LT3758 datasheet(22/36 Pages) LINER

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LT3758

3758f

APPLICATIONS INFORMATION

Flyback Converter: Input Capacitor Selection

The input capacitor in a ﬂyback converter is subject to

a large RMS current due to the discontinuous primary

current. To prevent large voltage transients, use a low

ESR input capacitor sized for the maximum RMS current.

The RMS ripple current rating of the input capacitors in

discontinuous operation can be determined using the

following equation:

RMS CIN DISCONTINUOUS

OUT MAX

IN MIN

(),

()

() •

≥

••

(•

)

•

−

MAX

SEPIC CONVERTER APPLICATIONS

The LT3758 can be conﬁgured as a SEPIC (single-ended

primary inductance converter), as shown in Figure 1. This

topology allows for the input to be higher, equal, or lower

than the desired output voltage. The conversion ratio as

a function of duty cycle is:

OUT

−

in continuous conduction mode (CCM).

In a SEPIC converter, no DC path exists between the input

and output. This is an advantage over the boost converter

for applications requiring the output to be disconnected

from the input source when the circuit is in shutdown.

Compared to the ﬂyback converter, the SEPIC converter

has the advantage that both the power MOSFET and the

output diode voltages are clamped by the capacitors (CIN,

CDC and COUT), therefore, there is less voltage ringing

across the power MOSFET and the output diodes. The

SEPIC converter requires much smaller input capacitors

than those of the ﬂyback converter. This is due to the fact

that, in the SEPIC converter, the inductor L1 is in series

with the input, and the ripple current ﬂowing through the

input capacitor is continuous.

SEPIC Converter: Switch Duty Cycle and Frequency

For a SEPIC converter operating in CCM, the duty cycle

of the main switch can be calculated based on the output

voltage (VOUT), the input voltage (VIN) and the diode

forward voltage (VD).

The maximum duty cycle (DMAX)occurswhentheconverter

has the minimum input voltage:

MAX

OUT

IN MIN

OUT

()

SEPIC Converter: Inductor and Sense Resistor Selection

As shown in Figure 1, the SEPIC converter contains two

inductors: L1 and L2. L1 and L2 can be independent, but

can also be wound on the same core, since identical volt-

ages are applied to L1 and L2 throughout the switching

cycle.

For the SEPIC topology, the current through L1 is the

converter input current. Based on the fact that, ideally, the

output power is equal to the input power, the maximum

average inductor currents of L1 and L2 are:

L MAX

IN MAX

O MAX

MAX

LMAX

()

(

•

−

))(

)

OMAX

In a SEPIC converter, the switch current is equal to IL1 +

IL2 when the power switch is on, therefore, the maximum

average switch current is deﬁned as:

SW MAX

L MAX

O MAX

MAX

()

() •

−

and the peak switch current is:

SW PEAK

O MAX

MAX

()

(

)

••

⎛

⎝⎜

⎞

⎠⎟

−

The constant χ in the preceding equations represents the

percentage peak-to-peak ripple current in the switch, rela-

tive to ISW(MAX), as shown in Figure 9. Then, the switch

ripple current ΔISW can be calculated by:

ΔISW = χ • ISW(MAX)

The inductor ripple currents ΔIL1 and ΔIL2 are identical:

ΔIL1 = ΔIL2 = 0.5 • ΔISW

The inductor ripple current has a direct effect on the

choice of the inductor value. Choosing smaller values of

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