MIC24045 datasheet(30/46 Pages) MICROCHIP

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MIC24045

DS20005568A-page 30

 2016 Microchip Technology Inc.

From the switching frequency, input voltage, output

voltage setting, and load current, the peak-to-peak

inductor current ripple and the peak inductor current

can be calculated as:

EQUATION 7-4:

EQUATION 7-5:

The capacitive ripple

VR,C and the ESR ripple

VR,ESR are given by:

EQUATION 7-6:

EQUATION 7-7:

The

total

peak-to-peak

output ripple is then

conservatively estimated as:

EQUATION 7-8:

The output capacitor value and the ESR should be

chosen so that

VR is within specifications. Capacitor

tolerance should be considered for worst-case calcula-

tions. In the case of ceramic output capacitors, factor

into account the decrease of effective capacitance

versus applied DC bias.

The worst-case load transient for output capacitor cal-

culation is an instantaneous 100% to 0% load release

when the inductor current is at its peak value. In this

case, all the energy stored in the inductor is absorbed

by the output capacitor while the converter stops

switching and keeps the low-side FET ON.

The peak output voltage overshoot (

VOUT) happens

when the inductor current has decayed to zero. This

can be calculated with Equation 7-9:

EQUATION 7-9:

Equation 7-10

calculates

the

minimum

output

capacitance value (COUT(MIN)) needed to limit the

output overshoot below

VOUT.

EQUATION 7-10:

The result from the minimum output capacitance value

for load transient is the most stringent requirement

found for capacitor value in most applications. Low

equivalent series resistance (ESR) ceramic output

capacitors, with X5R or X7R temperature characteris-

tics, are recommended.

For low-output voltage applications with demanding

load transient requirements, using a combination of

polarized and ceramic output capacitors may be most

convenient for smallest solution size.

7.6

Input Capacitor Selection

Two main requirements determine the size and charac-

teristics of the input capacitor:

• Steady-state ripple

• RMS current

The buck converter input current is a pulse train with

very fast rising and falling times so low-ESR ceramic

capacitors are recommended for input filtering,

because of their good high-frequency characteristics.

For ideal input filtering (assuming a DC input current

feeding the filtered buck power stage) and by neglect-

ing the capacitor ESR contribution to the input ripple

(typically possible for ceramic input capacitors), the

minimum capacitance value CIN(MIN) needed for a

given input peak-to-peak ripple voltage

Vr,IN can be

estimated as shown in Equation 7-11:

EQUATION 7-11:

I

L_PP

OUT

OUT VIN



–



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





L,PEAK

OUT

I

L_PP

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

V

R,C

I

L_PP



OUT



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

V

R,ESR

ESR

I

L_PP



V

R,C

V

R,ESR



V

OUT

--------------I

L,PEAK

OUT

–

OUT(MIN)

L,PEAK



V

OUT



OUT

–

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

IN(MIN)

OUT



–





V

r,IN



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

where:

D = the duty cycle at the given operating point

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