9

LT1373

Many engineers have heard that solid tantalum capacitors

are prone to failure if they undergo high surge currents.

This is historically true and type TPS capacitors are

specially tested for surge capability, but surge ruggedness

is not a critical issue with the

output capacitor. Solid

tantalum capacitors fail during very high

turn-on surges,

which do not occur at the output of regulators. High

discharge surges, such as when the regulator output is

dead shorted, do not harm the capacitors.

Single inductor boost regulators have large RMS ripple

current in the output capacitor, which must be rated to

handle the current. The formula to calculate this is:

Output Capacitor Ripple Current (RMS)

DC

1 – DC

Input Capacitors

The input capacitor of a boost converter is less critical due

to the fact that the input current waveform is triangular,

and does not contain large squarewave currents as is

found in the output capacitor. Capacitors in the range of

10

µF to 100µF with an ESR (effective series resistance) of

0.3

Ω or less work well up to a full 1.5A switch current.

Higher ESR capacitors may be acceptable at low switch

currents. Input capacitor ripple current for boost con-

verter is:

f = 250kHz switching frequency

The input capacitor can see a very high surge current when

a battery or high capacitance source is connected “live”,

and solid tantalum capacitors can fail under this condition.

Several manufacturers have developed a line of solid

tantalum capacitors specially tested for surge capability

(AVX TPS series, for instance), but even these units may

fail if the input voltage approaches the maximum voltage

rating of the capacitor. AVX recommends derating capaci-

tor voltage by 2:1 for high surge applications. Ceramic and

aluminum electrolytic capacitors may also be used and

have a high tolerance to turn-on surges.

Ceramic Capacitors

Higher value, lower cost ceramic capacitors are now

becoming available in smaller case sizes. These are tempt-

ing for switching regulator use because of their very low

ESR. Unfortunately, the ESR is so low that it can cause

loop stability problems. Solid tantalum capacitor ESR

generates a loop “zero” at 5kHz to 50kHz that is instrumen-

tal in giving acceptable loop phase margin. Ceramic ca-

pacitors remain capacitive to beyond 300kHz and usually

resonate with their ESL before ESR becomes effective.

They are appropriate for input bypassing because of their

high ripple current ratings and tolerance of turn-on surges.

Linear Technology plans to issue a Design Note on the use

of ceramic capacitors in the near future.

Output Diode

The suggested output diode (D1) is a 1N5818 Schottky or

its Motorola equivalent, MBR130. It is rated at 1A average

forward current and 30V reverse voltage. Typical forward

voltage is 0.42V at 1A. The diode conducts current only

during switch-off time. Peak reverse voltage for boost

converters is equal to regulator output voltage. Average

forward current in normal operation is equal to output

current.

Frequency Compensation

Loop frequency compensation is performed on the output

main pole is formed by the series capacitor and the output

impedance (

≈1MΩ) of the error amplifier. The pole falls in

the range of 5Hz to 30Hz. The series resistor creates a

“zero” at 2kHz to 10kHz, which improves loop stability and

transient response. A second capacitor, typically one tenth

the size of the main compensation capacitor, is sometimes

attenuated by the output divider and multiplied by the error

APPLICATIO S I FOR ATIO