LT3758 LINER [Linear Technology], LT3758 Datasheet - Page 23

• Manufacturer Part Number: LT3758
• Description: High Input Voltage, Boost, Flyback, SEPIC and Inverting Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
ΔI
loop gain (the converter will approach voltage mode).
Accepting larger values of ΔI
ductances, but results in higher input current ripple and
greater core losses. It is recommended that χ falls in the
range of 0.2 to 0.6.
Given an operating input voltage range, and having chosen
the operating frequency and ripple current in the induc-
tor, the inductor value (L1 and L2 are independent) of the
SEPIC converter can be determined using the following
equation:
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:
This maintains the same ripple current and energy storage
in the inductors. The peak inductor currents are:
The RMS inductor currents are:
Figure 9. The Switch Current Waveform of the SEPIC Converter
L
I
I
L
L
I
I
L1(PEAK)
L2(PEAK)
L RMS
SW
requires large inductances and reduces the current
1
=
1
=
(
Δ
V
L
IN MIN
I
2
SW
(
)
=
I
SW(MAX)
=
= I
= I
DT
0 5
•
I
S
. •
L MAX
)
f
L1(MAX)
L2(MAX)
1
V
•
(
IN MIN
D
Δ
(
MAX
I
SW
)
•
)
+ 0.5 • ΔI
+ 0.5 • ΔI
T
•
S
I
SW =
1
f
+
•
D
χ
MAX
I
12
L
SW(MAX)
2
L
allows the use of low in-
L1
L2
1
3758 F09
t
where
where
Based on the preceding equations, the user should choose
the inductors having suffi cient saturation and RMS cur-
rent ratings.
In a SEPIC converter, when the power switch is turned on,
the current fl owing through the sense resistor (I
the switch current.
Set the sense voltage at I
of the SENSE current limit threshold with a 20% margin.
The sense resistor value can then be calculated to be:
SEPIC Converter: Power MOSFET Selection
For the SEPIC confi guration, choose a MOSFET with a
V
input voltage by a safety margin (a 10V safety margin is
usually suffi cient).
The power dissipated by the MOSFET in a SEPIC con-
verter is:
The fi rst term in this equation represents the conduction
losses in the device, and the second term, the switching
loss. C
usually specifi ed in the MOSFET characteristics.
For maximum effi ciency, R
minimized. From a known power dissipated in the power
DC
P
+ 2 • (V
I
R
χ
χ
L RMS
FET
rating higher than the sum of the output voltage and
L
L
SENSE
2
1
(
2
RSS
=
=
= I
I
I
L MAX
L MAX
IN(MIN)
)
2
1
=
Δ
2
SW(MAX)
is the reverse transfer capacitance, which is
=
Δ
(
(
I
I
I
I
L
SW PEAK
L
L MAX
80
1
2
2
(
)
(
)
+ V
mV
• R
OUT
)
)
•
DS(ON)
)
1
SENSE(PEAK)
2
+
• I
DS(ON)
χ
L(MAX)
12
• D
2
L
2
MAX
and C
• C
to be the minimum
RSS
RSS
LT3758
• f/1A
should be
SENSE
23
3758f
) is