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LT3959

3959f

Based on the preceding equations, the user should choose

the inductors having sufficient saturation and RMS cur-

rent ratings.

SEPIC Converter: Output Diode Selection

To maximize efficiency, a fast switching diode with a low

forward drop and low reverse leakage is desirable. The

average forward current in normal operation is equal to

the output current.

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:

PD = IO(MAX) • VD

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

junction temperature is:

TJ = TA + PD • RθJA

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 output and input capacitors of the

SEPIC converter are similar to those of the boost converter.

Please refer to the Boost Converter, Output Capacitor

Selection and Boost Converter, Input Capacitor Selection

sections.

SEPIC Converter: Selecting the DC Coupling Capacitor

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

as shown in Figure 1) should be larger than the maximum

input voltage:

VCDC > VIN(MAX)

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 LT3959 can be configured as a dual-inductor inverting

topology, as shown in Figure 4. The VOUT to VIN ratio is:

VOUT –VD

VIN

= –

−D

In continuous conduction mode (CCM).

APPLICATIONS INFORMATION

CDC

VIN

CIN

COUT

VOUT

3959 F04

GND

LT3959

–

Figure 4. A Simplified Inverting Converter

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)occurswhentheconverter

has the minimum input voltage:

DMAX =

VOUT –VD

VOUT –VD –VIN(MIN)

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