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LTC1753

1753fa

Note that capacitor manufacturers’ ripple current ratings

are often based on only 2000 hours (three months)

lifetime at rated temperature. Further derating of the input

capacitor ripple current beyond the manufacturer’s speci-

fication is recommended to extend the useful life of the

circuit. Lower operating temperature will have the largest

effect on capacitor longevity.

The output capacitor in a buck converter sees much less

ripple current under steady-state conditions than the input

capacitor. Peak-to-peak current is equal to that in the

inductor, 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 LTC1753 can

adjust the inductor current to the new value. Output

capacitor ESR 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 will

result 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, the output capacitor is usually

chosen for ESR, not for capacitance value; a capacitor with

suitable ESR will usually have a larger capacitance value

than is needed for energy storage.

Electrolytic capacitors rated for use in switching power

supplies with specified ripple current ratings and ESR can

be used effectively in LTC1753 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 LTC1753 applications.

A common way to lower ESR and raise ripple current

capability is to parallel several capacitors. A typical LTC1753

application might exhibit 5A input ripple current. Sanyo

OS-CON part number 10SA220M (220

µF/10V) capacitors

feature 2.3A allowable ripple current at 85

°C; three in

parallel at the input (to withstand the input ripple current)

will meet the above requirements. Similarly, AVX

TPSE337M006R0100 (330

µF/6V) have a rated maximum

ESR of 0.1

Ω; seven in parallel will lower the net output

capacitor ESR to 0.014

Ω. For low cost application, Sanyo

MV-GX series of capacitors can be used with acceptable

performance. The small size, low profile Sanyo OS-CON

4SP820M comes with extremely low ESR (typically 0.008

Ω

at room temperature). This is an excellent choice for

output capacitor usage. However, due to the low ESR, it

requires attention to frequency compensation. Refer to

the Feedback Loop Compensation section for details.

Feedback Loop Compensation

The LTC1753 voltage feedback loop is compensated at the

COMP pin, attached to the output node of the internal gm

error amplifier. The feedback loop can generally be com-

pensated properly with an RC + C network from COMP to

GND as shown in Figure 7a.

Loop stability is affected by the values of the inductor,

output capacitor, output capacitor ESR, FET RDS(ON), error

amplifier transconductance and error amplifier compen-

sation network. The inductor and the output capacitor

create a double pole at the frequency:

fLC =

π√(LO)(COUT)

1753 F07a

DAC

LTC1753

SENSE

COMP

VFB

µF

ERR

Figure 7a. Compensation Pin Hook-Up

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