MCP19035-AAAAESLASHMF datasheet(20/44 Pages) MICROCHIP

20 / 44 page

MCP19035

DS22326B-page 20

 2012-2013 Microchip Technology Inc.

5.2.4

INPUT CAPACITOR SELECTION

The input capacitor is responsible for providing a low

impedance voltage source for the step-down converter.

This capacitor must be able to sustain high ripple

current, a consequence of the discontinuous input

current of the buck converter. A low equivalent series

resistance capacitor (ESR), preferably ceramic, is

recommended.

For

wide

temperature

range

applications,

multi-layer

X7R

dielectric

recommended, while for applications with limited

temperature range, a multi-layer X5R dielectric is

acceptable. A higher ESR will produce a higher voltage

ripple and higher power losses. The capacitor voltage

rating must be higher than the maximum operating

input voltage of the converter.

The

minimum

capacitance

determined

Equation 5-4:

EQUATION 5-4:

MINIMUM CAPACITANCE

FOR INPUT CAPACITOR

The maximum ripple current in the input capacitor

occurs when the duty cycle is 50%. This must be

considered worst case for calculating the input

capacitor.

The RMS current in the input capacitor is estimated

with Equation 5-5:

EQUATION 5-5:

RMS CURRENT IN THE

INPUT CAPACITOR

The input capacitor must be rated to sustain this RMS

current without considerable losses.

5.2.5

OUTPUT CAPACITOR SELECTION

The output capacitor is responsible for smoothing the

output voltage. It also plays an important role in the

stability of the control system. The voltage ripple across

the output capacitor is the sum of ripple voltages due to

the Equivalent Series Resistance (ESR) and the

voltage sag due to the load current that must be

supplied by the capacitor as the inductor is discharged.

A low ESR capacitor, preferably ceramic, is

recommended.

For

wide

temperature

range

applications,

multi-layer

X7R

dielectric

recommended, while for applications with limited

temperature range, a multi-layer X5R dielectric is

acceptable.

The output voltage ripple is estimated in Equation 5-6:

EQUATION 5-6:

OUTPUT VOLTAGE

RIPPLE

Minimum capacitance value is calculated according to

the demand of the load transient response. During a

transient load current, the excessive energy stored by

the inductor must be absorbed by the output capacitor

until the control loop sets the proper duty cycle.

Equation 5-7 calculates the minimum value for the

output capacitor value:

EQUATION 5-7:

OUTPUT CAPACITOR

MINIMUM VALUE

For applications that require low output voltage

overshoot during a step load, the value of the output

capacitor can become very large. In this case, it is

recommended to mix ceramic capacitors with

aluminum or polymer electrolytic capacitors to reach

the recommended value.

IN_MIN

OUT

–





Ripple

OUT

ESR



–





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

Where:

CIN_MIN = Minimum Capacitance of the Input

Capacitor (in Farad)

IOUT = Output Current (A)

D = Duty Cycle (for worst case this is 0.5)

fSW = Switching Frequency (Hz)

VRipple = Input Voltage Ripple (usually between

0.1V and 0.5V)

ESR = Equivalent Series Resistance of the

Capacitor (in Ohm)

RMS C



OUT

Ripple

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





OUT



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





–

Ripple

ESR

OUT



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







Where:

IRipple = Inductor Current Ripple (A)

VRipple = Output Voltage Ripple (V)

COUT = Output Capacitor (F)

ESR = Equivalent Series Resistance of the

Output Capacitor (Ohm)

OUT

–



OUT

–

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

Where:

IOH = Final Value of the Output Current

IOL = Initial Value of the Output Current

VOUT = Initial Output Voltage

Vf = Final Output Voltage

MCP19035-AAAAE/MF датащи(PDF) 20 Page - Microchip Technology

MCP19035-AAAAE/MF датащи(HTML) 20 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

датащи скачать

ссылки URL