MIC2102YML datasheet(21/37 Pages) MICREL

21 / 37 page

Micrel, Inc.

MIC2101/02

August 2012

M9999-080712-A

Figure 3a shows the operation of the MIC2101/02 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

since 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 MIC2101/02

converter.

Figure 3a. MIC2101/02 Load Transient Response

Unlike true current-mode control, the MIC2101/02 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

MIC2101/02 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

20mV~100mV over 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 to ensure proper operation. Please

refer to “Ripple Injection” subsection in Application

Information

for more details about the ripple injection

technique.

Discontinuous Mode (MIC2101 only)

In continuous mode, the inductor current is always

greater than zero; however, at light loads the MIC2101 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 3b. During this period, the efficiency is

optimized by shutting down all the non-essential circuits

and minimizing the supply current. The MIC2101 wakes

up and turns on the high-side MOSFET when the

feedback voltage VFB drops below 0.8V.

The MIC2101 has a zero crossing comparator (ZC

Detection) 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 MIC2101 automatically

powers down most of the IC circuitry and goes into a

low-power mode.

Once the MIC2101 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

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 3b shows the control loop timing in

discontinuous mode.

Figure 3b. MIC2101 Control Loop Timing

(Discontinuous Mode)

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