MIC28512 datasheet(19/34 Pages) MICROCHIP

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 2016 Microchip Technology Inc.

DS20005524A-page 19

MIC28512

FIGURE 4-2:

MIC28511 Control Loop

Timing.

Figure 4-3 shows the operation of the MIC28512 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 MIC28512 converter.

FIGURE 4-3:

MIC28512 Load Transient

Response.

Unlike true current-mode control, the MIC28512 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 MIC28512 control loop has the advantage

of eliminating the need for slope compensation.

In order to meet the stability requirements, the

MIC28512 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 is 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, if

the ESR of the output capacitor is very low, the output

voltage ripple and the feedback voltage ripple are not

necessarily in phase with the inductor current ripple. In

these cases, ripple injection is required to ensure

proper operation. Please refer to “Ripple Injection”

subsection in Application Information for more details

about the ripple injection technique.

4.2

Discontinuous Mode (MIC28512-1

Only)

In continuous mode, the inductor current is always

greater than zero; however, at light loads the

MIC28512-1 is able to force the inductor current to

operate in discontinuous mode. Discontinuous mode

occurs when the inductor current falls to zero, as

indicated by trace (IL) shown in Figure 4-4. During this

period, the efficiency is optimized by shutting down all

the non-essential circuits and minimizing the supply

current. The MIC28512-1 wakes up and turns on the

high-side MOSFET when the feedback voltage VFB

drops below 0.8V.

The MIC28512-1 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 MIC28521-1 automatically powers

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

mode.

Once the MIC28512-1 goes into discontinuous mode,

both DH and DL 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 4-4 shows the control loop timing in

discontinuous mode.

L(PP)

OUT(PP) = ESRC

× I

L(PP)

OUT

FB(PP) =

OUT(PP) ×

R1 + R2

TRIGGER ON-TIME IF V

FB IS BELOW VREF

ESTIMATED ON-TIME

OUT

REF

HSD

OUT

HSD

REF

OFF(MIN)

NO LOAD

FULL LOAD

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