LT8331 datasheet(21/30 Pages) AD

21 / 30 page

LT8331

Rev. C

For more information www.analog.com

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) •

VOUT + VD

VIN(MIN)

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

well for CDC.

INVERTING CONVERTER APPLICATIONS

The LT8331 can be configured as a dual-inductor invert-

ing topology, as shown in Figure 10. The VOUT to VIN

ratio is:

VOUT+ VD

VIN

1– D

Conversely, the minimum duty cycle (DMIN) occurs when

the converter operates at the maximum input voltage :

DMIN =

VOUT + VD

VOUT + VD+ VIN(MAX)

Be sure to check that DMAX and DMIN obey :

DMAX < 1 – Minimum Off-Time(MAX) • fOSC(MAX)

and

DMIN > Minimum On-Time(MAX) • fOSC(MAX)

where Minimum Off-Time, Minimum On-Time and fOSC

are specified in the Electrical Characteristics table.

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. 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.

Figure 10. A Simplified Inverting Converter

CDC

VIN

CIN

COUT

VOUT

8331 F10

GND

LT8331

–

+ –

APPLICATIONS INFORMATION

in continuous conduction mode (CCM).

Inverting Converter: Switch Duty Cycle and Frequency

For an inverting converter operating in CCM, the duty

cycle of the main switch can be calculated based on the

negative output voltage (VOUT) and the input voltage (VIN).

The maximum duty cycle (DMAX) occurs when the con-

verter has the minimum input voltage:

DMAX =

VOUT + VD

VOUT + VD + VIN(MIN)

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