LTC1530CS8 datasheet(14/24 Pages) LINER

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LTC1530

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

LTC1530 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 11A load step with a 0.05

Ω ESR output

capacitor results in a 550mV output voltage shift; this is

19.6% of the output voltage for a 2.8V 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 LTC1530 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. AVX TPS series surface mount

devices are popular surge tested tantalum capacitors that

work well in LTC1530 applications.

A common way to lower ESR and raise ripple current

capability is to parallel several capacitors. A typical LTC1530

application might exhibit 5A input ripple current. Sanyo

OS-CON capacitors, part number 10SA220M (220

µF/

10V), feature 2.3A allowable ripple current at 85

°C; three

in parallel at the input (to withstand the input ripple

current) meet the above requirements. Similarly, AVX

TPSE337M006R0100 (330

µF/6V) capacitors have a rated

maximum ESR of 0.1

Ω; seven in parallel lower the net

output capacitor ESR to 0.014

Ω. For low cost applica-

tions, the Sanyo MV-GX capacitor series can be used with

acceptable performance.

Feedback Loop Compensation

The LTC1530 voltage feedback loop is compensated at the

COMP pin, which is the output node of the gm error

amplifier. The feedback loop is generally compensated

with an RC + C network from COMP to GND as shown in

Figure 8a.

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:

L C

OUT

()

The ESR of the output capacitor and the output capacitor

value form a zero at the frequency:

ESR C

ESR

OUT

= ()( )( )

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:

and f

= ()()( )

ππ

respectively. Figure 8b shows the Bode plot of the overall

transfer function.

The compensation values used in this design are based on

the following criteria, fSW = 12fCO, fZ = fLC, fP = 5fCO. At the

closed-loop frequency fCO, the attenuation due to the LC

filter and the input resistor divider is compensated by the

gain of the PWM modulator and the gain of the error

amplifier (gmERR)(RC).

APPLICATIO S I FOR ATIO

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