LT8301 datasheet(17/38 Pages) AD

17 / 38 page

LT3757/LT3757A

Rev. G

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APPLICATIONS INFORMATION

For the bulk C component, which also contributes 1% to

the total ripple:

COUT ≥

IO(MAX)

0.01• VOUT • f

The output capacitor in a boost regulator experiences

high RMS ripple currents, as shown in Figure 6. The RMS

ripple current rating of the output capacitor can be deter-

mined using the following equation:

IRMS(COUT) ≥IO(MAX) •

DMAX

−DMAX

Multiple capacitors are often paralleled to meet ESR

requirements. Typically, once the ESR requirement is sat-

isfied, the capacitance is adequate for filtering and has the

required RMS current rating. Additional ceramic capaci-

tors in parallel are commonly used to reduce the effect of

parasitic inductance in the output capacitor, which reduces

high frequency switching noise on the converter output.

Boost Converter: Input Capacitor Selection

The input capacitor of a boost converter is less critical

than the output capacitor, due to the fact that the inductor

is in series with the input, and the input current wave-

form is continuous. The input voltage source impedance

determines the size of the input capacitor, which is typi-

cally in the range of 10µF to 100µF. A low ESR capacitor

is recommended, although it is not as critical as for the

output capacitor.

The RMS input capacitor ripple current for a boost con-

verter is:

IRMS(CIN) = 0.3 • ∆IL

FLYBACK CONVERTER APPLICATIONS

The LT3757 can be configured as a flyback converter for

the applications where the converters have multiple out-

puts, high output voltages or isolated outputs. Figure 7

shows a simplified flyback converter.

The flyback converter has a very low parts count for mul-

tiple outputs, and with prudent selection of turns ratio,

can have high output/input voltage conversion ratios with

a desirable duty cycle. However, it has low efficiency due

to the high peak currents, high peak voltages and con-

sequent power loss. The flyback converter is commonly

used for an output power of less than 50W.

The flyback converter can be designed to operate either

in continuous or discontinuous mode. Compared to con-

tinuous mode, discontinuous mode has the advantage of

smaller transformer inductances and easy loop compen-

sation, and the disadvantage of higher peak-to-average

current and lower efficiency. In the high output voltage

applications, the flyback converters can be designed

to operate in discontinuous mode to avoid using large

transformers.

Figure 7. A Simplified Flyback Converter

RSENSE

NP:NS

VIN

CIN

CSN

VSN

SUGGESTED

RCD SNUBBER

ISW

VDS

3757 F07

GATE

GND

LT3757

SENSE

–

RSN

DSN

–

COUT

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