MIC45404 datasheet(21/32 Pages) MICROCHIP

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DS20005478A-page 21

MIC45404

5.5

Permissible MIC45404 Settings

Combinations

The MIC45404 allowable settings are constrained by

the values in Table 5-4.

5.6

Output Capacitor Selection

Two main requirements determine the size and

characteristics of the output capacitor, CO:

• Steady-state ripple

• Maximum voltage deviation during load transient

For steady-state ripple calculation, both the ESR and

the capacitive ripple contribute to the total ripple ampli-

tude. The MIC45404 utilizes a low loss inductor, whose

nominal value is 1.2 µH. 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 5-4:

EQUATION 5-5:

The capacitive ripple,

Vr,C, and the ESR ripple,

Vr,ESR, are given by:

EQUATION 5-6:

EQUATION 5-7:

The

total peak-to-peak

output

ripple is then

conservatively estimated as:

EQUATION 5-8:

The output capacitor value and ESR should be chosen

such that

VR is within specifications. Capacitor toler-

ance should be considered for worst-case calculations.

In 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 (

VO) happens

when the inductor current has decayed to zero. This

can be calculated with Equation 5-9:

EQUATION 5-9:

Equation 5-10

calculates

the

minimum

output

capacitance value (CO(MIN)) needed to limit the output

overshoot below

VO.

EQUATION 5-10:

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 ratings, are

recommended.

For low output voltage applications with demanding

load transient requirements, using a combination of

polarized and ceramic output capacitors may be the

most convenient option for smallest solution size.

TABLE 5-4:

PERMISSIBLE MIC45404

SETTINGS COMBINATIONS

Output Voltage

Frequency

3.3V

790 kHz

2.5V (2.49V)

1.8V

565 kHz

1.5V

1.2V

400 kHz

1.0V

0.9V

0.8V

0.7V

I

L_PP

--------

–



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









L,P EAK

I

L_PP

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

V

R,C

I

L_PP



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

V

R,ESR

ESR

I

L_PP



V

R,C

V

R,ESR



V

-------

L,PEAK



–

O MIN



 2

L,PEAK

V



–

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