Datasheet

LT3958
18
3958f
APPLICATIONS INFORMATION
The snubber resistor value (R
SN
) can be calculated by the
following equation:
R
SN
= 2•
V
2
SN
− V
SN
•V
OUT
•
N
P
N
S
I
2
SW(PEAK)
•L
LK
•ƒ
L
LK
is the leakage inductance of the primary winding,
which is usually specifi ed in the transformer character-
istics. L
LK
can be obtained by measuring the primary
inductance with the secondary windings shorted. The
snubber capacitor value (C
SN
) can be determined using
the following equation:
C
SN
=
V
SN
ΔV
SN
•R
SN
•ƒ
where ΔV
SN
is the voltage ripple across C
SN
. A reasonable
ΔV
SN
is 5% to 10% of V
SN
. The reverse voltage rating of
D
SN
should be higher than the sum of V
SN
and V
IN(MAX)
.
Flyback Converter: Output Diode Selection
The output diode in a fl yback converter is subject to large
RMS current and peak reverse voltage stresses. A fast
switching diode with a low forward drop and a low reverse
leakage is desired. Schottky diodes are recommended if
the output voltage is below 100V.
Approximate the required peak repetitive reverse voltage
rating V
RRM
using:
V
RRM
>
N
S
N
P
•V
IN(MAX)
+ V
OUT
The power dissipated by the diode is:
P
D
= I
O(MAX)
• V
D
and the diode junction temperature is:
T
J
= T
A
+ P
D
• R
θJA
The R
θJA
to be used in this equation normally includes the
R
θJC
for the device, plus the thermal resistance from the board
to the ambient temperature in the enclosure. T
J
must not
exceed the diode maximum junction temperature rating.
Flyback Converter: Output Capacitor Selection
The output capacitor of the fl yback converter has a similar
operation condition as that of the boost converter. Refer to
the Boost Converter: Output Capacitor Selection section
for the calculation of C
OUT
and ESR
COUT
.
The RMS ripple current rating of the output capacitors
in continuous operation can be determined using the
following equation:
I
RMS(COUT),CONTINUOUS
≈ I
O(MAX)
•
D
MAX
1−D
MAX
Flyback Converter: Input Capacitor Selection
The input capacitor in a fl yback converter is subject to
a large RMS current due to the discontinuous primary
current. To prevent large voltage transients, use a low
ESR input capacitor sized for the maximum RMS current.
The RMS ripple current rating of the input capacitors in
continuous operation can be determined using the fol-
lowing equation:
I
RMS(CIN),CONTINUOUS
≈
P
OUT(MAX)
V
IN(MIN)
• η
•
1−D
MAX
D
MAX
SEPIC CONVERTER APPLICATIONS
The LT3958 can be confi gured as a SEPIC (single-ended
primary inductance converter), as shown in Figure 1. This
topology allows for the input to be higher, equal, or lower
than the desired output voltage. The conversion ratio as
a function of duty cycle is:
V
OUT
+ V
D
V
IN
=
D
1−D
in continuous conduction mode (CCM).
In a SEPIC converter, no DC path exists between the input
and output. This is an advantage over the boost converter
for applications requiring the output to be disconnected
from the input source when the circuit is in shutdown.