LT8331 datasheet(19/24 Pages) AD

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LT8334

Rev. 0

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APPLICATIONS INFORMATION

Inverting Converter: Inductor, Output Diode and Input

Capacitor Selections

The selections of the inductor, output diode and input

capacitor of an inverting converter are similar to those

of the SEPIC converter. Please refer to the corresponding

SEPIC converter sections.

Inverting Converter: Output Capacitor Selection

The inverting converter requires much smaller output

capacitors than those of the boost, flyback and SEPIC

converters for similar output ripples. Since in the inverting

converters, the inductor L2 is in series with the output,

and the ripple current flowing through the output capaci-

tors are continuous. The output ripple voltage is produced

by the ripple current of L2 flowing through the ESR and

bulk capacitance of the output capacitor (Equation 38).

ΔVOUT(P–P) = ΔIL2 • ESRCOUT +

8 • fOSC • COUT

⎛

⎝

⎜

⎞

⎠

⎟ (38)

After specifying the maximum output ripple, the user can

select the output capacitors according to Equation 38.

The ESR can be minimized by using high quality X5R or

X7R dielectric ceramic capacitors. In many applications,

ceramic capacitors are sufficient to limit the output volt-

age ripple.

The RMS ripple current rating of the output capacitor

needs to be greater than (Equation 39).

IRMS(COUT) > 0.3 • ∆IL2

(39)

Inverting Converter: Selecting the DC

Coupling Capacitor

The DC voltage rating of the DC coupling capacitor

(CDC, as shown in Figure 8) should be larger than the

maximum input voltage minus the output voltage (nega-

tive voltage), Equation 40.

VCDC > VIN(MAX) +| VOUT|

(40)

CDC has nearly a rectangular current waveform.

During the switch off-time, the current through CDC is

IIN, while approximately –IO flows during the on-time.

The RMS rating of the coupling capacitor is determined

with Equation 41.

IRMS(CDC) >IO(MAX) •

DMAX

1 − DMAX

(41)

A low ESR and ESL, X5R or X7R ceramic capacitor works

well for CDC.

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