Datasheet

LT3759
21
3759fc
For more information www.linear.com/3759
The inductor ripple currents ΔI
L1
and ΔI
L2
are identical:
DI
L1
= DI
L2
= 0.5 • D I
SW
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)
DI
SW
• f
OSC
• D
MAX
=
R
SENSE
• V
IN(MIN)
0.01V • f
OSC
• D
MAX
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 minimum of
the SENSE current limit threshold with a 20% margin. The
sense resistor value can then be calculated to be:
R
SENSE
=
40mV
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
+ (V
IN(MIN)
+ V
OUT
)
2
• I
SW(MAX)
• C
RSS
•
f
OSC
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.
Figure 8. The Switch Current Waveform of a SEPIC Converter
3759 F08
∆I
SW
=
χ
• I
SW(MAX)
I
SW
t
DT
S
I
SW(MAX)
T
S
APPLICATIONS INFORMATION
The inductor ripple current has a direct effect on the
choice of the inductor value. Choosing smaller values of
ΔI
L
requires large inductances and reduces the current
loop gain (the converter will approach voltage mode).
Accepting larger values of ΔI
L
allows the use of low in-
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 0.4.
Choose an inductor value based on operating frequency,
input and output voltage to provide a current mode ramp
on SENSE during the switch on-time of approximately
10mV magnitude. 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 • DI
SW
• f
OSC
• D
MAX
=
R
SENSE
• V
IN(MIN)
0.5 • 0.01V • f
OSC
• D
MAX
For most SEPIC applications, the equal inductor values
will fall in the range of 1µH to 100µH.