MIC28512 datasheet(24/34 Pages) MICROCHIP

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MIC28512

DS20005524A-page 24

 2016 Microchip Technology Inc.

As described in the Theory of Operation subsection of

the Functional Description, the MIC28512 requires at

least 20 mV peak-to-peak ripple at the FB pin for the gM

amplifier and the error comparator to operate properly.

Also, the ripple on FB pin should be in phase with the

inductor current. Therefore, the output voltage ripple

caused by the output capacitors value should be much

smaller than the ripple caused by the output capacitor

ESR. If low-ESR capacitors, such as ceramic

capacitors, are selected as the output capacitors, a

ripple injection method should be applied to provide the

enough feedback voltage ripple. Refer to the Ripple

Injection subsection for details.

The voltage rating of the capacitor should be twice the

output voltage for a tantalum and 20% greater for

aluminum electrolytic or OS-CON. The output capacitor

RMS current is calculated in Equation 5-11.

EQUATION 5-11:

The power dissipated in the output capacitor is

calculated by using:

EQUATION 5-12:

5.5

Input Capacitor Selection

The input capacitor for the power stage input VIN

should be selected for ripple current rating and voltage

rating. Tantalum input capacitors may fail when

subjected to high inrush currents, caused by turning the

input supply on. A tantalum input capacitor’s voltage

rating should be at least two times the maximum input

voltage to maximize reliability. Aluminum electrolytic,

OS-CON, and multilayer polymer film capacitors can

handle the higher inrush currents without voltage

de-rating. The input voltage ripple will primarily depend

on the input capacitor’s ESR. The peak input current is

equal to the peak inductor current, so:

EQUATION 5-13:

The input capacitor must be rated for the input current

ripple. The RMS value of input capacitor current is

determined at the maximum output current. Assuming

the peak-to-peak inductor current ripple is low:

EQUATION 5-14:

The power dissipated in the input capacitor is:

EQUATION 5-15:

5.6

Ripple Injection

The VFB ripple required for proper operation of the

MIC28512’s gM amplifier and error comparator is

20 mV to 100 mV. However, the output voltage ripple is

generally designed as 1% to 2% of the output voltage.

If the feedback voltage ripple is so small that the gM

amplifier and error comparator can’t sense it, then the

MIC28512 will lose control and the output voltage is not

regulated. In order to have some amount of VFB ripple,

a ripple injection method is applied for low output

voltage ripple applications.

The applications are divided into three situations

according to the amount of the feedback voltage ripple:

• Enough ripple at the feedback voltage due to the

large ESR of the output capacitors (Figure 5-4).

The converter is stable without any ripple

injection.

FIGURE 5-4:

Enough Ripple at FB.

The feedback voltage ripple is:

EQUATION 5-16:

• Inadequate ripple at the feedback voltage due to

the small ESR of the output capacitors.

The output voltage ripple is fed into the FB pin

through a feed-forward capacitor, CFF in this

situation, as shown in Figure 5-5. The typical CFF

value is selected by using Equation 5-17.

OUT RMS



LPP





------------------

DISS COUT



COUT RMS



ESR

COUT





LPK



ESR

CIN



CIN RMS



OUT MAX



–







DISS CIN



CIN RMS



ESR

CIN



FB PP





--------------------

ESR

OUT



LPP







Where:

∆IL(PP)

Peak-to-Peak Value of the Inductor

Current Ripple

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