MIC2102YML datasheet(27/37 Pages) MICREL

27 / 37 page

Micrel, Inc.

MIC2101/02

August 2012

M9999-080712-A

The total output ripple is a combination of the ESR and

output capacitance. The total ripple is calculated in

Equation 20:

2

L(PP)

OUT

L(PP)

OUT(pp)

OUT

ESR

∆I

∆V





















Eq. 20

where:

D = duty cycle

COUT = output capacitance value

fsw = switching frequency

As described in the “Theory of Operation” subsection in

Functional Description

, the MIC2101/02 requires at least

20mV 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” subsection 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 in Equation 21:

∆I

L(PP)

(RMS)

COUT



Eq. 21

The power dissipated in the output capacitor is:

OUT

(RMS)

)

DISS(C

ESR





Eq. 22

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:

∆VIN = IL(pk) × ESRCIN

Eq. 23

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:

OUT(max)

CIN(RMS)







Eq. 24

The power dissipated in the input capacitor is:

PDISS(CIN) = ICIN(RMS)

2 × ESRCIN

Eq. 25

Voltage Setting Components

The MIC2101/02 requires two resistors to set the output

voltage as shown in Figure 7:

Figure 7. Voltage-Divider Configuration

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