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LT8362

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APPLICATIONS INFORMATION

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. This is due to the

fact that, in the inverting converter, the inductor L2 is

in series with the output, and the ripple current flowing

through the output capacitors are continuous. The output

ripple voltage is produced by the ripple current of L2 flow-

ing through the ESR and bulk capacitance of the output

capacitor:

ΔVOUT(P–P) = ΔIL2 • ESRCOUT +

8 • fOSC • COUT

⎛

⎝

⎜

⎞

⎠

⎟

After specifying the maximum output ripple, the user can

select the output capacitors according to the preceding

equation.

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:

IRMS(COUT) > 0.3 • ∆IL2

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 (negative voltage):

VCDC > VIN(MAX) +| VOUT|

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 by the fol-

lowing equation:

IRMS(CDC) >IO(MAX) •

DMAX

1 − DMAX

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

well for CDC.

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