MIC33M350 datasheet(21/32 Pages) MICROCHIP

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DS20006348B-page 21

MIC33M350

5.0

APPLICATION INFORMATION

5.1

Output Voltage Sensing

To achieve accurate output voltage regulation, the

VOUT pin (internal feedback divider top terminal) should

be Kelvin-connected as close as possible to the point

of regulation top terminal. Since both the internal

reference and the internal feedback divider’s bottom

terminal refer to AGND, it is important to minimize

voltage drops between the AGND and the point of

regulation return terminal (typically the ground terminal

of the output capacitor which is closest to the load).

5.2

Output Capacitor Selection

The type of the output capacitor is usually determined

by its Equivalent Series Resistance (ESR). Voltage and

RMS current capability are two other important factors

for selecting the output capacitor. Recommended

capacitor types are ceramic, low-ESR aluminum

electrolytic, OS-CON, and POSCAP. 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 using Equation 5-1.

EQUATION 5-1:

The peak-to-peak inductor current ripple can be

calculated with the formula in Equation 5-2.

EQUATION 5-2:

The total output ripple is a combination of the ESR and

output capacitance.The total ripple is calculated in

Equation 5-3.

EQUATION 5-3:

The output capacitor RMS current is calculated in

Equation 5-4.

EQUATION 5-4:

The power dissipated in the output capacitor is:

EQUATION 5-5:

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 can fail when

subjected to high inrush currents, caused by turning on

the input supply. 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

derating. The input voltage ripple depends on the input

capacitor’s ESR. The peak input current is equal to the

peak inductor current, as shown in Equation 5-6.

EQUATION 5-6:

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 5-7:

The power dissipated in the input capacitor is

calculated in Equation 5-8.

EQUATION 5-8:

ESRC

OUT

VOUT PP





ILPP





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



Where:

VOUT(PP)

Peak-to-peak output voltage ripple

IL(PP)

Peak-to-peak inductor current ripple

I

L(PP)

VOUT VIN(MAX) VOUT

–





VIN(MAX) fSW



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

Where:

L = 0.47 µH

VOUT PP





ILPP





COUT fSW



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





 2

ILPP





ESRC

OUT





 2

Where:

COUT

Output Capacitance Value

fSW

Switching Frequency

OUT RMS



ILPP





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

PDISS COUT



ICOUT RMS



ESRCOUT





ILPK

 CESR



ICIN RMS

 IOUT MAX



D1 D

–







PDISS CIN



ICIN RMS



CESR



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