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LTC3783

3783f

Layout section for more information on component place-

ment). Lab breadboards generally suffer from excessive

series inductance (due to inter-component wiring), and

these parasitics can make the switching waveforms look

significantly worse than they would be on a properly

designed PC board.

The output capacitor in a boost regulator experiences high

RMS ripple currents. The RMS output capacitor ripple

current is:

RMS COUT

OUT MAX

OUT

IN MIN

()

(

)

()

•

–

Note that the ripple current ratings from capacitor manu-

facturers are often based on only 2000 hours of life. This

makes it advisable to further derate the capacitor or to

choose a capacitor rated at a higher temperature than

required. Several capacitors may also be placed in parallel

to meet size or height requirements in the design.

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 hence, the input current wave-

form is continuous (see Figure 10). The input voltage

source impedance determines the size of the input capaci-

tor, which is typically in the range of 10

µF 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:

RMS CIN

IN MIN

MAX

()

.•

•

Please note that the input capacitor can see a very high

surge current when a battery is suddenly connected to the

input of the converter, and solid tantalum capacitors can

fail catastrophically under these conditions. Be sure to

specify surge-tested capacitors!

Boost Converter Design Example

The design example given here will be for the circuit shown

in Figure 1. The input voltage is 12V, and the output voltage

is 25V at a maximum load current of 0.7A (1A peak).

1. The duty cycle is:

OUT

–. –

25 0 4 12

25 0 4

2. The operating frequency is chosen to be 1MHz to

maximize the PWM dimming range. From Figure 2, the

resistor from the FREQ pin to ground is 6k.

3. An inductor ripple current of 40% of the maximum load

current is chosen, so the peak input current (which is also

the minimum saturation current) is:

IN PEAK

OUT MAX

MAX

()

•

–

.•

⎛

⎝⎜

⎞

⎠⎟

10 53

–.

The inductor ripple current is:

∆ =

−

OUT MAX

MAX

χ •. •

()

10 53

And so the inductor value is:

AMHz

µH

IN MIN

MAX

()

•

.•

•.

∆

053

4. RSENSE should be:

SENSE

SENSE MAX

IN PEAK

0 5 150

.•

()

Ω

OPERATIO

IIN

Figure 10. Inductor and Input Currents

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