MIC2182 datasheet(12/28 Pages) MICREL

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MIC2182

Micrel

MIC2182

June 2000

A hysteretic comparator is used in place of the PWM error

amplifier and a current-limit comparator senses the inductor

current. A one-shot starts the switching cycle by momentarily

turning on the low side MOSFET to insure the high side drive

boost capacitor, Cbst, is fully charged. The high side MOS-

FET is turned on and current ramps up in the inductor, L1.

The high side drive is turned off when either the peak voltage

on the input of the current-sense comparator exceeds the

threshold, typically 35mV, or the output voltage rises above

the hysteretic threshold of the output voltage comparator.

Once the high side MOSFET is turned off, the load current

discharges the output capacitor, causing V

OUT to fall. The

cycle repeats when V

OUT falls below the lower threshold, –

1%.

The maximum peak inductor current depends on the skip-

mode current-limit threshold and the value of the current-

sense resistor, R

CS.

35mV

inductor(peak)

sense

Figure 6 shows the improvement in efficiency that skip mode

makes when at lower output currents.

100

0.01

0.1

100

OUTPUT CURRENT (A)

Skip

PWM

Figure 6. Efficiency

VDD

VIN + VDD

VIN

VSW

Reset

Pulse

VHSD

VLSD

ILOAD

IL1

Figure 5a. PWM-Mode Timing

VDD

+1%

–1%

VNOMINAL

VIN

VOUT

VSW

VLSD

VHSD

Vone-shot

VDD

ILIM(skip)

IL1

VOUT

IOUT

Figure 5b. Skip-Mode Timing

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