MIC26901 datasheet(21/39 Pages) MICROCHIP

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

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DS20007047B-page 21

MIC26901

The maximum value of ESR is calculated:

EQUATION 5-7:

The total output ripple is a combination of the ESR and

output capacitance. The total ripple is calculated in

Equation 5-8:

EQUATION 5-8:

As described in the Theory of Operation section, the

MIC26901 requires at least 20 mV peak-to-peak ripple

at the FB pin to make the gm amplifier and the error

comparator behave properly. Also, the output voltage

ripple 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. Please refer to the Ripple

Injection section for more 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 below:

EQUATION 5-9:

The power dissipated in the output capacitor is:

EQUATION 5-10:

5.3

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 5-11:

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 5-12:

The power dissipated in the input capacitor is:

EQUATION 5-13:

5.4

Ripple Injection

The VFB ripple required for proper operation of the

MIC26901 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.

For a low output voltage, such as a 1V, the output

voltage ripple is only 10 mV to 20 mV, and the feedback

voltage ripple is less than 20 mV. If the feedback

voltage ripple is so small that the gm amplifier and error

comparator can’t sense it, then the MIC26901 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.

ESRC OUT





VOUT PP







IL PP







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



Where:

ΔVOUT(PP) = Peak-to-peak output voltage ripple

ΔIL(PP) = Peak-to-peak inductor current ripple

VOUT PP







IL PP







COUT fSW



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









IL PP







ESRC OUT









2

Where:

COUT = Output capacitance value

fSW = Switching frequency

ICOUT RMS





IL PP







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

PDISS COUT





ICOUT RMS





2 ESR

COUT



VIN



IL PK





ESRCIN



ICIN RMS





IOUT MAX





1 D

–









PDISS CIN





ICIN RMS





2 ESR

CIN



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