MIC28303 datasheet(22/38 Pages) MICROCHIP

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MIC28303

DS20005464A-page 22

 2016 Microchip Technology Inc.

charge the bootstrap capacitor (CBST) because the

feedback voltage is still below VREF. Then, the next

ON-time period is triggered due to the low feedback

voltage. Therefore, the switching frequency changes

during the load transient, but returns to the nominal

fixed frequency once the output has stabilized at the

new load current level. With the varying duty cycle and

switching frequency, the output recovery time is fast

and the output voltage deviation is small.

FIGURE 4-2:

MIC28303 Load Transient

Response

Unlike true current-mode control, the MIC28303 uses

the output voltage ripple to trigger an ON-time period.

The output voltage ripple is proportional to the inductor

current ripple if the ESR of the output capacitor is large

enough.

In order to meet the stability requirements, the

MIC28303 feedback voltage ripple should be in phase

with the inductor current ripple and are large enough to

be sensed by the gm amplifier and the error

comparator. The recommended feedback voltage

ripple is 20 mV ~ 100 mV over the full input voltage

range. If a low ESR output capacitor is selected, then

the feedback voltage ripple may be too small to be

sensed by the gm amplifier and the error comparator.

Also, the output voltage ripple and the feedback

voltage ripple are not necessarily in phase with the

inductor current ripple if the ESR of the output capacitor

is very low. In these cases, ripple injection is required

ensure

proper

operation.

Please

refer

“Section 5.6, Ripple Injection” for more details about

the ripple injection technique.

4.2

Discontinuous Mode (MIC28303-1

Only)

In continuous mode, the inductor current is always

greater than zero; however, at light loads, the

MIC28303-1 is able to force the inductor current to

operate in discontinuous mode. Discontinuous mode is

where the inductor current falls to zero, as indicated by

trace (IL) shown in Figure 4-3. During this period, the

efficiency is optimized by shutting down all the

non-essential circuits and minimizing the supply

current. The MIC28303-1 wakes up and turns on the

high-side MOSFET when the feedback voltage VFB

drops below 0.8V.

The MIC28303-1 has a zero crossing comparator (ZC)

that monitors the inductor current by sensing the

voltage drop across the low-side MOSFET during its

ON-time. If the VFB > 0.8V and the inductor current

goes

slightly

negative,

then

the

MIC28303-1

automatically powers down most of the IC’s circuitry

and goes into a low-power mode.

Once the MIC28303-1 goes into discontinuous mode,

both DL and DH are low, which turns off the high-side

and low-side MOSFETs. The load current is supplied

by the output capacitors and VOUT drops. If the drop of

VOUT causes VFB to go below VREF, then all the circuits

will wake up into normal continuous mode. First, the

bias currents of most circuits reduced during the

discontinuous mode are restored, and then a tON pulse

is triggered before the drivers are turned on to avoid

any possible glitches. Finally, the high-side driver is

turned on. Figure 4-3 shows the control loop timing in

discontinuous mode.

FIGURE 4-3:

MIC28303-1 Control Loop

Timing (Discontinuous Mode)

OUT

NO LOAD

FULL LOAD

REF

OFF(min)

L CROSSES 0 and VFB > 0.8

DISCONTINUOUS MODE STARTS

FB < 0.8. WAKE UP FROM

DISCONTINUOUS MODE

ESTIMATED ON-TIME

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