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LT8603

8603f

For more information www.linear.com/LT8603

APPLICATIONS INFORMATION

For a coupled inductor:

IL(PEAK) = 1+











 •IOUT(MAX) • 1+

VOUT + VD

VBATT(MIN)













For uncoupled inductors:

IL1(PEAK) = 1+











 •IOUTMAX •

OUT

+ VD

VBATT(MIN)

IL2(PEAK) = 1+











 •IOUT(MAX) •

VBATT(MIN) + VD

VBATT(MIN)

RSENSE can then be calculated by:

RSENSE =

0.04

IL1(PEAK)

The chosen inductor(s) should be rated for the maximum

expected current.

SEPIC: MOSFET Selection

For the SEPIC configuration, the selected MOSFET should

have a BVDSS rating exceeding, with margin, the maxi-

mum input voltage plus the maximum output voltage plus

the maximum diode forward voltage. The maximum input

voltage should include careful consideration of possible

transient conditions. In addition, the chosen MOSFET

should be compatible with the LT8603’s nominal gate

drive of 4.6V and have a low value of RDS(ON) for best

efficiency. The current rating for the MOSFET must be

greater than the peak switch current of:

ISW(PEAK) = 1+











 •IOUT(MAX) •

1– DMAX

Finally, the maximum gate drive current required by the

external MOSFET should not exceed the 40mA capability

of the LT8603. The current drawn by the gate driver is

given by:

IDRIVE = Qg • fSW

Figure 8. SEPIC Converter Switching Waveforms

8a. Input Inductor Current

IIN

IL1

OFF

8b. Output Inductor Current

IL2

8c. DC Coupling Capacitor Current

IIN

IC1

8e. Output Ripple Voltage

VOUT

(AC)

ΔVESR

RINGING DUE TO

TOTAL INDUCTANCE

(BOARD + CAP)

ΔVCOUT

8d. Diode Current

ID1

8603 F08

SEPIC: Diode Selection

For the rectifier diode, SD1, the peak reverse voltage that

the diode must withstand is:

VBATT(MAX) + VOUT

The average forward current is equal to the output current

and the peak current is given by:

ID(PEAK) = 1+











 •IOUT(MAX) •

VOUT + VD

VBATT(MIN)

+ 1













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