LTC1624CS datasheet(17/28 Pages) LINER

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LTC1624

µH inductance is obtained with a current rating of 4A.

Splitting the two windings creates two 10

µH inductors

with a current rating of 2A each. Therefore substitute

(2)(10

µH) = 20µH for L in the equations.

Specify the maximum inductor current to safely handle

IL(PEAK). Make sure the inductor’s saturation current rat-

ing (current when inductance begins to fall) exceeds the

maximum current rating set by RSENSE.

SEPIC Converter: RSENSE Selection for Maximum

Output Current

RSENSE is chosen based on the required output current.

Remember the LTC1624 current comparator has a maxi-

mum threshold of 160mV/RSENSE. The current compara-

tor threshold sets the peak of the inductor current, yielding

a maximum average output current IOUT(MAX) equal to

IL1(PEAK) less half the peak-to-peak ripple current, ∆IL,

divided by the output-input voltage ratio (see equation for

IL1(PEAK)).

Allowing a margin for variations in the LTC1624 (without

considering variation in RSENSE), assuming 30% ripple

current in the inductor, yields:

SENSE

OUT MAX

IN MIN

OUT













()

100

SEPIC Converter: Output Diode

The output diode conducts current only during the switch

off-time. Peak reverse voltage for SEPIC converters is

equal to VOUT + VIN. Average forward current in normal

operation is equal to output current. Peak current is:

D PEAK

OUT MAX

OUT

IN MIN

()

( )

()













+∆

Schottky diodes such as MBR130LT3 are recommended.

SEPIC Converter: Input and Output Capacitors

The output capacitor is normally chosen by its effective

series resistance (ESR), because this is what determines

APPLICATIONS INFORMATION

output ripple voltage. The input capacitor needs to be sized

to handle the ripple current safely.

Since the output capacitor’s ESR affects efficiency, use

low ESR capacitors for best performance. SEPIC regula-

tors, like step-down regulators, have a triangular current

waveform but have maximum ripple at VIN(MAX). The input

capacitor ripple current is:

RIPPLE RMS

()=

∆

The output capacitor ripple current is:

RIPPLE RMS

OUT

()=

The output capacitor ripple voltage (RMS) is:

VOUT(RIPPLE) = 2(∆IL)(ESR)

The input capacitor can see a very high surge current when

a battery is suddenly connected, and solid tantalum

capacitors can fail under this condition. Be sure to specify

surge tested capacitors.

SEPIC Converter: Coupling Capacitor (C1)

The coupling capacitor C1 in Figure 7 sees a nearly

rectangular current waveform. During the off-time the

current through C1 is IOUT(VOUT/VIN) while approximately

–IOUT flows though C1 during the on-time. This current

waveform creates a triangular ripple voltage on C1:

∆V

kHz C

OUT

200

= ()( )





















The maximum voltage on C1 is then:

VC1(MAX) = VIN + ∆VC1/2 (typically close to VIN(MAX)).

The ripple current though C1 is:

RIPPLE C

OUT

()=

The maximum ripple current occurs at IOUT(MAX) and

VIN(MIN). The capacitance of C1 should be large enough so

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