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LT1370

500kHz High Efficiency 6A Switching Regulator

Linear Technology 

LT1370

UU W U

APPLICATIO S I FOR ATIO

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 10µF to

100µF with an ESR of 0.1Ω or less work well up to full 6A

switch current. Higher ESR capacitors may be acceptable

at low switch currents. Input capacitor ripple current for a

boost converter is :

IRIPPLE

=

0.3(VIN)(VOUT – VIN)

(f)(L)(VOUT)

f = 500kHz 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 tantalum capaci-

tors 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 during a high surge. AVX recommends derating

capacitor voltage by 2:1 for high surge applications.

Ceramic, OS-CON 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 instru-

mental in giving acceptable loop phase margin. Ceramic

capacitors remain capacitive to beyond 300kHz and usu-

ally 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.

Output Diode

The suggested output diode (D1) is a Motorola MBRD835L.

It is rated at 8A average forward current and 35V reverse

voltage. Typical forward voltage is 0.4V at 3A. The diode

conducts current only during switch OFF time. Peak re-

verse voltage for boost converters is equal to regulator

output voltage. Average forward current in normal opera-

tion is equal to output current.

Frequency Compensation

Loop frequency compensation is performed on the output

of the error amplifier (VC pin) with a series RC network.

The main pole is formed by the series capacitor and the

output impedance (≈500kΩ) of the error amplifier. The

pole falls in the range of 2Hz to 20Hz. The series resistor

creates a “zero” at 1kHz to 5kHz, which improves loop

stability and transient response. A second capacitor, typi-

cally one-tenth the size of the main compensation capaci-

tor, is sometimes used to reduce the switching frequency

ripple on the VC pin. VC pin ripple is caused by output

voltage ripple attenuated by the output divider and multi-

plied by the error amplifier. Without the second capacitor,

VC pin ripple is:

VC

Pin

Ripple

=

1.245(VRIPPLE)(gm)(RC)

(VOUT)

VRIPPLE = Output ripple (VP–P)

gm = Error amplifier transconductance

( ≈1500µmho)

RC = Series resistor on VC pin

VOUT = DC output voltage

To prevent irregular switching, VC pin ripple should be

kept below 50mVP–P. Worst-case VC pin ripple occurs at

maximum output load current and will also be increased if

poor quality (high ESR) output capacitors are used. The

addition of a 0.0047µF capacitor on the VC pin reduces

switching frequency ripple to only a few millivolts. A low

value for RC will also reduce VC pin ripple, but loop phase

margin may be inadequate.

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