LT1432CS8 datasheet(12/28 Pages) LINER

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LT1432

Switching Waveforms in Burst Mode

In burst mode, the LT1432 amplifier is converted to a

comparator with hysteresis. This causes its VC pin current

drive to be either zero (output low), or full “on” at about

0.8mA (output high). The LT1271 therefore is either driven

to full on condition or forced into complete micropower

shutdown. This makes a dramatic reduction in quiescent

current losses because the switching regulator chip draws

supply current only during the relatively short “on” peri-

ods. This burst mode results in a battery drain of only

1.2mA with zero output load, even though the nominal

quiescent current of the switcher chip is 7mA. This low

battery drain is accomplished at the expense of higher

output ripple voltage, but the ripple is still well within the

normal requirements for logic chips.

Figure 11 shows burst mode output ripple at load currents

of 0 (top trace), and 50mA (bottom trace). Ripple ampli-

tude is nominally set by the 100mV hysteresis built into the

LT1432, but in most applications, other effects come into

play which can significantly modify this value. The first is

delay in turning off the switcher. This causes the output to

overshoot slightly and therefore increases output ripple.

Delay is caused by the compensation capacitors used to

maintain a stable loop in the normal mode. Another effect,

however, is the ESR of the output capacitor. The surge

current from the switcher creates a step across the capaci-

tor ESR which prematurely trips the LT1432 comparator,

reducing ripple amplitude. A second delay occurs in

turning the switcher back on when the output falls below

its lower level. This delay is somewhat longer, but because

the output normally falls at a much slower rate than it rises,

this delay is not significant until output current exceeds

10mA. Falling rate is set by the output capacitor (including

any secondary filter capacitor), and the actual load cur-

rent, dVOUT/dt = IOUT/COUT. The slope in the top traces

implies a load current of approximately 2mA. This is the

sum of the 1mA output quiescent current of the LT1432

and the 1mA drawn by the VC pin and shunted through the

internal Schottky diode during the switcher “off” period.

The bottom trace at IOUT = 50mA shows increased ripple

caused by turn-on delay. Note that ripple frequency has

increased from 50Hz to about 600Hz and amplitude has

APPLICATI

I FOR ATIO

100mV/DIV

5ms/DIV

Figure 11. Burst Mode Output Ripple Voltage

100mV/DIV

5ms/DIV

more than doubled. Figure 12 shows the same conditions

except that a 47k

Ω resistor is connected from the LT1271

VIN pin to the VC pin to provide more start-up current.

These additions reduce ripple amplitude at 50mA load

current to a value only slightly higher than the no-load

condition.

Although it is difficult to see in Figures 11 and 12, there is

a narrow spike on the leading edge of the ripple caused by

the burst current and capacitor ESR. Figure 13 shows this

spike in more detail, both with and without an output filter.

Figure 12. Burst Mode Output Ripple Voltage

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