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LT3758

3758f

APPLICATIONS INFORMATION

MOSFET, its junction temperature can be obtained using

the following equation:

TJ = TA + PFET •

θJA = TA + PFET • (θJC + θCA)

TJ must not exceed the MOSFET maximum junction

temperature rating. It is recommended to measure the

MOSFET temperature in steady state to ensure that absolute

maximum ratings are not exceeded.

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, and the peak current is equal to:

DPEAK

O MAX

MAX

()

(

)

••

⎛

⎝⎜

⎞

⎠⎟

−

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

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:

RMS CDC

O MAX

OUT

IN MIN

()

(

)

()

•

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

well for CDC.

INVERTING CONVERTER APPLICATIONS

The LT3758 can be conﬁgured as a dual-inductor invert-

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

ratio is:

OUT

−

=−

−

in continuous conduction mode (CCM).

Figure 10. A Simpliﬁed Inverting Converter

RSENSE

CDC

VIN

CIN

COUT

VOUT

3758 F10

GATE

GND

LT3758

SENSE

–

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:

MAX

OUT

IN MIN

−

−−

()

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