LT3759 Datasheet, PDF (21/32 Pages) Linear Technology – Wide Input Voltage Range

LT3759
APPLICATIONS INFORMATION
The inductor ripple currents ΔI_L1 and ΔI_L2 are identical:
DI_L1= DI_L2 = 0.5 • DI_SW
I_SW
I_SW(MAX)
DT_S
)I_SW = ^HzI_SW(MAX)
T_S
t
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Figure 8. The Switch Current Waveform of a SEPIC Converter
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.
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.
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