LT1432CS8 datasheet(18/28 Pages) LINER

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LT1432

APPLICATI

I FOR ATIO

in mind when adding an output filter is that if the filter

capacitor is small, it may allow large output perturbations

if large load transients occur. This effect should be care-

fully checked before finalizing any filter design. For more

details on output filters, consult Application Notes 19

and 44.

Output Capacitor

To avoid overheating, the output capacitor must be large

enough to handle the ripple current generated by the main

inductor. It must also have low enough effective series

resistance (ESR) to meet output ripple voltage require-

ments. RMS ripple current in the output capacitor is given

by:

VV – V

3.5V (f)(L)

RIPPLE(RMS)

OUT

()

(use maximum VIN)

For VIN = 15V, f = 60kHz, L = 50µH,

5(15 – 5)

3.5 15 60E

50E

0.32A

RIPPLE(RMS)

3–6

RMS

() 





Ripple current ratings are specified on capacitors intended

for switching applications, but the number is subject to

much manipulation. The high frequency number is greater

than the low frequency value, and theoretically one can

multiply the ripple number by significant amounts at

temperatures below the typical 85

°Cor105°Cratingpoint.

The problem is that the ripple ratings are already unreal-

istically high at the rated temperature because they are

typically based on a 2000 hour life. I assume this is an

unacceptable lifetime number, so the ripple rating must be

reduced to extend life. The net result of all this fiddling

with the numbers is generally a headache, but it is prob-

ably conservative to use the stated high frequency rating

at temperatures below 60

°C for a 105°C capacitor, and

assume that the unit will last at least 50,000 hours.

Remember to factor in

actual operating time at elevated

temperatures. Laptop computers, for instance, might be

expected to operate no more than four hours a day on

Input Supply Bypass Capacitor

The input capacitor on a step-down (buck) switching

regulator must handle switching currents with a peak-to-

peak amplitude at least equal to the output current. The

RMS value of capacitor current is approximately equal to:

V – V

RMS

OUT

1/2

()

[]

This formula has a maximum at VIN = 2VOUT, where IRMS

is equal to IOUT/2. This simple worst case condition is

commonly used for design because even significant de-

viations from VIN/2 do not offer much relief. A 2A output

(transient loads can be ignored if they last less than 30

seconds) therefore requires an input capacitor with a 1A

ripple current rating.

Don’t cheat, and read the output

capacitor section for details on ripple current! The input

capacitor may well be the largest component in the switch-

ing regulator. Spend time playing with aspect ratios of

various capacitor families and don’t hesitate to parallel

several units to achieve a low profile.

Output Voltage Ripple

Output voltage ripple is determined by the main inductor

value, switching frequency, input voltage, and the ESR

(effective series resistance) of the output capacitor. The

following formula assumes a load current high enough to

establish continuous current in the inductor.

Output Ripple Voltage = Vp-p

()( )

V – V

ESR

V (f)(L)

OUT

p-p

With VIN = 12V, ESR = 0.05Ω, f = 60kHz, and L = 50µH

5(12 – 5)(0.05)

12 60E

50E

= 48.6mV

p-p

3–6

p-p

= ()















If low output ripple voltage is a requirement, larger output

capacitors and/or inductors may not be the answer. An

output filter can be added at modest cost which will

attenuate ripple much more space-effectively than an

oversized output capacitor or inductor. The thing to keep

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