MIC45205 datasheet(18/40 Pages) MICROCHIP

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MIC45205

DS20005798B-page 18



2017 - 2022 Microchip Technology Inc.

5.0

APPLICATION INFORMATION

5.1

Setting the Switching Frequency

The MIC45205 switching frequency can be adjusted by

changing the value of resistors R1 and R2.

FIGURE 5-1:

Switching Frequency

Adjustment.

Equation 5-1 gives the estimated switching frequency:

EQUATION 5-1:

FIGURE 5-2:

Switching Frequency vs. R2.

5.2

Output Capacitor Selection

The type of the output capacitor is usually determined

by the application and its equivalent series resistance

(ESR). Voltage and RMS current capability are two

other important factors for selecting the output

capacitor. Recommended capacitor types are MLCC,

OS-CON and POSCAP. The output capacitor’s ESR is

usually the main cause of the output ripple. The

MIC45205 requires ripple injection and the output

capacitor ESR affects the control loop from a stability

point of view.

The maximum value of ESR is calculated as in

Equation 5-2:

EQUATION 5-2:

The total output ripple voltage is a combination of ripple

voltages due to the ESR and output capacitance. The

total output ripple voltage is calculated in Equation 5-3:

EQUATION 5-3:

As described in the Theory of Operation subsection in

the Functional Description, the MIC45205 requires at

least 20 mV 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 enough feedback voltage ripple. Please refer

to the Ripple Injection subsection in the Application

Information section for more details.

VIN

MIC45205

BST

PGND

FREQ

fSW fO

R1 R2

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



Where:

600 kHz (typical per the Electrical

Characteristics table).

100 kΩ is recommended.

Needs to be selected in order to set the

required switching frequency.

100

200

300

400

500

600

700

800

10.00

100.00

1000.00

10000.00

R2 (k )

OUT = 5V

IN = 12V

R1 = 100k

ESRCOUT

VOUT PP







IL PP







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



Where:

ΔVOUT(PP) Peak-to-peak output voltage ripple

ΔIL(PP)

Peak-to-peak inductor current

ripple

VOUT PP







IL PP







COUT fSW



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









IL PP







ESRCOUT





2

Where:

COUT Output Capacitance Value

fSW Switching Frequency

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