MIC2126 datasheet(18/34 Pages) MICROCHIP

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MIC2125/6

DS20005459B-page 18

 2015 Microchip Technology Inc.

FIGURE 4-2:

MIC2125/6 Load Transient

Response

Unlike true current-mode control, the MIC2125/6 uses

the output voltage ripple to trigger an ON-time period.

In order to meet the stability requirements, the

MIC2125/6 feedback voltage ripple should be in phase

with the inductor current ripple and large enough to be

sensed by the gm amplifier. The recommended

feedback voltage ripple is 20 mV ~ 100 mV over the 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. 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. For these

applications, ripple injection is required to ensure

proper operation. Refer to the Ripple Injection section

under Application Information for details about the

ripple injection technique.

4.2

Discontinuous Conduction Mode

(MIC2125 Only)

The MIC2125 operates in discontinuous conduction

mode at light load. The MIC2125 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 > 0.6V and the

inductor current goes slightly negative, the MIC2125

turns off both the high-side and low-side MOSFETs.

During this period, the efficiency is optimized by

shutting down all the non-essential circuits and the load

current is supplied by the output capacitor. The control

circuitry wakes up when the feedback voltage falls

below VREF and triggers a tON pulse. Figure 4-3 shows

the control loop timing in discontinuous conduction

mode.

FIGURE 4-3:

MIC2125 Control Loop

Timing (Discontinuous Conduction Mode)

The typical no load supply current during discontinuous

conduction mode is only about 340 μA, allowing the

MIC2125 to achieve high efficiency at light load

operation.

4.3

Soft-Start

Soft-start reduces the power supply inrush current at

startup by controlling the output voltage rise time. The

MIC2125/6 implements an internal digital soft-start by

ramping up the reference voltage VREF from 0 to 100%

in about 7 ms. Once the soft-start is completed, the

related circuitry is disabled to reduce the current

consumption.

ESTIMATED ON TIME

LSD

HSD

REF

L CROSSES 0 AND VFB > 0.6.

DISCONTINUOUS CONDUCTION MODE STARTS.

FB > 0.8. WAKE UP FROM

DISCONTINUOUS CONDUCTION MODE.

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