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LTC1871

1871fe

APPLICATIONS INFORMATION

Remember to keep the diode lead lengths short and to

observe proper switch-node layout (see Board Layout

Checklist) to avoid excessive ringing and increased dis-

sipation.

Boost Converter: Output Capacitor Selection

Contributions of ESR (equivalent series resistance), ESL

(equivalent series inductance) and the bulk capacitance

must be considered when choosing the correct component

for a given output ripple voltage. The effects of these three

parameters (ESR, ESL and bulk C) on the output voltage

ripple waveform are illustrated in Figure 12e for a typical

boost converter.

The choice of component(s) begins with the maximum

acceptable ripple voltage (expressed as a percentage of

the output voltage), and how this ripple should be divided

between the ESR step and the charging/discharging ΔV.

For the purpose of simplicity we will choose 2% for the

maximum output ripple, to be divided equally between the

ESR step and the charging/discharging ΔV. This percent-

age ripple will change, depending on the requirements

of the application, and the equations provided below can

easily be modiﬁed.

For a 1% contribution to the total ripple voltage, the ESR

of the output capacitor can be determined using the fol-

lowing equation:

ESRCOUT

0.01• VO

IIN(PEAK)

where:

IIN(PEAK)= 1+

•

IO(MAX)

1– DMAX

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

the total ripple:

COUT

IO(MAX)

0.01• VO • f

For many designs it is possible to choose a single capacitor

type that satisﬁes both the ESR and bulk C requirements

for the design. In certain demanding applications, however,

the ripple voltage can be improved signiﬁcantly by con-

necting two or more types of capacitors in parallel. For

example, using a low ESR ceramic capacitor can minimize

the ESR step, while an electrolytic capacitor can be used

to supply the required bulk C.

Once the output capacitor ESR and bulk capacitance have

been determined, the overall ripple voltage waveform

should be veriﬁed on a dedicated PC board (see Board

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, as shown in Figure 12. The RMS

output capacitor ripple current is:

IRMS(COUT) IO(MAX) •

VO –VIN(MIN)

VIN(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.

Manufacturers such as Nichicon, United Chemicon and

Sanyo should be considered for high performance through-

hole capacitors. The OS-CON semiconductor dielectric

capacitor available from Sanyo has the lowest product of

ESR and size of any aluminum electrolytic, at a somewhat

higher price.

In surface mount applications, multiple capacitors may

have to be placed in parallel in order to meet the ESR or

RMS current handling requirements of the application.

Aluminum electrolytic and dry tantalum capacitors are

both available in surface mount packages. In the case of

tantalum, it is critical that the capacitors have been surge

tested for use in switching power supplies. An excellent

choice is AVX TPS series of surface mount tantalum. Also,

ceramic capacitors are now available with extremely low

ESR, ESL and high ripple current ratings.

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