LT8331 datasheet(18/24 Pages) AD

18 / 24 page

LT8334

Rev. 0

For more information www.analog.com

It is recommended that the peak repetitive reverse voltage

rating VRRM is higher than VOUT + VIN(MAX) by a safety

margin (a 10V safety margin is usually sufficient). The

power dissipated by the diode is given by Equation 30.

PD = IO(MAX) • VD

(30)

where VD is diode’s forward voltage drop, and the diode

junction temperature given by Equation 31.

TJ = TA + PD • RθJA

(31)

The RθJA used in this equation normally includes the RθJC

for the device, plus the thermal resistance from the board

to the ambient temperature in the enclosure. TJ must not

exceed the diode maximum junction temperature rating.

SEPIC Converter: Output and Input Capacitor Selection

The selections of the inductor, output diode and input

capacitor of an inverting converter are like those of the

SEPIC converter. Please refer to the corresponding SEPIC

converter sections.

SEPIC Converter: Selecting the DC Coupling Capacitor

The DC voltage rating of the DC coupling capacitor (CDC,

as shown in Figure 6) should be larger than the maximum

input voltage (Equation 32).

VCDC > VIN(MAX)

(32)

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 33.

IRMS(CDC) > ID(MAX) •

VOUT + VD

VIN(MIN)

(33)

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

well for CDC.

APPLICATIONS INFORMATION

INVERTING CONVERTER APPLICATIONS

The LT8334 can be configured as a dual-inductor invert-

ing topology, as shown in Figure 8. The VOUT to VIN ratio

is given by Equation 34.

⎜VOUT⎜ − VD

VIN

− D

(34)

in continuous conduction mode (CCM)

CDC

CIN

COUT

VOUT

GND

LT8334

VIN

8334 F08

–

Figure 8. A Simplified Inverting Converter

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 (Equation 35).

DMAX =

⎜VOUT⎜ − VD

⎜VOUT⎜ − VD − VIN(MIN)

(35)

Conversely, the minimum duty cycle (DMIN) occurs when

the converter operates at the maximum input voltage

(Equation 36).

DMIN =

⎜VOUT⎜ + VD

⎜VOUT⎜ + VD + VIN(MAX)

(36)

Be sure to check that DMAX and DMIN obey Equation 37.

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

and

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

(37)

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

are specified in the Electrical Characteristics table.

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