V = 78V
ZENER(MAX)
8300f
18
Linear [ Linear ]
