MIC2104 datasheet(23/42 Pages) MICROCHIP

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DS20005899A-page 23

MIC2103/4

less than VREF. This causes 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 MIC2103/4 converter.

FIGURE 4-2:

MIC2103/4 Load Transient

Response.

Unlike true current-mode control, the MIC2103/4 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

MIC2103/4 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 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 the Ripple Injection

subsection in Application Information for more details

about the ripple injection technique.

4.2

Discontinuous Mode (MIC2103

Only)

In continuous mode, the inductor current is always

greater than zero. However, at light loads, the MIC2103

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

MIC2103 wakes up and turns on the high-side

MOSFET when the feedback voltage VFB drops below

0.8V.

The MIC2103 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 MIC2103

automatically powers down most of the IC circuitry and

goes into a low-power mode.

Once the MIC2103 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, 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:

MIC2103 Control Loop

Timing (Discontinuous Mode).

OUT

REF

FULL LOAD

NO LOAD

OFF(min)

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