MIC28304 datasheet(23/39 Pages) MICREL

23 / 39 page

Micrel, Inc.

MIC28304

Figure 1. MIC28304 Control Loop Timing

Figure 2 shows the operation of the MIC28304 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 the

bootstrap capacitor (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.

Figure 2. MIC28304 Load Transient Response

Unlike true current-mode control, the MIC28304 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 MIC28304

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 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 (MIC28304-1 only)

In continuous mode, the inductor current is always

greater than zero; however, at light loads, the MIC28304-

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 3. During this period, the efficiency is

optimized by shutting down all the non-essential circuits

and minimizing the supply current. The MIC28304-1

wakes up and turns on the high-side MOSFET when the

feedback voltage VFB drops below 0.8V.

The MIC28304-1 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 MIC28304-1 automatically powers

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

mode.

Once the MIC28304-1 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

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

shows

the

control

loop

timing

discontinuous mode.

March 25, 2014

Revision 1.1

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