MIC2182 datasheet(15/40 Pages) MICROCHIP

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DS20006644A-page 15

MIC2182

FIGURE 4-3:

PWM-Mode Timing.

FIGURE 4-4:

Skip-Mode Timing.

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 MOSFET 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 35 mV, 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 VOUT to fall. The cycle repeats when VOUT falls

below the lower threshold, –1%.

The maximum peak inductor current in Skip Mode

depends on the skip-mode current-limit threshold and

the value of the current-sense resistor, RCS.

EQUATION 4-1:

Figure 4-5 shows the improvement in efficiency that

skip mode makes at lower output currents.

VDD

VIN + VDD

VIN

VSW

Reset

Pulse

VHSD

VLSD

ILOAD

IL1

VDD

+1%

–1%

VNOMINAL

VIN

VOUT

VSW

VLSD

VHSD

Vone-shot

VDD

IL(PKMAX)_SKIP

IL1

VOUT

IOUT

IL(PKMAX)_SKIP 35mV

RCS

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