MOS6020 datasheet(19/34 Pages) MICROCHIP

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DS22285A-page 19

MCP16321/2

5.0

APPLICATION INFORMATION

5.0.1

TYPICAL APPLICATIONS

The MCP16321/2 synchronous step-down converter

operates over a wide input range, up to 24V maximum.

Typical applications include generating a bias or VDD

voltage for PIC® microcontrollers, digital control system

bias supply for AC-DC converters and 12V industrial

input and similar applications.

5.0.2

ADJUSTABLE OUTPUT VOLTAGE

CALCULATIONS

To calculate the resistor divider values for the

MCP16321/2 adjustable version, use Equation 5-1.

RTOP is connected to VOUT, RBOT is connected to

SGND, and both are connected to the VFB input pin.

EQUATION 5-1:

RESISTOR DIVIDER

CALCULATION

EXAMPLE 5-1:

2.0V RESISTOR DIVIDER

EXAMPLE 5-2:

4.2V RESISTOR DIVIDER

The error amplifier is internally compensated to ensure

loop stability. External resistor dividers, inductance and

output capacitance, all have an impact on the control

system and should be selected carefully and evaluated

for stability. A 10 kΩ resistor is recommended as a

good trade-off for quiescent current and noise

immunity.

5.0.3

GENERAL DESIGN EQUATIONS

The step-down converter duty cycle can be estimated

using Equation 5-2 while operating in Continuous

Inductor Current Mode. This equation accounts for the

forward drop of two internal N-Channel MOSFETS. As

load current increases, the voltage drop in both internal

switches will increase, requiring a larger PWM duty

cycle to maintain the output voltage regulation. Switch

voltage drop is estimated by multiplying the switch

current times the switch resistance or RDSON.

EQUATION 5-2:

CONTINUOUS INDUCTOR

CURRENT DUTY CYCLE

5.0.4

INPUT CAPACITOR SELECTION

The step-down converter input capacitor must filter the

high-input ripple current, as a result of pulsing or

chopping the input voltage. The MCP16321/2 input

voltage pin is used to supply voltage for the power train

and as a source for internal bias. A low equivalent

series resistance (ESR), preferably a ceramic

capacitor,

recommended.

The

necessary

capacitance is dependent upon the maximum load

current and source impedance. Three capacitor

parameters to keep in mind are the voltage rating,

equivalent series resistance and the temperature

rating. For wide temperature range applications, a

multilayer X7R dielectric is recommended, while for

applications with limited temperature range, a

multilayer X5R dielectric is acceptable. The input

capacitor voltage rating must be VIN plus margin.

5.0.5

OUTPUT CAPACITOR SELECTION

The output capacitor provides a stable output voltage

during sudden load transients, and reduces the output

voltage ripple. As with the input capacitor, X5R and

X7R ceramic capacitors are well suited for this applica-

tion.

The MCP16321/2 is internally compensated, so the

output capacitance range is limited. See TABLE 5-1:

“Capacitor Value Range”

for the recommended out-

put capacitor range.

The amount and type of output capacitance and

equivalent series resistance will have a significant

effect on the output ripple voltage and system stability.

The range of the output capacitance is limited due to

the integrated compensation of the MCP16321/2.

The output voltage capacitor rating should be a

minimum of VOUT plus margin.

VOUT =2.0V

VFB =0.9V

RBOT =10 kΩ

RTOP =12.2 kΩ (standard value = 12.3 kΩ)

VOUT = 2.007V (using standard values)

VOUT =4.2V

VFB =0.9V

RBOT =10 kΩ

RTOP =36.7 kΩ (standard value = 36.5 kΩ)

VOUT = 4.185V (using standard values)

OUT

TOP

BOT

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

⎝⎠

⎛⎞

TABLE 5-1:

CAPACITOR VALUE RANGE

Parameter

Min

Max

CIN

8µF

None

OUT

LSW

DSONL

()

HSW

DSONH

()

–

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