MIC2174 datasheet(16/27 Pages) MICREL

16 / 27 page

Micrel, Inc.

MIC2174/MIC2174C

September 2010

M9999-091310-C

where:

TH = temperature of wire under full load

T20°C = ambient temperature

RWINDING(20°C) = room temperature winding resistance

(usually specified by the manufacturer)

Output Capacitor Selection

The type of the output capacitor is usually determined by

its ESR (equivalent series resistance). Voltage and RMS

current capability are two other important factors for

selecting the output capacitor. Recommended capacitors

are tantalum, low-ESR aluminum electrolytic, OS-CON

and POSCAPS. The output capacitor’s ESR is usually

the main cause of the output ripple. The output capacitor

ESR also affects the control loop from a stability point of

view. The maximum value of ESR is calculated:

L(PP)

OUT(pp)

ΔI

ΔV

ESR

OUT

≤

(19)

where:

ΔVOUT(pp) = peak-to-peak output voltage ripple

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

The total output ripple is a combination of the ESR and

output capacitance. The total ripple is calculated below:

()2

L(PP)

OUT

L(PP)

OUT(pp)

OUT

ESR

ΔI

ΔV

⎟

⎠

⎞

⎜

⎝

⎛

(20)

where:

D = duty cycle

COUT = output capacitance value

fSW = switching frequency

As described in the “Theory of Operation” subsection in

Functional Description

, the MIC2174/MIC2174C requires

at least 20mV peak-to-peak ripple at the FB pin to make

the gm amplifier and the error comparator to behavior

properly. Also, the output voltage ripple should be in

phase with the inductor current. Therefore, the output

voltage ripple caused by the output capacitor COUT

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 FB 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 below:

ΔI

L(PP)

(RMS)

COUT

(21)

The power dissipated in the output capacitor is:

OUT

(RMS)

)

DISS(C

ESR

(22)

Input Capacitor Selection

The input capacitor for the power stage input VHSD

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:

ΔVIN = IL(pk) × ESRCIN

(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)

−

≈

(24)

The power dissipated in the input capacitor is:

PDISS(CIN) = ICIN(RMS)

2 × ESRCIN

(25)

External Schottky Diode (Optional)

An external freewheeling diode, which is not necessary,

is used to keep the inductor current flow continuous

while both MOSFETs are turned off. This dead-time

prevents current from flowing unimpeded through both

MOSFETs and is typically 30ns. The diode conducts

twice during each switching cycle. Although the average

current through this diode is small, the diode must be

able to handle the peak current.

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