Datasheet

LT3757/LT3757A
23
3757afe
For more information www.linear.com/LT3757
Given an operating input voltage range, and having cho-
sen 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 fol-
lowing equation:
L1= L2 =
V
IN(MIN)
0.5 • ΔI
SW
• f
• 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
• f
• 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 • ∆I
L1
I
L2(PEAK)
= I
L2(MAX)
+ 0.5 • ∆I
L2
The RMS inductor currents are:
I
L1(RMS)
=I
L1(MAX)
• 1+
c
2
L1
12
where:
c
L1
=
Δ
I
L1
I
L1(MAX)
I
L2(RMS)
=I
L2(MAX)
• 1+
c
2
L2
12
where:
c
L2
=
Δ
I
L2
I
L2 (MAX)
Based on the preceding equations, the user should choose
the inductors having sufficient saturation and RMS cur-
rent ratings.
In a SEPIC converter, when the power switch is turned on,
the current flowing through the sense resistor (I
SENSE
) is
the switch current.
Set the sense voltage at I
SENSE(PEAK)
to be the minimum
of the SENSE current limit threshold with a 20% margin.
The sense resistor value can then be calculated to be:
R
SENSE
=
80 mV
I
SW(PEAK)
SEPIC Converter: Power MOSFET Selection
For the SEPIC configuration, choose a MOSFET with a
V
DC
rating higher than the sum of the output voltage and
input voltage by a safety margin (a 10V safety margin is
usually sufficient).
The power dissipated by the MOSFET in a SEPIC converter
is:
P
FET
= I
2
SW(MAX)
• R
DS(ON)
• D
MAX
+ 2 • (V
IN(MIN)
+ V
OUT
)
2
• I
L(MAX)
• C
RSS
• f /1A
The first term in this equation represents the conduction
losses in the device, and the second term, the switching
loss. C
RSS
is the reverse transfer capacitance, which is
usually specified in the MOSFET characteristics.
For maximum efficiency, R
DS(ON)
and C
RSS
should be
minimized. From a known power dissipated in the power
MOSFET, its junction temperature can be obtained using
the following equation:
T
J
= T
A
+ P
FET
• θ
JA
= T
A
+ P
FET
• (θ
JC
+ θ
CA
)
T
J
must not exceed the MOSFET maximum junction
temperature rating. It is recommended to measure the
MOSFET temperature in steady state to ensure that abso-
lute maximum ratings are not exceeded.
applicaTions inForMaTion
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