MCP16331 datasheet(19/48 Pages) MICROCHIP

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DS20005308D-page 19

MCP16331

5.0

APPLICATION INFORMATION

5.1

Typical Applications

The MCP16331 step-down converter operates over a

wide input voltage range, up to 50V maximum. Typical

applications include generating a bias or VDD voltage for

the PIC® microcontroller product line, digital control

system bias supply for AC-DC converters, 24V industrial

input and similar applications.

5.2

Adjustable Output Voltage

Calculations

To calculate the resistor divider values for the

MCP16331, Equation 5-1 can be used. RTOP is

connected to VOUT, RBOT is connected to GND and

both are connected to the VFB input pin.

EQUATION 5-1:

EXAMPLE 5-1:

EXAMPLE 5-2:

The transconductance error amplifier gain is controlled

by its internal impedance. The external resistor divider

have no effect on system gain, so a wide range of values

can be used. A 10 k

 bottom resistor is recommended

as a good trade-off for quiescent current and noise

immunity.

5.3

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 also counts the

forward drop of the freewheeling diode and internal

N-Channel MOSFET switch voltage drop. As the load

current increases, the switch voltage drop and diode

voltage drop increase as well, 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 (RDSON).

EQUATION 5-2:

CONTINUOUS INDUCTOR

CURRENT DUTY CYCLE

The MCP16331 device features an integrated slope

compensation to prevent the bimodal operation of the

PWM duty cycle. Internally, half of the inductor current

downslope is summed with the internal current sense

signal. For the proper amount of slope compensation,

it is recommended to keep the inductor down-slope

current constant, by varying the inductance with VOUT,

where K = 0.22 V/µH.

EQUATION 5-3:

For VOUT = 3.3V, an inductance of 15 µH is

recommended.

RTOP

RBOT

VOUT

VFB

-------------1

–







VOUT =3.3V

VFB =0.8V

RBOT =10 k

RTOP = 31.25 k (standard value = 31.6 k)

VOUT = 3.328V (using standard value)

VOUT =5.0V

VFB =0.8V

RBOT =10 k

RTOP = 52.5 k (standard value = 52.3 k)

VOUT = 4.98V (using standard value)

TABLE 5-1:

RECOMMENDED INDUCTOR

VALUES

VOUT

LSTANDARD

2.0V

0.20

10 µH

3.3V

0.22

15 µH

5.0V

0.23

22 µH

12V

0.21

56 µH

15V

0.22

68 µH

24V

0.24

100 µH

VOUT VDiode



VIN

ISW RDSON





–



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

KVOUT L



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