LT1432CS8 datasheet(9/28 Pages) LINER

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LT1432

APPLICATI

I FOR ATIO

1A/DIV

5V/DIV

1A/DIV

5V/DIV

0.5A/DIV

Figure 5

5V/DIV

often allow the power-down sequence to consist of simul-

taneous turn-off of load current and switch to burst mode.

Power-up is accomplished by switching to normal mode

and simultaneously increasing load current to the lowest

possible value (30mA to 500mA), followed by a short

pause and return to full load current.

Full Shutdown

When the mode pin is driven high, full shutdown of the

regulator occurs. Regulator input current will then consist

of the LT1432 shutdown current (

≈15µA) plus the switch

leakage of the switching IC (

≈1µA to 25µA). Mode input

current (

≈15µA at 5V) must also be considered. Startup

from shutdown can be in either normal or burst mode, but

one should always check startup overshoot, especially if

the output capacitor or frequency compensation compo-

nents have been changed.

Switching Waveforms in Normal Mode

The waveforms in Figures 3 through 10 were taken with

an input voltage of 12V. Figure 3 shows the classic buck

converter waveforms of switch output voltage (5V/DIV) at

the top and switch current (1A/DIV) underneath, at an

output current of 2A. The regulator is operating in “con-

tinuous” mode as evidenced by the fact that switch

current does not start at zero at switch turn-on. Instead,

it jumps to an initial value, then continues to slope upward

during the duration of switch on time. The slope of the

current waveform is determined by the difference be-

tween input and output voltage, and the value of inductor

used.

V– V

OUT

= ()

According to theory, the average switch current during

switch on time should be equal to the 2A output current

and this is confirmed in the photograph. The peak switch

current, however, is about 2.4A.This peak current must

be considered when calculating maximum available load

current because both the LT1432 and the LT1070 family

current limit on instantaneous switch current.

µs/DIV

Figure 4

Figure 3

µs/DIV

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