HCF1305-2R2-R datasheet(17/39 Pages) MICROCHIP

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DS20007043B-page 17

MIC24054

Figure 4-2 shows the operation of the MIC24054 during

a load transient. The output voltage drops due to the

sudden load increase, which causes the VFB to be less

than VREF. This will cause the error comparator to

trigger an ON-time period. At the end of the ON-time

period, a minimum OFF-time, tOFF(MIN), is generated to

charge 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 in MIC24054 converter.

FIGURE 4-2:

Load Transient Response.

Unlike true current-mode control, the MIC24054 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. The MIC24054 control loop has the advantage

of eliminating the need for slope compensation.

In order to meet stability requirements, the MIC24054

feedback voltage ripple should be in phase with the

inductor current ripple and large enough to be sensed

by the gm amplifier and the error comparator. The

recommended

feedback

voltage

ripple

20 mV~100 mV. 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 to ensure proper operation. Please refer to the

Ripple Injection section for more details about the

ripple injection technique.

4.3

Discontinuous Mode

In continuous mode, the inductor current is always

greater than zero; however, at light loads, the

MIC24054 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 MIC24054 wakes up and turns on the

high-side MOSFET when the feedback voltage (VFB)

drops below 0.8V.

The MIC24054 has a zero crossing comparator 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 MIC24054 automatically powers

down most of the IC circuitry and goes into a low-power

mode.

Once the MIC24054 goes into discontinuous mode,

both LSD and HSD 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, 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:

Control Loop Timing

(Discontinuous Mode).

During discontinuous mode, the bias current of most

circuits are reduced. As a result, the total power supply

current during discontinuous mode is only about

450 µA, allowing the MIC24054 to achieve high

efficiency in light load applications.

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