MIC45212-2 データシートの表示（PDF） - Microchip Technology

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MIC45212-2

26V, 14A DC-to-DC Power Module

Microchip Technology 

MIC45212-1/-2

IL

IOUT

VOUT

IL(PP)

VOUT(PP) = ESRCOUT IL(PP)

VFB

VREF

VFB(PP)

=

VOUT(PP)



RFB2

RFB1 + RFB2

DH Trigger ON-Time if VFB is Below VREF

Estimated ON-time

FIGURE 4-2:

Timing.

MIC45212 Control Loop

Figure 4-3 shows the operation of the MIC45212 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 trig-

ger 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 feed-

back 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. Note that the

instantaneous switching frequency during load tran-

sient remains bounded and cannot increase arbitrarily.

The minimum is limited by tON + tOFF(MIN). Because the

variation in VOUT is relatively limited during load transient,

tON stays virtually close to its steady-state value.

IOUT

No Load

Full Load

VOUT

VFB

VREF

DH

FIGURE 4-3:

Response.

tOFF(MIN)

MIC45212 Load Transient

DS20005607A-page 16

Unlike true Current mode control, the MIC45212 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

MIC45212 feedback voltage ripple should be in phase

with the inductor current ripple, and is large enough to

be sensed by the gM amplifier and the error compara-

tor. 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 feed-

back 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 Section 5.5 “Ripple

Injection” in Section 5.0 “Application Information”

for more details about the ripple injection technique.

4.2 Discontinuous Mode

(MIC45212-1 only)

In Continuous mode, the inductor current is always

greater than zero; however, at light loads, the

MIC45212-1 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 4-4. During this period, the effi-

ciency is optimized by shutting down all the non-essential

circuits and minimizing the supply current as the

switching frequency is reduced. The MIC45212-1

wakes up and turns on the high-side MOSFET when

the feedback voltage, VFB, drops below 0.8V.

The MIC45212-1 has a Zero-Crossing (ZC) 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 MIC45212-1 automati-

cally powers down most of the IC circuitry and goes into

a Low-Power mode.

Once the MIC45212-1 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 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.

 2017 Microchip Technology Inc.

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