LTC3830ES8 datasheet(16/24 Pages) LINER

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LTC3830/LTC3830-1

sn3830 3830fs

The output capacitor in a buck converter under steady-

state conditions sees much less ripple current than the

input capacitor. Peak-to-peak current is equal to inductor

ripple current, usually 10% to 40% of the total load

current. Output capacitor duty places a premium not on

power dissipation but on ESR. During an output load

transient, the output capacitor must supply all of the

additional load current demanded by the load until the

LTC3830 adjusts the inductor current to the new value.

ESR in the output capacitor results in a step in the output

voltage equal to the ESR value multiplied by the change in

load current. An 5A load step with a 0.05

Ω ESR output

capacitor results in a 250mV output voltage shift; this is

7.6% of the output voltage for a 3.3V supply! Because of

the strong relationship between output capacitor ESR and

output load transient response, choose the output capaci-

tor for ESR, not for capacitance value. A capacitor with

suitable ESR will usually have a larger capacitance value

than is needed to control steady-state output ripple.

Electrolytic capacitors rated for use in switching power

supplies with specified ripple current ratings and ESR can

be used effectively in LTC3830 applications. OS-CON

electrolytic capacitors from Sanyo and other manufactur-

ers give excellent performance and have a very high

performance/size ratio for electrolytic capacitors. Surface

mount applications can use either electrolytic or dry

tantalum capacitors. Tantalum capacitors must be surge

tested and specified for use in switching power supplies.

Low cost, generic tantalums are known to have very short

lives followed by explosive deaths in switching power

supply applications. Other capacitors that can be used

include the Sanyo POSCAP and MV-WX series.

A common way to lower ESR and raise ripple current

capability is to parallel several capacitors. A typical

LTC3830 application might exhibit 5A input ripple cur-

rent. Sanyo OS-CON capacitors, part number 10SA220M

(220

µF/10V), feature 2.3A allowable ripple current at

°C; three in parallel at the input (to withstand the input

ripple current) meet the above requirements. Similarly,

Sanyo POSCAP 4TPB470M (470

µF/4V) capacitors have

a maximum rated ESR of 0.04

Ω; three in parallel lower

the net output capacitor ESR to 0.013

Ω.

Feedback Loop Compensation

The LTC3830 voltage feedback loop is compensated at the

COMP pin, which is the output node of the error amplifier.

The feedback loop is generally compensated with an RC +

C network from COMP to GND as shown in Figure 10a.

Loop stability is affected by the values of the inductor, the

output capacitor, the output capacitor ESR, the error

amplifier transconductance and the error amplifier com-

pensation network. The inductor and the output capacitor

create a double pole at the frequency:

OUT

=π

[]

(

)(

)

The ESR of the output capacitor and the output capacitor

value form a zero at the frequency:

ESR C

ESR

OUT

=π

[]

(

)(

)

The compensation network used with the error amplifier

must provide enough phase margin at the 0dB crossover

frequency for the overall open-loop transfer function. The

zero and pole from the compensation network are:

fZ = 1/[2π(RC)(CC)] and

fP = 1/[2π(RC)(C1)] respectively

APPLICATIO S I FOR ATIO

3830 F10a

LTC3830

VREF

SENSE–

SENSE+

VFB

COMP

ERR

Figure 10a. Compensation Pin Hook-Up

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