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LTC3615

3615f

Output Capacitor COUT Selection

The selection of COUT is typically driven by the required

ESR to minimize voltage ripple and load step transients

(low-ESR ceramic capacitors are discussed in the next

section). Typically, once the ESR requirement is satisfied,

the capacitance is adequate for filtering. The output ripple

∆VOUT is determined by:

∆

ESR

OUT

≤









•

wherefSW=operatingfrequency,COUT=outputcapacitance

and ∆IL = ripple current in the inductor. The output ripple

is highest at maximum input voltage since ∆IL increases

with input voltage.

In surface mount applications, multiple capacitors may

have to be paralleled to meet the capacitance, ESR or RMS

currenthandlingrequirementoftheapplication.Aluminum

electrolytic, special polymer, ceramic and dry tantalum

capacitors are all available in surface mount packages.

Tantalum capacitors have the highest capacitance density,

but can have higher ESR and must be surge tested for

use in switching power supplies. Aluminum electrolytic

capacitors have significantly higher ESR, but can often

be used in extremely cost-sensitive applications provided

that consideration is given to ripple current ratings and

long term reliability.

Ceramic Input and Output Capacitors

Ceramic capacitors have the lowest ESR and can be cost

effective, but also have the lowest capacitance density,

high voltage and temperature coefficients, and exhibit

audible piezoelectric effects. In addition, the high-Q of

ceramic capacitors along with trace inductance can lead

to significant ringing.

Capacitors are tempting for switching regulator use

because of their very low ESR. Great care must be taken

when using only ceramic input and output capacitors.

Ceramic caps are prone to temperature effects which re-

quirethedesignertocheckloopstabilityovertheoperating

temperaturerange.Tominimizetheirlargetemperatureand

voltage coefficients, only X5R or X7R ceramic capacitors

should be used.

When a ceramic capacitor is used at the input, and the

power is being supplied through long wires, such as from

a wall adapter, a load step at the output can induce ringing

at the VIN pin. At best, this ringing can couple to the output

and be mistaken as loop instability. At worst, the ringing

at the input can be large enough to damage the part.

Since the ESR of a ceramic capacitor is so low, the input

and output capacitor must instead fulfill a charge storage

requirement.Duringaloadstep,theoutputcapacitormust

instantaneously supply the current to support the load

until the feedback loop raises the switch current enough

to support the load. The time required for the feedback

loop to respond is dependent on the compensation com-

ponents and the output capacitor size. Typically, three to

four cycles are required to respond to a load step, but only

in the first cycle does the output drop linearly. The output

droop, VDROOP, is usually about two to three times the

linear drop of the first cycle. Thus, a good place to start

is with the output capacitor size of approximately:

OUT

DROOP

≈

2 5

. •

•

∆

More capacitance may be required depending on the duty

cycle and load step requirements. In most applications,

the input capacitor is merely required to supply high

frequency bypassing, since the impedance to the supply

is very low.

applicaTions inForMaTion

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