LTC3822 datasheet(20/32 Pages) LINER

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LTC3852

3852f

COUT Selection

The selection of COUT is primarily determined by the effec-

tive series resistance, ESR, to minimize voltage ripple. The

output ripple,

DVOUT,incontinuousmodeisdeterminedby:

ΔV

OUT ≅ΔIL ESR +

8fC

OUT

⎛

⎝⎜

⎞

⎠⎟

where f = operating frequency, COUT = output capacitance

and

DIL = ripple current in the inductor. The output ripple

is highest at maximum input voltage since

DIL increases

with input voltage. Typically, once the ESR requirement

for COUT has been met, the RMS current rating gener-

ally far exceeds the IRIPPLE(P-P) requirement. With

DIL =

0.3IOUT(MAX) and allowing 2/3 of the ripple to be due to

ESR, the output ripple will be less than 50mV at maximum

VIN and:

OUT Required ESR < 2.2RSENSE

OUT >

8fR

SENSE

The ﬁrst condition relates to the ripple current into the ESR

of the output capacitance while the second term guarantees

that the output capacitance does not signiﬁcantly discharge

during the operating frequency period due to ripple current.

The choice of using smaller output capacitance increases

the ripple voltage due to the discharging term but can be

compensated for by using capacitors of very low ESR to

maintain the ripple voltage at or below 50mV. The ITH pin

OPTI-LOOP compensation components can be optimized

to provide stable, high performance transient response

regardless of the output capacitors selected.

The selection of output capacitors for applications with

large load current transients is primarily determined by the

voltage tolerance speciﬁcations of the load. The resistive

component of the capacitor, ESR, multiplied by the load

current change, plus any output voltage ripple must be

within the voltage tolerance of the load.

The required ESR due to a load current step is:

ESR ≤

ΔV

ΔI

APPLICATIONS INFORMATION

where

DIisthechangeincurrentfromfullloadtozeroload

(or minimum load) and

DV is the allowed voltage devia-

tion (not including any droop due to ﬁnite capacitance).

The amount of capacitance needed is determined by the

maximum energy stored in the inductor. The capacitance

must be sufﬁcient to absorb the change in inductor

current when a high current to low current transition

occurs. The opposite load current transition is generally

determined by the control loop OPTI-LOOP components,

so make sure not to over compensate and slow down

the response. The minimum capacitance to assure the

inductors’ energy is adequately absorbed is:

OUT >

ΔI

()2

ΔV

()V

OUT

where

DI is the change in load current.

Manufacturers such as Nichicon, United Chemi-Con and

Sanyo can be considered for high performance through-

hole capacitors. The OS-CON semiconductor electrolyte

capacitor available from Sanyo has the lowest (ESR)(size)

product of any aluminum electrolytic at a somewhat

higher price. An additional ceramic capacitor in parallel

with OS-CON capacitors is recommended to reduce the

inductance effects.

In surface mount applications, ESR, RMS current handling

and load step speciﬁcations may require multiple capacitors

in parallel. Aluminum electrolytic, dry tantalum and

special polymer capacitors are available in surface mount

packages. Special polymer surface mount capacitors offer

very low ESR but have much lower capacitive density per

unit volume than other capacitor types. These capacitors

offer a very cost-effective output capacitor solution and are

an ideal choice when combined with a controller having

high loop bandwidth. Tantalum capacitors offer the highest

capacitance density and are often used as output capacitors

for switching regulators having controlled soft-start.

Several excellent surge-tested choices are the AVX TPS,

AVX TPSV or the KEMET T510 series of surface mount

tantalums, available in case heights ranging from 1.5mm

to 4.1mm. Aluminum electrolytic capacitors can be used

in cost-driven applications, provided that consideration

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