Datasheet

LT3957A
18
3957afa
APPLICATIONS INFORMATION
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 flyback 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 following equation:
I
RMS(CIN),CONTINUOUS
≈
P
OUT(MAX)
V
IN(MIN)
• η
•
1−D
MAX
D
MAX
SEPIC CONVERTER APPLICATIONS
The LT3957A can be configured 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.
Compared to the flyback converter, the SEPIC converter
has the advantage that both the power MOSFET and the
output diode voltages are clamped by the capacitors (C
IN
,
C
DC
and C
OUT
), therefore, there is less voltage ringing
across the power MOSFET and the output diodes. The
SEPIC converter requires much smaller input capacitors
than those of the flyback converter. This is due to the fact
that, in the SEPIC converter, the current through inductor
L1 (which is series with the input) is continuous.
SEPIC Converter: Switch Duty Cycle and Frequency
For a SEPIC converter operating in CCM, the duty cycle
of the main switch can be calculated based on the output
voltage (V
OUT
), the input voltage (V
IN
) and the diode
forward voltage (V
D
).
The maximum duty cycle (D
MAX
) occurs when the converter
has the minimum input voltage:
D
MAX
=
V
OUT
+ V
D
V
IN(MIN)
+ V
OUT
+ V
D
SEPIC Converter: The Maximum Output Current
Capability and Inductor Selection
As shown in Figure 1, the SEPIC converter contains two
inductors: L1 and L2. L1 and L2 can be independent, but can
also be wound on the same core, since identical voltages
are applied to L1 and L2 throughout the switching cycle.
For the SEPIC topology, the current through L1 is the
converter input current. Based on the fact that, ideally, the
output power is equal to the input power, the maximum
average inductor currents of L1 and L2 are:
I
L1(MAX)
=I
IN(MAX)
=I
O(MAX)
•
D
MAX
1−D
MAX
I
L2(MAX)
=I
O(MAX)
Due to the current limit of its internal power switch, the
LT3957A should be used in a SEPIC converter whose
maximum output current (I
O(MAX)
) is less than the output
current capability by a sufficient margin (10% or higher
is recommended):
I
O(MAX)
< 1−D
MAX
()
•5A−0.5 • ΔI
SW
()
The inductor ripple currents ΔI
L1
and ΔI
L2
are identical:
ΔI
L1
= ΔI
L2
= 0.5 • ΔI
SW
The inductor ripple current ΔI
SW
has a direct effect on the
choice of the inductor value and the converter’s maximum
output current capability. Choosing smaller values of ΔI
SW
requires large inductances and reduces the current loop
gain (the converter will approach voltage mode). Accepting
larger values of ΔI
SW
allows the use of low inductances,