Datasheet

LT3957A
19
3957afa
APPLICATIONS INFORMATION
but results in higher input current ripple and greater core
losses and reduces output current capability.
Given an operating input voltage range, and having chosen
the operating frequency and ripple current in the inductor,
the inductor value (L1 and L2 are independent) of the SEPIC
converter can be determined using the following equation:
L1 = L2 =
V
IN(MIN)
0.5 • ΔI
SW
•ƒ
•D
MAX
For most SEPIC applications, the equal inductor values
will fall in the range of 1µH to 100µH.
By making L1 = L2, and winding them on the same core, the
value of inductance in the preceding equation is replaced
by 2L, due to mutual inductance:
L =
V
IN(MIN)
ΔI
SW
•ƒ
•D
MAX
This maintains the same ripple current and energy storage
in the inductors. The peak inductor currents are:
I
L1(PEAK)
= I
L1(MAX)
+ 0.5 • ΔI
L1
I
L2(PEAK)
= I
L2(MAX)
+ 0.5 • ΔI
L2
The maximum RMS inductor currents are approximately
equal to the maximum average inductor currents.
Based on the preceding equations, the user should choose
the inductors having sufficient saturation and RMS cur-
rent ratings.
SEPIC Converter: Output Diode Selection
To maximize efficiency, a fast switching diode with a low
forward drop and low reverse leakage is desirable. The
average forward current in normal operation is equal to
the output current.
It is recommended that the peak repetitive reverse voltage
rating V
RRM
is higher than V
OUT
+ V
IN(MAX)
by a safety
margin (a 10V safety margin is usually sufficient).
The power dissipated by the diode is:
P
D
= I
O(MAX)
• V
D
where V
D
is diode’s forward voltage drop, and the diode
junction temperature is:
T
J
= T
A
+ P
D
• R
θJA
The R
θJA
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.
SEPIC Converter: Output and Input Capacitor Selection
The selections of the output and input capacitors of the
SEPIC converter are similar to those of the boost converter.
Please refer to the Boost Converter: Output Capacitor
Selection and Boost Converter: Input Capacitor Selection
sections.
SEPIC Converter: Selecting the DC Coupling Capacitor
The DC voltage rating of the DC coupling capacitor (C
DC
,
as shown in Figure 1) should be larger than the maximum
input voltage:
V
CDC
> V
IN(MAX)
C
DC
has nearly a rectangular current waveform. During
the switch off-time, the current through C
DC
is I
IN
, while
approximately –I
O
flows during the on-time. The RMS
rating of the coupling capacitor is determined by the fol-
lowing equation:
I
RMS(CDC)
>I
O(MAX)
•
V
OUT
+ V
D
V
IN(MIN)
A low ESR and ESL, X5R or X7R ceramic capacitor works
well for C
DC
.
INVERTING CONVERTER APPLICATIONS
The LT3957A can be configured as a dual-inductor inverting
topology, as shown in Figure 8. The V
OUT
to V
IN
ratio is:
V
OUT
− V
D
V
IN
=−
D
1−D
in continuous conduction mode (CCM).