參數(shù)資料
型號: LTC1871IMS-1
廠商: LINEAR TECHNOLOGY CORP
元件分類: 穩(wěn)壓器
英文描述: SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PDSO10
封裝: PLASTIC, MSOP-10
文件頁數(shù): 17/36頁
文件大?。?/td> 523K
代理商: LTC1871IMS-1
LTC1871-1
24
18711fb
APPLICATIONS INFORMATION
SEPIC Converter Applications
The LTC1871-1 is also well suited to SEPIC (single-ended
primary inductance converter) converter applications. The
SEPIC converter shown in Figure 16 uses two inductors.
The advantage of the SEPIC converter is the input voltage
may be higher or lower than the output voltage, and the
output is short-circuit protected.
The rst inductor, L1, together with the main switch,
resembles a boost converter. The second inductor, L2,
together with the output diode D1, resembles a yback or
buck-boost converter. The two inductors L1 and L2 can be
independent but can also be wound on the same core since
identical voltages are applied to L1 and L2 throughout the
switching cycle. By making L1 = L2 and winding them on
the same core the input ripple is reduced along with cost
and size. All of the SEPIC applications information that
follows assumes L1 = L2 = L.
SEPIC Converter: Duty Cycle Considerations
For a SEPIC converter operating in a continuous conduction
mode (CCM), the duty cycle of the main switch is:
D
=
VO + VD
VIN + VO + VD
where VD is the forward voltage of the diode. For convert-
ers where the input voltage is close to the output voltage
the duty cycle is near 50%.
The maximum output voltage for a SEPIC converter is:
VO(MAX) = VIN + VD
() DMAX
1– DMAX
–VD
1
1– DMAX
The maximum duty cycle of the LTC1871-1 is typically
92%.
SEPIC Converter: The Peak and Average
Input Currents
The control circuit in the LTC1871-1 is measuring the input
current (either using the RDS(ON) of the power MOSFET
or by means of a sense resistor in the MOSFET source),
so the output current needs to be reected back to the
input in order to dimension the power MOSFET properly.
Based on the fact that, ideally, the output power is equal
to the input power, the maximum input current for a SEPIC
converter is:
IIN(MAX) =IO(MAX)
DMAX
1– DMAX
The peak input current is:
IIN(PEAK) = 1+
2
IO(MAX)
DMAX
1– DMAX
The maximum duty cycle, DMAX, should be calculated at
minimum VIN.
Figure 17. SEPIC Converter Switching Waveforms
17a. Input Inductor Current
IIN
IL1
SW
ON
SW
OFF
17b. Output Inductor Current
IO
IL2
17c. DC Coupling Capacitor Current
IO
IIN
IC1
17e. Output Ripple Voltage
VOUT
(AC)
ΔVESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
ΔVCOUT
17d. Diode Current
IO
ID1
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